Molecular cloning and functional expression of chromaffin cell scinderin indicates that it belongs to the family of Ca(2+)-dependent F-actin severing proteins

Comprehensive Analysis Based on the TCGA Database Identified SCIN as a Key DNA Methylation-Driver Gene in Epstein-Barr Virus-Associated Gastric Cancer

An important feature of EBV-associated gastric cancer (EBVaGC) is extensive methylation of viral and host genomes. This study aims to analyze DNA methylation-driven genes (DMDG) in EBVaGC through bioinformatics methods, providing an important bioinformatics basis for the differential diagnosis and treatment of potential methylation biomarkers in EBVaGC. We downloaded the mRNA expression profiles and methylation datasets of EBVaGC and EBV-negative gastric cancer (EBVnGC) through the TCGA database to screen methylated-differentially expressed genes (MDEGs). DNA methylation-driver genes were identified based on MethylMix algorithm and key genes were further identified by LASSO regression and Random Forest algorithm. Then, we performed gene enrichment analysis for key genes and validated them by GEO database. Gene expression differences in EBVaGC and EBVnGC cell lines was determined by quantitative real-time PCR (qRT-PCR) and western blotting and in GT38 cell and SNU719 cell which all treated by 5-Aza-CdR. Finally, the effect of key gene on the migration and proliferation capacity of EBVaGC cells was determined by Transwells assay and Cell counting Kit-8 (CCK-8) assay. We obtained a total of 687 hypermethylation-low expression genes (Hyper-LGs) and further obtained 53 DNA methylation-driver genes based on the MethylMix algorithm. A total of six key genes (SCIN, ETNK2, PCDH20, PPP1R3C, MATN2, and HOXA5) were identified by LASSO regression and Random Forest algorithm. Among them, SCIN expression was significantly lower in EBVaGC cell lines than in EBVnGC cell lines, and its expression was significantly recovered in EBVaGC cell lines treated with 5-Aza-CdR. Overexpression of SCIN can promote the proliferation and migration capacity of EBVaGC cells. Our study will provide some bioinformatics basis for the study of EBVaGC-related methylation. SCIN may be used as potential methylation biomarkers for the diagnosis and treatment of EBVaGC.

The actin-binding protein Adseverin mediates neutrophil polarization and migration

Through the process of chemotactic migration, neutrophils are able to recruit to an inflammatory site and eliminate pathogens, thus playing a vital role in host defense. The process of neutrophil chemotaxis is mediated by dynamic actin reorganization and polymerization. Adseverin, an actin-binding protein and member of the Gelsolin superfamily of proteins, has been hypothesized to regulate goal directed movement through the capping and severing of actin filaments, but has never been investigated in the context of neutrophil chemotaxis. Using an Adseverin knockout mouse model, we show that Adseverin plays a role in subcortical F-actin assembly at the leading edge during chemotaxis through the generation of free barbed ends on existing actin filaments. In addition, in the absence of Adseverin, neutrophils show reduced speed of migration in vitro and in vivo. This study indicates that Adseverin is a regulator of actin filament generation during neutrophil chemotaxis.

Adseverin knockdown inhibits osteoclastogenesis in RAW264.7 cells

Osteoclastogenesis is a complex process that is highly dependent on the dynamic regulation of the actin cytoskeleton. Adseverin (Ads), a member of the gelsolin superfamily of actin-binding proteins, regulates actin remodeling by severing and capping actin filaments. The objective of the present study was to characterize the role of Ads during osteoclastogenesis by assessing Ads expression and using a knockdown strategy. Immunoblot analyses were used to examine Ads expression during osteoclastogenesis. A stable Ads knockdown macrophage cell line was generated using a retroviral shRNA construct. Osteoclast differentiation was morphologically examined via cell staining with osteoclast specific markers and light microscopy. The results showed that Ads expression was significantly increased in response to receptor activator of nuclear factor-κB ligand during osteoclastogenesis, and Ads was highly expressed in mature osteoclasts. Ads-knockdown macrophages showed major osteoclastogenesis defects, most likely caused by a pre-osteoclast fusion defect. These results indicate that Ads deficiency in monocytes inhibits osteoclastogenesis. Thus, in future studies it could be noteworthy to investigate the function of Ads in bone marrow monocytes during osteoclastogenesis.

The cortical acto-Myosin network: from diffusion barrier to functional gateway in the transport of neurosecretory vesicles to the plasma membrane

Dysregulation of regulated exocytosis is linked to an array of pathological conditions, including neurodegenerative disorders, asthma, and diabetes. Understanding the molecular mechanisms underpinning neuroexocytosis including the processes that allow neurosecretory vesicles to access and fuse with the plasma membrane and to recycle post-fusion, is therefore critical to the design of future therapeutic drugs that will efficiently tackle these diseases. Despite considerable efforts to determine the principles of vesicular fusion, the mechanisms controlling the approach of vesicles to the plasma membrane in order to undergo tethering, docking, priming, and fusion remain poorly understood. All these steps involve the cortical actin network, a dense mesh of actin filaments localized beneath the plasma membrane. Recent work overturned the long-held belief that the cortical actin network only plays a passive constraining role in neuroexocytosis functioning as a physical barrier that partly breaks down upon entry of Ca(2+) to allow secretory vesicles to reach the plasma membrane. A multitude of new roles for the cortical actin network in regulated exocytosis have now emerged and point to highly dynamic novel functions of key myosin molecular motors. Myosins are not only believed to help bring about dynamic changes in the actin cytoskeleton, tethering and guiding vesicles to their fusion sites, but they also regulate the size and duration of the fusion pore, thereby directly contributing to the release of neurotransmitters and hormones. Here we discuss the functions of the cortical actin network, myosins, and their effectors in controlling the processes that lead to tethering, directed transport, docking, and fusion of exocytotic vesicles in regulated exocytosis.

There is more than one way to model an elephant. Experiment-driven modeling of the actin cytoskeleton

Mathematical modeling has established its value for investigating the interplay of biochemical and mechanical mechanisms underlying actin-based motility. Because of the complex nature of actin dynamics and its regulation, many of these models are phenomenological or conceptual, providing a general understanding of the physics at play. But the wealth of carefully measured kinetic data on the interactions of many of the players in actin biochemistry cries out for the creation of more detailed and accurate models that could permit investigators to dissect interdependent roles of individual molecular components. Moreover, no human mind can assimilate all of the mechanisms underlying complex protein networks; so an additional benefit of a detailed kinetic model is that the numerous binding proteins, signaling mechanisms, and biochemical reactions can be computationally organized in a fully explicit, accessible, visualizable, and reusable structure. In this review, we will focus on how comprehensive and adaptable modeling allows investigators to explain experimental observations and develop testable hypotheses on the intracellular dynamics of the actin cytoskeleton.

New insights into the role of the cortical cytoskeleton in exocytosis from neuroendocrine cells

The cortical cytoskeleton is a dense network of filamentous actin (F-actin) that participates in the events associated with secretion from neuroendocrine cells. This filamentous web traps secretory vesicles, acting as a retention system that blocks the access of vesicles to secretory sites during the resting state, and it mediates their active directional transport during stimulation. The changes in the cortical cytoskeleton that drive this functional transformation have been well documented, particularly in cultured chromaffin cells. At the biochemical level, alterations in F-actin are governed by the activity of molecular motors like myosins II and V and by other calcium-dependent proteins that influence the polymerization and cross-linking of F-actin structures. In addition to modulating vesicle transport, the F-actin cortical network and its associated motor proteins also influence the late phases of the secretory process, including membrane fusion and the release of active substances through the exocytotic fusion pore. Here, we discuss the potential interactions between the F-actin cortical web and proteins such as SNAREs during secretion. We also discuss the role of the cytoskeleton in organizing the molecular elements required to sustain regulated exocytosis, forming a molecular structure that foments the efficient release of neurotransmitters and hormones.

Dual effect of cell-cell contact disruption on cytosolic calcium and insulin secretion

Cell-to-cell interactions play an important role in insulin secretion. Compared with intact islets, dispersed pancreatic beta-cells show increased basal and decreased glucose-stimulated insulin secretion. In this study, we used mouse MIN6B1 cells to investigate the mechanisms that control insulin secretion when cells are in contact with each other or not. RNAi-mediated silencing of the adhesion molecule E-cadherin in confluent cells reduced glucose-stimulated secretion to the levels observed in isolated cells but had no impact on basal secretion. Dispersed cells presented high cytosolic Ca(2+) activity, depolymerized cytoskeleton and ERK1/2 activation in low glucose conditions. Both the increased basal secretion and the spontaneous Ca(2+) activity were corrected by transient removal of Ca(2+) or prolonged incubation of cells in low glucose, a procedure that restored the ability of dispersed cells to respond to glucose (11-fold stimulation). In conclusion, we show that dispersed pancreatic beta-cells can respond robustly to glucose once their elevated basal secretion has been corrected. The increased basal insulin secretion of dispersed cells is due to spontaneous Ca(2+) transients that activate downstream Ca(2+) effectors, whereas engagement of cell adhesion molecules including E-cadherin contributes to the greater secretory response to glucose seen in cells with normal intercellular contacts.

Multiple roles of the furrow deepening Ca2+ transient during cytokinesis in zebrafish embryos

The generation of a required series of localized Ca(2+) transients during cytokinesis in zebrafish embryos suggests that Ca(2+) plays a necessary role in regulating this process. Here, we report that cortical actin remodeling, characterized by the reorganization of the contractile band and the formation during furrow deepening of pericleavage F-actin enrichments (PAEs), requires a localized increase in intracellular Ca(2+), which is released from IP(3)-sensitive stores. We demonstrate that VAMP-2 vesicle fusion at the deepening furrow also requires Ca(2+) released via IP(3) receptors, as well as the presence of PAEs and the action of calpains. Finally, by expressing a dominant-negative form of the kinesin-like protein, kif23, we demonstrate that its recruitment to the furrow region is required for VAMP-2 vesicle transport; and via FRAP analysis, that kif23 localization is also Ca(2+)-dependent. Collectively, our data demonstrate that a localized increase in intracellular Ca(2+) is involved in regulating several key events during furrow deepening and subsequent apposition.

Duplicated gelsolin family genes in zebrafish: a novel scinderin-like gene (scinla) encodes the major corneal crystallin

We have previously identified a gelsolin-like protein (C/L-gelsolin) as a corneal crystallin in zebrafish. Here we show by phylogenetic analysis that there are at least six genes encoding gelsolin-like proteins based on their gelsolin domains in zebrafish: gsna and gsnb group with the vertebrate gelsolin gene, scina and scinb group with the scinderin (adseverin) gene, and scinla (C/L-gelsolin) and scinlb are novel scinderin-like genes. RT-PCR showed that scinla, scinlb, and gsnb are preferentially expressed in the adult cornea whereas gsna is expressed to a similar extent in cornea, lens, brain, and heart; scina and scinb expression were detectable only in whole zebrafish and not in these adult tissues. Quantitative RT-PCR and 2-dimensional polyacrylamide gel electrophoresis followed by MALDI/TOF mass spectroscopy confirmed high expression of beta-actin and scinla, moderate expression of scinlb, and very low expression of gsna and gsnb in the cornea. Finally, transgenic zebrafish carrying a green fluorescent protein reporter transgene driven by a 4 kb scinla promoter fragment showed expression in the cornea, snout, dorsal fin, and tail fin of 3-day-old zebrafish larvae. Our data suggest that scinla and scinlb are diverged paralogs of the vertebrate scinderin gene and show that scinla encodes the zebrafish corneal crystallin previously called C/L-gelsolin.

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

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

Exocytosis in neuroendocrine cells: new tasks for actin

Most secretory cells undergoing calcium-regulated exocytosis in response to cell surface receptor stimulation display a dense subplasmalemmal actin network, which is remodeled during the exocytotic process. This review summarizes new insights into the role of the cortical actin cytoskeleton in exocytosis. Many earlier findings support the actin-physical-barrier model whereby transient depolymerization of cortical actin filaments permits vesicles to gain access to their appropriate docking and fusion sites at the plasma membrane. On the other hand, data from our laboratory and others now indicate that actin polymerization also plays a positive role in the exocytotic process. Here, we discuss the potential functions attributed to the actin cytoskeleton at each major step of the exocytotic process, including recruitment, docking and fusion of secretory granules with the plasma membrane. Moreover, we present actin-binding proteins, which are likely to link actin organization to calcium signals along the exocytotic pathway. The results cited in this review are derived primarily from investigations of the adrenal medullary chromaffin cell, a cell model that is since many years a source of information concerning the molecular machinery underlying exocytosis.

Expression of various scinderin domains in chromaffin cells indicates that this protein acts as a molecular switch in the control of actin filament dynamics and exocytosis

Stimulation-induced chromaffin cell cortical F-actin disassembly allows the movement of vesicles towards exocytotic sites. Scinderin (Sc), a Ca2+-dependent protein, controls actin dynamics. Sc six domains have three actin, two PIP2 and two Ca2+-binding sites. F-actin severing activity of Sc is Ca2+-dependent, whereas Sc-evoked actin nucleation is Ca2+-independent. Sc domain role in secretion was studied by co-transfection of human growth hormone (hGH) reporter gene and green fluorescent protein (GFP)-fusion Sc constructs. Cells over-expressing actin severing Sc1-6 or Sc1-2 (first and second actin binding sites) constructs, increased F-actin disassembly and hGH release upon depolarization. Over-expression of nucleating Sc5-6, Sc5 or ScABP3 (third actin site) constructs decreased F-actin disassembly and hGH release upon stimulation. Over-expression of ScL5-6 or ScL5 (lack of third actin site) produced no changes. During secretion, actin sites 1 and 2 are involved in F-actin severing, whereas site 3 is responsible for nucleation (polymerization). Sc functions as a molecular switch in the control of actin (disassembly left arrow over right arrow assembly) and release (facilitation left arrow over right arrow inhibition). The position of the switch (severing left arrow over right arrow nucleation) may be controlled by [Ca2+]i. Thus, increase in [Ca2+]i produced by stimulation-induced Ca2+ entry would increase Sc-evoked cortical F-actin disassembly. Decrease in [Ca2+]i by either organelle sequestration or cell extrusion would favor Sc-evoked actin nucleation.

Secretory granule exocytosis

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

Pathways that control cortical F-actin dynamics during secretion

Chromaffin cells possess a mesh of filamentous actin underneath the plasma membrane which acts as a barrier to the chromaffin vesicles access to exocytotic sites. Disassembly of cortical F-actin in response to stimulation allows the movement of vesicles from the reserve pool to the release-ready vesicle pool and, therefore, to exocytotic sites. The dynamics of cortical F-actin is controlled by two mechanisms: a) stimulation-induced Ca2+ entry and scinderin activation and b) protein kinase C (PKC) activation and MARCKS phosphorylation as demonstrated here by experiments with recombinant proteins, antisense olygodeoxynucleotides and vector mediated transient expressions. Under physiological conditions (i.e., cholinergic receptor stimulation followed by Ca2+ entry), mechanism (a) is the most important for the control of cortical F-actin network whereas when Ca2+ is released from intracellular stores (i.e., histamine stimulation) cortical F-actin is regulated mainly by mechanism b.

The role of different Scinderin domains in the control of F-actin cytoskeleton during exocytosis

Scinderin (Sc), a Ca(2+)-regulated actin-binding protein, has been previously shown to control submembranous actin dynamics in regulated secretion. The results of the present study suggest the possibility that Sc might act as a molecular switch in the control of cortical F-actin dynamics during secretion.

Molecular biology of the chromaffin cell

A large number of molecular biology studies have been performed on chromaffin cells, and many genes involved in catecholamine synthesis, storage, and release have been cloned and their function determined. Catecholamine synthesis takes place in different cellular compartments, and enzymes involved in this process are subject to a fine regulation, as demonstrated by recent studies on their gene promoters. Genes coding for such intravesicular proteins as chromogranin A, B, and secretogranin II (chromogranin C) are also regulated in response to a variety of stimuli. Chromogranin gene promoters and transcription factors involved in their regulation have been elucidated. This review serves as an introduction to the studies described in the chapters to follow.

Expression of scinderin in megakaryoblastic leukemia cells induces differentiation, maturation, and apoptosis with release of plateletlike particles and inhibits proliferation and tumorigenesis

Rapid proliferation of atypical megakaryoblasts is a characteristic of megakaryoblastic leukemia. Cells from patients with this disorder and cell lines established from this type of leukemia showed the presence of gelsolin but the absence of scinderin expression, 2 filamentous actin-severing proteins present in normal megakaryocytes and platelets. Vector-mediated expression of scinderin in the megakaryoblastic cell line MEG-01 induced a decrease in both F-actin and gelsolin. This was accompanied by increased Rac2 expression and by activation of the PAK/MEKK.SEK/JNK/c-jun, c-fos transduction pathway. The Raf/MEK/ERK pathway was also activated in these cells. Transduction pathway activation was followed by cell differentiation, polyploidization, maturation, and apoptosis with release of platelet-like particles. Particles expressed surface CD41a antigen (glycoprotein IIb/IIIa or fibrinogen receptor), had dense bodies, high-affinity serotonin transport, and circular array of microtubules. Treatment of particles with thrombin induced serotonin release and aggregation that was blocked by CD41a antibodies. PAC-1 antibodies also blocked aggregation. Exposure of cells to PD98059, a blocker of MEK, inhibited antigen CD41a expression, increases in cell volume, and number of protoplasmic extensions. Cell proliferation and cell ability to form tumors in nude mice were also inhibited by the expression of scinderin. MEG-01 cells expressing scinderin had the same fate in vivo as in culture. Thus, when injected into nude mice, they entered apoptosis and released platelet-like particles. The lack of scinderin expression in megakaryoblastic leukemia cells seems to be responsible for their inability to enter into differentiation and maturation pathways characteristic of their normal counterparts.

Chromaffin cell F-actin disassembly and potentiation of catecholamine release in response to protein kinase C activation by phorbol esters is mediated through myristoylated alanine-rich C kinase substrate phosphorylation

The large majority of chromaffin vesicles are excluded from the plasma membrane by a cortical F-actin network. Treatment of chromaffin cells with phorbol 12-myristate 13-acetate produces disassembly of cortical F-actin, increasing the number of vesicles at release sites (Vitale, M. L., Seward, E. P., and Trifaró, J. M. (1995) Neuron 14, 353-363). Here, we provide evidence for involvement of myristoylated alanine-rich protein kinase C substrate (MARCKS), a protein kinase C substrate, in chromaffin cell secretion. MARCKS binds and cross-links F-actin, the latter is inhibited by protein kinase C-induced MARCKS phosphorylation. MARCKS was found in chromaffin cells by immunoblotting. MARCKS was also detected by immunoprecipitation. In intact or permeabilized cells MARCKS phosphorylation increased upon stimulation with 10(-7) m phorbol 12-myristate 13-acetate. This was accompanied by cortical F-actin disassembly and potentiation of secretion. MARCKS phosphorylation, cortical F-actin disassembly, and potentiation of Ca(2+)-evoked secretion were inhibited by a peptide (MARCKS phosphorylation site domain sequence (MPSD)) with amino acid sequence corresponding to MARCKS phosphorylation site. MPSD was phosphorylated in the process. A similar peptide (alanine-substituted phosphorylated site domain) with four serine residues of MPSD substituted by alanines was ineffective. These results provide the first evidence for MARCKS involvement in chromaffin cell secretion and suggest that regulation of cortical F-actin cross-linking might be involved in this process.

ATP induces intracellular calcium increases and actin cytoskeleton disaggregation via P2x receptors

The consequences of purinoceptor activation on calcium signalling, inositol phosphate metabolism, protein secretion and the actin cytoskeleton were demonstrated in the WRK-1 cell line. Extracellular ATP was used as a secretagogue to induce a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)), acting via P2x purinergic receptors, which causes actin skeleton disaggregation and protein secretion. ATP bound specifically to purinergic receptors, with Ki of 0.8 microM. The magnitude order for binding of different nucleotides was alpha beta-Met-ATP >or= dATPalphaS > ATP >or= ADP > UTP > AMP > suramin. No increase in inositol phosphates (IPs) was observed after ATP application suggesting that the purinergic sites in WRK-1 cells are not of a P2y type. ATP (1-100 microM) caused a concentration-dependent increase in [Ca(2+)](i)(EC(50)= 30 microM). The responses were reproducible without any desensitization over several applications. The response to ATP was abolished when extracellular calcium ([Ca(2+)](e)) was reduced to 100 nM. A non-specific purinergic antagonist, suramin, reversibly inhibited the ATP-response suggesting that ATP is able to bind to P2x purinergic sites to trigger Ca(2+) entry and increase of [Ca(2+)](i). ATP induced a concentration-dependent disaggregation of actin and exocytotic release of proteins both, which were dependent upon [Ca(2+)](e). Similarly, alpha,beta-Met-ATP, a potent P2x agonist also stimulated Ca(2+) mobilization, actin network destructuration, and protein release. In the isolated rat neurohypophysial nerve terminals, ATP was shown to act as a physiological stimulus for vasopressin release via Ca(2+) entry through a P2x receptor [6]. Here, we show that in these nerve terminals, ATP is also able to induce actin disaggregation by a Ca(2+) dependent mechanism. Thus, actin cytoskeleton alterations induced by ATP through activation of P2x receptors could be a prelude to exocytosis.

An antisense oligodeoxynucleotide targeted to chromaffin cell scinderin gene decreased scinderin levels and inhibited depolarization-induced cortical F-actin disassembly and exocytosis

Chromaffin cell secretion requires cortical F-actin disassembly and it has been suggested that scinderin, a Ca2+ dependent F-actin severing protein, controls cortical actin dynamics. An antisense oligodeoxynucleotide targeting the scinderin gene was used to decrease the expression of the protein and access its role in secretion. Treatment with 2 microM scinderin antisense oligodeoxynucleotide for 4 days produced a significant decrease in scinderin expression and its mRNA levels. The expression of gelsolin, another F-actin severing protein, was not affected. Scinderin decrease was accompanied by concomitant and parallel decreases in depolarization-evoked cortical F-actin disassembly and exocytosis. Similar treatment with a mismatched oligodeoxynucleotide produced no effects. Scinderin antisense oligodeoxynucleotide treatment was also a very effective inhibitor of exocytosis in digitonin-permeabilized cells stimulated with increasing concentrations of Ca2+. This ruled out scinderin antisense interference with stimulation-induced depolarization or Ca2+ channel activation. Scinderin antisense treatment decreased the maximum (B(max)) secretory response to Ca2+ without modifying the affinity (K(m)) of the cation for the exocytotic machinery. Moreover, the antisense treatment did not affect norepinephrine uptake or the expression of dopamine ss-hydroxylase, suggesting that the number and function of chromaffin vesicles was not modified. In addition, scinderin antisense treatment did not alter the expression of proteins involved in vesicle-plasma membrane fusion, such as synaptophysin, synaptotagmin or syntaxin, indicating a lack of effects on the fusion machinery components. These observations strongly suggest that scinderin is a key player in the events involved in the secretory process.

Calcium regulation of gelsolin and adseverin: a natural test of the helix latch hypothesis

The gelsolin family of actin filament binding proteins have highly homologous structures. Gelsolin and adseverin, also known as scinderin, are the most similar members of this family, with adseverin lacking a C-terminal helix found in gelsolin. This helix has been postulated to serve as a calcium-sensitive latch, keeping gelsolin inactive. To test this hypothesis, we have analyzed the kinetics of severing by gelsolin, adseverin, and a gelsolin truncate which lacks the C-terminal latch. We find that the relationship between severing rate and calcium ion concentration differs between gelsolin and adseverin, and suggest that calcium controls one rate-limiting step in the activation of adseverin and two in the activation of gelsolin. In contrast, both proteins are activated equally by protons, and have identical severing kinetics at pHs below 6.3. The temperature sensitivity of severing by adseverin and gelsolin is remarkably different, with gelsolin increasing its severing rate 8-fold per 10 degrees C increase in temperature and adseverin increasing its rate only 2-fold per 10 degrees C increase in temperature. Analysis of the gelsolin construct lacking the C-terminal helix demonstrates that this helix is responsible for the regulatory differences between gelsolin and adseverin. These results support the C-terminal latch hypothesis for the calcium ion activation of gelsolin.

Two pathways control chromaffin cell cortical F-actin dynamics during exocytosis

Neurosecretory cells including chromaffin cells possess a mesh of filamentous actin underneath the plasma membrane. We have proposed that the F-actin network acts as a barrier to the secretory vesicles blocking their access to exocytotic sites at the plasma membrane. Disassembly of cortical F-actin in chromaffin cells in response to stimulation is thought to allow the free movement of secretory vesicles to exocytotic sites. Moreover, experiments by us using morphometric analysis of resting and stimulated chromaffin cells together with membrane capacitance measurements have shown that cortical F-actin controls the traffic of vesicles from the vesicle reserve compartment to the release-ready vesicle compartment. The dynamics of the cortical F-actin is controlled by two pathways: A) stimulation-induced Ca(2+) entry and scinderin activation; and B) protein kinase C (PKC) activation and MARCKS (myristoylated alanine-rich C kinase substrate) phosphorylation. When chromaffin cells are stimulated through nicotinic receptors, cortical F-actin disassembly is mainly through the intervention of pathway A, since in the presence of PKC inhibitors, F-actin disassembly in response to cholinergic stimulation is only blocked by 20%. Pathway A involves the activation of scinderin by Ca(2+) with a consequent F-actin severing. Pathway B is fully activated by phorbol esters and in this case PKC blockers inhibit by 100% the disruption of cortical F-actin. This pathway operates through MARCKS. A peptide with amino acid sequence corresponding to the phosphorylation site domain of MARCKS, which also corresponds to its actin binding site, blocks PMA potentiation of Ca(2+)-induced catecholamine release. The results suggest that under physiological conditions (i.e., nicotinic receptor stimulation) pathway A is the principal mechanism for the control of cortical F-actin dynamic changes.

Multiple forms of phospholipase D in plants: the gene family, catalytic and regulatory properties, and cellular functions

Multiple Phospholipase D (PLD) genes have been identified in plants and encode isoforms with distinct regulatory and catalytic properties. Elucidation of the genetic and biochemical heterogeneity has provided important clues as to the regulation and function of this family of enzymes. Polyphosphoinositides, Ca(2+), and G-proteins are possible cellular regulators for PLD activation. PLD-mediated hydrolysis of membrane lipids increases in response to various stresses. Recent studies suggest that PLD plays a role in the signaling and production of hormones involved in plant stress responses.

Scinderin, a Ca2+-dependent actin filament severing protein that controls cortical actin network dynamics during secretion

Secretory vesicles are localized in specific compartments within neurosecretory cells. These are different pools in which vesicles are in various states of releasability. The transit of vesicles between compartments is controlled and regulated by Ca2+, scinderin and the cortical F-actin network. Cortical F-actin disassembly is produced by the filament severing activity of scinderin. This Ca2+-dependent activity of scinderin together with its Ca2+-independent actin nucleating activity, control cortical F-actin dynamics during the secretory cycle. A good understanding of the interaction of actin with scinderin and of the role of this protein in secretion has been provided by the analysis of the molecular structure of scinderin together with the use of recombinant proteins corresponding to its different domains.

Scinderin and cortical F-actin are components of the secretory machinery

Secretory vesicle exocytosis is the mechanism of release of neurotransmitters and neuropeptides. Secretory vesicles are localized in at least two morphologically and functionally distinct compartments: the reserve pool and the release-ready pool. Filamentous actin networks play an important role in this compartmentalization and in the trafficking of vesicles between these compartments. The cortical F-actin network constitutes a barrier (negative clamp) to the movement of secretory vesicles to release sites, and it must be locally disassembled to allow translocation of secretory vesicles in preparation for exocytosis. The disassembly of the cortical F-actin network is controlled by scinderin (a Ca2+-dependent F-actin severing protein) upon activation by Ca2+ entering the cells during stimulation. There are several factors that regulate scinderin activation (i.e., Ca2+ levels, phosphatidylinositol 4,5-bisphosphate (PIP2), etc.). The results suggest that scinderin and the cortical F-actin network are components of the secretory machinery.Key words: F-actin, scinderin, exocytosis, cytoskeleton, chromaffin cell.

In vivo, villin is required for Ca(2+)-dependent F-actin disruption in intestinal brush borders

Villin is an actin-binding protein localized in intestinal and kidney brush borders. In vitro, villin has been demonstrated to bundle and sever F-actin in a Ca(2+)-dependent manner. We generated knockout mice to study the role of villin in vivo. In villin-null mice, no noticeable changes were observed in the ultrastructure of the microvilli or in the localization and expression of the actin-binding and membrane proteins of the intestine. Interestingly, the response to elevated intracellular Ca(2+) differed significantly between mutant and normal mice. In wild-type animals, isolated brush borders were disrupted by the addition of Ca(2+), whereas Ca(2+) had no effect in villin-null isolates. Moreover, increase in intracellular Ca(2+) by serosal carbachol or mucosal Ca(2+) ionophore A23187 application abolished the F-actin labeling only in the brush border of wild-type animals. This F-actin disruption was also observed in physiological fasting/refeeding experiments. Oral administration of dextran sulfate sodium, an agent that causes colonic epithelial injury, induced large mucosal lesions resulting in a higher death probability in mice lacking villin, 36 +/- 9.6%, compared with wild-type mice, 70 +/- 8.8%, at day 13. These results suggest that in vivo, villin is not necessary for the bundling of F-actin microfilaments, whereas it is necessary for the reorganization elicited by various signals. We postulate that this property might be involved in cellular plasticity related to cell injury.

Calcium-dependent actin filament-severing protein scinderin levels and localization in bovine testis, epididymis, and spermatozoa

We assessed the levels and localization of the actin filament-severing protein scinderin, in fetal and adult bovine testes, and in spermatozoa during and following the epididymal transit. We performed immunoblots on seminiferous tubules and interstitial cells isolated by enzymatic digestion, and on bovine chromaffin cells, spermatozoa, aorta, and vena cava. Immunoperoxidase labeling was done on Bouin's perfusion-fixed testes and epididymis tissue sections, and on spermatozoa. In addition, immunofluorescence labeling was done on spermatozoa. Immunoblots showed one 80-kDa band in chromaffin cells, fetal and adult tubules, interstitial cells, spermatozoa, aorta, and vena cava. Scinderin levels were higher in fetal than in adult seminiferous tubules but showed no difference between fetal and adult interstitial cells. Scinderin levels were higher in epididymal than in ejaculated spermatozoa. Scinderin was detected in a region corresponding with the subacrosomal space in the round spermatids and with the acrosome in the elongated spermatids. In epididymal spermatozoa, scinderin was localized to the anterior acrosome and the equatorial segment, but in ejaculated spermatozoa, the protein appeared in the acrosome and the post-equatorial segment of the head. In Sertoli cells, scinderin was detected near the cell surface and within the cytoplasm, where it accumulated near the base in a stage-specific manner. In the epididymis, scinderin was localized next to the surface of the cells; in the tail, it collected near the base of the principal cells. In Sertoli cells and epididymal cells, scinderin may contribute to the regulation of tight junctional permeability and to the release of the elongated spermatids by controlling the state of perijunctional actin. In germ cells, scinderin may assist in the shaping of the developing acrosome and influence the fertility of the spermatozoa.

Calcium sensors in regulated exocytosis

Neurotransmitter release, hormone secretion and a variety of other secretory process are tightly regulated with exocytotic fusion of secretory vesicles being triggered by a rise in cytosolic Ca2+ concentration. A series of proteins that act as part of a conserved core machinery for vesicle docking and fusion throughout the cell have been identified. In regulated exocytosis this core machinery must be controlled by Ca(2+)-sensor proteins that allow rapid activation of the fusion process following elevation of cytosolic Ca2+ concentration. The properties of such Ca2+ sensors are known from physiological studies but their molecular identity remains to be unequivocally established. The multiple Ca(2+)-dependent steps in the exocytotic pathway suggest the likely involvement of several Ca(2+)-binding proteins with distinct properties. Functional evidence for the role of various Ca(2+)-binding proteins and their possible sites of action is accumulating but a definitive identification of the major Ca(2+)-sensor in the final step of Ca(2+)-triggered membrane fusion in different cell types awaits further analysis.

The regulation of neurotransmitter secretion by protein kinase C

The effect of protein kinase C (PKC) on the release of neurotransmitters from a number preparations, including sympathetic nerve endings, brain slices, synaptosomes, and neuronally derived cell lines, is considered. A comparison is drawn between effects of activation of PKC on neurotransmitter release from small synaptic vesicles and large dense-cored vesicles. The enhancement of neurotransmitter release is discussed in relation to the effect of PKC on: 1. Rearrangement of the F-actin-based cytoskeleton, including the possible role of MARCKS in this process, to allow access of large dense-cored vesicles to release sites on the plasma membrane. 2. Phosphorylation of key components in the SNAP/SNARE complex associated with the docking and fusion of vesicles at site of secretion. 3. Ion channel activity, particularly Ca2+ channels.

Localization by segmental deletion analysis and functional characterization of a third actin-binding site in domain 5 of scinderin

Scinderin is a Ca2+-dependent actin filament severing protein present in a variety of secretory cells. Previous work suggests that scinderin-evoked cortical F-actin disassembly is required for secretion because local disassembly of cortical cytoskeleton allows secretory vesicle exocytosis (Vitale, M. L., Rodríguez Del Castillo, A., Tchakarov, L., and Trifaró, J.-M. (1991) J. Cell Biol. 113, 1057-1067). Scinderin has six domains each containing three internal sequence motifs, two actin, and two phosphatidylinositol disphosphate-binding sites in domains 1 and 2. In this paper we report the presence of another actin-binding site at the NH2-terminal of domain 5 (Sc511-518). This site binds actin in a Ca2+-independent manner and a recombinant fragment (Sc5-6 or Sc502-715) containing this site binds to actin-DNase-I-Sepharose 4B beads, co-sediments with actin and is able to nucleate actin assembly. Recombinant ScL5-6, a fusion protein devoid of the actin-binding site (Sc519-715), did not exhibit these properties. Moreover, Sc-ABP3, a peptide constructed with sequence (RLFQVRRNLASIT) identical to Sc511-523 blocked the binding of Sc5-6 to actin. Sc5-6 and Sc-ABP3 also prevented the actin severing activity of recombinant full-length scinderin (r-Sc) and inhibited the potentiation by r-Sc of Ca2+-evoked release of serotonin from permeabilized platelets. On the other hand, ScL5-6 failed to block the effect of r-Sc on platelet serotonin release. Sc1-4,6, a construct devoid of domain 5, was able to sever but unable to nucleate actin, indicating that an actin nucleation site of scinderin was in domain 5. The results suggest that scinderin, in addition to binding actin on sites present in domains 1 and 2, must bind actin on a third site in domain 5 to sever and nucleate actin effectively.

Molecular heterogeneity of phospholipase D (PLD). Cloning of PLDgamma and regulation of plant PLDgamma, -beta, and -alpha by polyphosphoinositides and calcium

Phospholipase D (PLD) has emerged as an important enzyme involved in signal transduction, vesicle trafficking, and membrane metabolism. This report describes the cloning and expression of a new Arabidopsis PLD cDNA, designated PLDgamma, and the regulation of PLDgamma, -beta, and -alpha by phosphatidylinositol 4,5-bisphosphate (PIP2) and Ca2+. The PLDgamma cDNA is 3.3 kilobases in length and codes for an 855-amino acid protein of 95,462 Da with a pI of 6.9. PLDgamma shares a 66% amino acid sequence identity with PLDbeta, but only a 41% identity with PLDalpha. A potential N-terminal myristoylation site is found in PLDgamma, but not in PLDalpha and -beta. Catalytically active PLDgamma was expressed in Escherichia coli, and its activity requires polyphosphoinositides. Both PLDgamma and -beta are most active at microM Ca2+ concentrations, whereas the optimal PLDalpha activity requires mM Ca2+ concentrations. Binding studies showed that the PLDs bound PIP2 in the order of PLDbeta > PLDgamma > PLDalpha. This binding ability correlates with the degree of conservation of a basic PIP2-binding motif located near the putative catalytic site. The binding of [3H]PIP2 was saturable and could be competitively decreased by addition of unlabeled PIP2. Neomycin inhibited the activities of PLDgamma and -beta, but not PLDalpha. These results demonstrate that PLD is encoded by a heterogeneous gene family and that direct polyphosphoinositide binding is required for the activities of PLDgamma and -beta, but not PLDalpha. The different structural and biochemical properties suggest that PLDalpha, -beta, and -gamma are regulated differently and may mediate unique cellular functions.

Recombinant scinderin enhances exocytosis, an effect blocked by two scinderin-derived actin-binding peptides and PIP2

The cortical F-actin cytoskeleton represents a negative control for secretion, and it must be locally disassembled to allow chromaffin vesicle exocytosis. Recombinant scinderin (a Ca(2+)-dependent F-actin-severing protein) potentiated Ca(2+)-evoked F-actin disassembly and exocytosis in permeabilized chromaffin cells, an effect blocked by peptides Sc-ABP1 and Sc-ABP2 (with sequences corresponding to two actin-binding sites of scinderin), exogenous gamma-actin, or phosphatidylinositol 4,5-bisphosphate (PIP2). PIP2 effect was blocked by peptide Sc-PIP2BP (with sequence corresponding to a PIP2-binding site of scinderin). Truncated scinderin254-715 (lacking actin-severing domains) did not potentiate exocytosis. Sc-ABP1, Sc-ABP2, and gamma-actin also inhibited exocytosis in the absence of recombinant scinderin, suggesting an inhibition of endogenous scinderin. Results suggest that scinderin-evoked cortical F-actin disassembly is required for secretion and that scinderin is an important component of the exocytotic machinery.