Microtubules can modulate pseudopod activity from a distance inside macrophages

Roles for 3' Phosphoinositides in Macropinocytosis

The distinct movements of macropinosome formation and maturation have corresponding biochemical activities which occur in a defined sequence of stages and transitions between those stages. Each stage in the process is regulated by variously phosphorylated derivatives of phosphatidylinositol (PtdIns) which reside in the cytoplasmic face of the membrane lipid bilayer. PtdIns derivatives phosphorylated at the 3' position of the inositol moiety, called 3' phosphoinositides (3'PIs), regulate different stages of the sequence. 3'PIs are synthesized by numerous phosphoinositide 3'-kinases (PI3K) and other lipid kinases and phosphatases, which are themselves regulated by small GTPases of the Ras superfamily. The combined actions of these enzymes localize four principal species of 3'PI to distinct domains of the plasma membrane or to discrete organelles, with distinct biochemical activities confined to those domains. Phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P₃) and phosphatidylinositol (3,4)-bisphosphate (PtdIns(3,4)P₂) regulate the early stages of macropinosome formation, which include cell surface ruffling and constrictions of circular ruffles which close into macropinosomes. Phosphatidylinositol 3-phosphate (PtdIns3P) regulates macropinosome fusion with other macropinosomes and early endocytic organelles. Phosphatidylinositol (3,5)-bisphosphate (PtdIns(3,5)P₂) mediates macropinosome maturation and shrinkage, through loss of ions and water, and subsequent traffic to lysosomes. The different characteristic rates of macropinocytosis in different cell types indicate levels of regulation which may be governed by the cell's capacity to generate 3'PIs.

A versatile and customizable low-cost 3D-printed open standard for microscopic imaging

Modern microscopes used for biological imaging often present themselves as black boxes whose precise operating principle remains unknown, and whose optical resolution and price seem to be in inverse proportion to each other. With UC2 (You. See. Too.) we present a low-cost, 3D-printed, open-source, modular microscopy toolbox and demonstrate its versatility by realizing a complete microscope development cycle from concept to experimental phase. The self-contained incubator-enclosed brightfield microscope monitors monocyte to macrophage cell differentiation for seven days at cellular resolution level (e.g. 2 μm). Furthermore, by including very few additional components, the geometry is transferred into a 400 Euro light sheet fluorescence microscope for volumetric observations of a transgenic Zebrafish expressing green fluorescent protein (GFP). With this, we aim to establish an open standard in optics to facilitate interfacing with various complementary platforms. By making the content and comprehensive documentation publicly available, the systems presented here lend themselves to easy and straightforward replications, modifications, and extensions.

Open standard microscopy is urgently needed to give low-cost solutions to researchers and to overcome the reproducibility crisis in science. Here the authors present a 3D-printed, open-source modular microscopy toolbox UC2 (You. See. Too.) for a few hundred Euros.

Dorsal ruffles enhance activation of Akt by growth factors

In fibroblasts, platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) stimulate the formation of actin-rich, circular dorsal ruffles (CDRs) and phosphatidylinositol 3-kinase (PI3K)-dependent phosphorylation of Akt. To test the hypothesis that CDRs increase synthesis of phosphorylated Akt1 (pAkt), we analyzed the contributions of CDRs to Akt phosphorylation in response to PDGF and EGF. CDRs appeared within several minutes of growth factor addition, coincident with a peak of pAkt. Microtubule depolymerization with nocodazole blocked CDR formation and inhibited phosphorylation of Akt in response to EGF but not PDGF. Quantitative immunofluorescence showed increased concentrations of Akt, pAkt and phosphatidylinositol (3,4,5)-trisphosphate (PIP₃), the phosphoinositide product of PI3K that activates Akt, concentrated in CDRs and ruffles. EGF stimulated lower maximal levels of pAkt than did PDGF, which suggests that Akt phosphorylation requires amplification in CDRs only when PI3K activities are low. Accordingly, stimulation with low concentrations of PDGF elicited lower levels of Akt phosphorylation, which, like responses to EGF, were inhibited by nocodazole. These results indicate that when receptor signaling generates low levels of PI3K activity, CDRs facilitate local amplification of PI3K and phosphorylation of Akt.This article has an associated First Person interview with the first author of the paper.

Exit mechanisms of the intracellular bacterium Ehrlichia

Background

The obligately intracellular bacterium Ehrlichia chaffeensis that resides in mononuclear phagocytes is the causative agent of human monocytotropic ehrlichiosis. Ehrlichia muris and Ixodes ovatus Ehrlichia (IOE) are agents of mouse models of ehrlichiosis. The mechanism by which Ehrlichia are transported from an infected host cell to a non-infected cell has not been demonstrated.

Methodology/Principal Findings

Using fluorescence microscopy and transmission and scanning electron microscopy, we demonstrated that Ehrlichia was transported through the filopodia of macrophages during early stages of infection. If host cells were not present in the vicinity of an Ehrlichia-infected cell, the leading edge of the filopodium formed a fan-shaped structure filled with the pathogen. Formation of filopodia in the host macrophages was inhibited by cytochalasin D and ehrlichial transport were prevented due to the absence of filopodia formation. At late stages of infection the host cell membrane was ruptured, and the bacteria were released.

Conclusions/Significance

Ehrlichia are transported through the host cell filopodium during initial stages of infection, but are released by host cell membrane rupture during later stages of infection.

Arf6 and microtubules in adhesion-dependent trafficking of lipid rafts

Integrin-mediated adhesion regulates membrane binding sites for Rac1 within lipid rafts. Detachment of cells from the substratum triggers the clearance of rafts from the plasma membrane through caveolin-dependent internalization. The small GTPase Arf6 and microtubules also regulate Rac-dependent cell spreading and migration, but the mechanisms are poorly understood. Here we show that endocytosis of rafts after detachment requires F-actin, followed by microtubule-dependent trafficking to recycling endosomes. When cells are replated on fibronectin, rafts exit from recycling endosomes in an Arf6-dependent manner and return to the plasma membrane along microtubules. Both of these steps are required for the plasma membrane targeting of Rac1 and for its activation. These data therefore define a new membrane raft trafficking pathway that is crucial for anchorage-dependent signalling.

Hypoparathyroidism, retardation, and dysmorphism syndrome: impaired early growth and increased susceptibility to severe infections due to hyposplenism and impaired polymorphonuclear cell functions

Hypoparathyroidism, retardation, and dysmorphism (HRD) syndrome is the first reported disease caused by a defect in the tubulin folding and assembly pathway. We aimed to summarize our experience with a cohort of patients with HRD, analyze their growth, and evaluate patients' polymorphonuclear cell (PMN) functions. The records of 22 HRD patients in a single medical center were reviewed. Growth during infancy and early childhood were analyzed by the Infancy-Childhood-Puberty (ICP) growth model. PMN functions were compared with healthy controls. Twelve patients died and many hospitalizations due to infections and convulsions were recorded. Growth measurements, expressed as weight and height SD scores in boys at a mean age of 4 y were -13.1+/-3.8 and -8.7+/-1 and -16.6+/-4.4 and -9.5+/-2.4, respectively, in girls at a mean age of 6.4 y. Chemotactic migration, random migration, and phagocytosis of PMN from HRD patients were significantly lower than that of PMN from healthy controls. No significant differences were found in superoxide production of PMN from patients compared with controls. Functional hyposplenism has been demonstrated in most of the studied patients. The defect in the tubulin folding and assembly pathway, previously described in HRD, has grave consequences on growth and PMN functions.

From blood to brain: amoeboid microglial cell, a nascent macrophage and its functions in developing brain

Amoeboid microglial cells (AMC) in the developing brain are active macrophages. The macrophagic nature of these cells has been demonstrated by many methods, such as the localization of various hydrolytic enzymes and the presence of complement type 3 surface receptors in them. More importantly is the direct visualization of these cells engaged in the phagocytosis of degenerating cells at the ultrastructural level. Further evidence of them being active macrophages is the avid internalization of tracers administered by the intravenous or intraperitoneal routes in developing rats. The potential involvement of AMC in immune functions is supported by the induced expression of major histocompatibility complex class I and II antigens on them when challenged by lipopolysaccharide or interferon-gamma. Immunosuppressive drugs, such as glucocorticoids and immune function-enhancing drugs like melatonin, affect the expression of surface receptors and antigens and the release of cytokines by AMC. Recent studies in our laboratory have shown the expression of insulin-like growth factors, endothelins, 2',3'-cyclic nucleotide 3'-phosphodiesterase, and N-methyl-D-asparate receptors. This along with the release of chemokines, such as stromal derived factor-1a and monocyte chemoattractant protein-1, suggests multiple functional roles of AMC in early brain development.

Microtubules regulate PI-3K activity and recruitment to the phagocytic cup during Fcgamma receptor-mediated phagocytosis in nonelicited macrophages

Phagocytosis is a complex sequence of events involving coordinated remodeling of the plasma membrane with the underlying cytoskeleton. Although the role of the actin cytoskeleton is becoming increasingly elucidated, the role of microtubules (MTs) remains poorly understood. Here, we examine the role of MTs during FcgammaR-mediated phagocytosis in RAW264.7 mouse macrophages. We observe that MTs extend into the phagosomal cups. The MT-depolymerizing agents, colchicine and nocodazole, cause a sizeable reduction in phagocytosis of large particles in RAW264.7 cells. Phagocytosis in primed macrophages is unaffected by MT-depolymerizing agents. However, activation of macrophages coincides with an increased population of drug-stable MTs, which persist in functional phagocytic cups. Scanning electron microscopy analysis of unprimed macrophages reveals that pseudopod formation is reduced markedly following colchicine treatment, which is not a consequence of cell rounding. MT depolymerization in these cells does not affect particle binding, Syk, or Grb2-associated binder 2 recruitment or phosphotyrosine accumulation at the site of phagocytosis. Ras activation also proceeds normally in macrophages treated with colchicine. However, MT disruption causes a decrease in accumulation of AKT-pleckstrin homology-green fluorescent protein, a probe that binds to PI-3K products at the sites of particle binding. A corresponding decline in activated AKT is observed in colchicine-treated cells using immunoblotting with a phospho-specific-AKT (ser473) antibody. Furthermore, the translocation of the p85alpha regulatory subunit of PI-3K is reduced at the phagocytic cup in colchicine-treated cells. These findings suggest that MTs regulate the recruitment and localized activity of PI-3K during pseudopod formation.

Systems Cell Biology Knowledge Created from High Content Screening

High content screening (HCS), the large-scale automated analysis of the temporal and spatial changes in cells and cell constituents in arrays of cells, has the potential to create enormous systems cell biology knowledge bases. HCS is being employed along with the continuum of the early drug discovery process, including lead optimization where new knowledge is being used to facilitate the decision-making process. We demonstrate methodology to build new systems cell biology knowledge using a multiplexed HCS assay, designed with the aid of knowledge-mining tools, to measure the phenotypic response of a panel of human tumor cell types to a panel of natural product-derived microtubule-targeted anticancer agents and their synthetic analogs. We show how this new systems cell biology knowledge can be used to design a lead compound optimization strategy for at least two members of the panel, (-)-laulimalide and (+)-discodermolide, that exploits cell killing activity while minimally perturbing the regulation of the cell cycle and the stability of microtubules. Furthermore, this methodology can also be applied to basic biomedical research on cells.

Drug-Eluting Stents

Advances in catheter and stent design have made stent implantation the standard coronary angioplasty procedure. Unfortunately, in-stent restenosis continues to plague this procedure, with the optimum binary restenosis rates reaching ~10% to 20%. In the past few years, it has become clear that in-stent restenosis is largely due to the migration and proliferation of vascular smooth muscle cells to form a neointima. To address this issue, stents coated with drug-delivery vehicles have been developed to deliver antiproliferative therapeutics. Two drugs, rapamycin and taxol, have been the lead compounds for testing the idea of a drug-eluting stent. These drugs have been successful largely because of the solid mechanistic understanding of their effects and extensive preclinical examination. The result of these years of work is that the rapamycin-coated stent entered the US market in April of 2003, and the taxol-coated stent appears poised to follow soon.

Actomyosin transports microtubules and microtubules control actomyosin recruitment during Xenopus oocyte wound healing

BACKGROUND

Interactions between microtubules and actin filaments (F-actin) are critical for cellular motility processes ranging from directed cell locomotion to cytokinesis. However, the cellular bases of these interactions remain poorly understood. We have analyzed the role of microtubules in generation of a contractile array comprised of F-actin and myosin-2 that forms around wounds made in Xenopus oocytes.

RESULTS

After wounding, microtubules are transported to the wound edge in association with F-actin that is itself recruited to wound borders via actomyosin-powered cortical flow. This transport generates sufficient force to buckle and break microtubules at the wound edge. Transport is complemented by local microtubule assembly around wound borders. The region of microtubule breakage and assembly coincides with a zone of actin assembly, and perturbation of the microtubule cytoskeleton disrupts this zone as well as local recruitment of the Arp2/3 complex and myosin-2.

CONCLUSIONS

The results reveal transport of microtubules in association with F-actin that is pulled to wound borders via actomyosin-based contraction. Microtubules, in turn, focus zones of actin assembly and myosin-2 recruitment at the wound border. Thus, wounding triggers the formation of a spatially coordinated feedback loop in which transport and assembly of microtubules maintains actin and myosin-2 in close proximity to the closing contractile array. These results are surprisingly reminiscent of recent findings in locomoting cells, suggesting that similar feedback interactions may be generally employed in a variety of fundamental cell motility processes.

Phagocytosis and the microtubule cytoskeleton

Phagocytosis is a critical host defense mechanism used by macrophages and neutrophils to clear invading pathogens. The complex sequence of events resulting in internalization and degradation of the pathogens is a coordinated process involving lipids, signaling proteins, and the cytoskeleton. Here, we examine the role of the microtubule cytoskeleton in supporting both the engulfment of pathogens and their elimination within phagolysosomes.

Involvement of cellular cytoskeleton components in antibody-induced internalization of viral glycoproteins in pseudorabies virus-infected monocytes

Addition of pseudorabies virus (PrV)-specific polyclonal immunoglobulins to PrV-infected monocytes induces internalization of plasma membrane-anchored viral glycoproteins and this may interfere with antibody-dependent cell lysis. We investigated the role of actin, microtubules, clathrin, and dynein, the major cellular components involved in physiological endocytosis during this virological internalization. Porcine monocytes were infected in vitro for 13 h and afterward treated with different concentrations of colchicine, cytochalasin D, latrunculin B, and amantadine-HCl, which inhibit polymerization of microtubules, actin/clathrin, actin, and clathrin, respectively. This resulted in a significant reduction of internalization compared to the nontreated control, indicating that these components are involved in the process. A double labeling was performed during the internalization process and a clear colocalization of actin, microtubules, clathrin, and dynein with the viral glycoproteins was observed at different stages during the internalization process. We conclude that these cellular components are used by PrV to generate the antibody-induced internalization of viral glycoproteins.

Neurotrophins and the dynamic regulation of the neuronal cytoskeleton

The morphology of neuronal axons and dendrites is dependent on the dynamics of the cytoskeleton. An understanding of neurodevelopment and adult neuroplasticity must therefore include a detailed description of the intrinsic and extrinsic mechanisms that regulate the organization and dynamics of actin filaments and microtubules. In this paper we review recent advances in the understanding of the dynamic regulation of neuronal morphology by interactions among cytoskeletal components and the regulation of the cytoskeleton by neurotrophins.

Regional regulation of microtubule dynamics in polarized, motile cells

Microtubules are known to be required for locomotion of mammalian cells, and recent experiments demonstrate that suppression of microtubule dynamic turnover reduces the rate of cell motility and induces wandering of growth cones [Liao et al., 1995: J Cell Sci. 108:3473-3483; Tanaka et al., 1995: J Cell Biol. 128:139-155]. To determine how microtubule dynamic instability behavior contributes to directed cell locomotion, the behavior of individual microtubules has been directly observed and quantified at leading and lateral edges of hepatocyte growth factor-treated motile cells. Microtubules extended into newly formed protrusions at the leading edge; these "pioneer" microtubules [Waterman-Storer and Salmon, 1997: J Cell Biol. 139:417-434] showed persistent growth when compared with microtubules in non-leading, lateral edges. The percentage of total observation time spent in the growth phase was 68.2% at the leading edge compared with 32.0% in non-leading edges, and net microtubule elongation was observed in lamellipodia at the leading edge. The frequency of catastrophe transitions was threefold greater and the average number of transitions/microtubule/min was twofold greater in non-leading edges, as compared with the leading edge. These observations demonstrate that pioneer microtubules that enter newly formed lamellipodia at the leading edge of motile cells are characterized by persistent growth excursions, and directly demonstrate that the frequency of catastrophe transitions can be regionally regulated in polarized motile cells. The data indicate that region specific differences in the organization and dynamics of actin filaments may regulate microtubule dynamic instability behavior in vivo.

Microtubule dynamics in serum-starved and serum-stimulated Swiss 3T3 mouse fibroblasts: implications for the relationship between serum-induced contractility and microtubules

It has been established that cell contractility can be stimulated with low or depolymerizing doses of microtubule (MT) poisons. In addition, low doses of nocodazole and vinblastine have recently been shown to decrease MT dynamics in vivo. In this study, investigated whether there is a direct, or reciprocal feedback-type relationship between contractility and microtubule dynamics, by examining MT dynamic behavior in live cells under conditions where contractility is known to be altered. Quiescent, serum-starved Swiss 3T3 mouse fibroblasts have been shown to be weakened in their contractility; serum stimulation increases cell contractility and causes the formation of stress fibers and adhesion plaques. Growing (control), quiescent (Go), and serum-stimulated cells were injected with rhodamine-tubulin, and MT dynamics were determined by analysis of MT length changes obtained from digitized images of the extreme periphery of the cells, where the MT ends were readily apparent. The MTs in quiescent cells were less dynamic than those in control cells: the growth and shortening rates were reduced by 30% and 45%, respectively. Dynamicity decreased by 47%, and the MTs spent more time in pause. After serum stimulation, MT growth rate, dynamicity, and time spent in pause returned to control cell levels. Although the shortening rate increased by 28%, it remained significantly lower than in control cells. In this system, the serum-induced increase in contractility was accompanied by an increase in MT dynamics. However, increased contractility stimulated with low doses of MT poisons is known to be accompanied by a decrease in MT dynamics. These results suggest that the relationship between MT dynamics and contractility is an indirect one.

Myoseverin, a microtubule-binding molecule with novel cellular effects

A new microtubule-binding molecule, myoseverin, was identified from a library of 2,6,9-trisubstituted purines in a morphological differentiation screen. Myoseverin induces the reversible fission of multinucleated myotubes into mononucleated fragments. Myotube fission promotes DNA synthesis and cell proliferation after removal of the compound and transfer of the cells to fresh growth medium. Transcriptional profiling and biochemical analysis indicate that myoseverin alone does not reverse the biochemical differentiation process. Instead, myoseverin affects the expression of a variety of growth factor, immunomodulatory, extracellular matrix-remodeling, and stress response genes, consistent with the activation of pathways involved in wound healing and tissue regeneration.

Polymerizing microtubules activate site-directed F-actin assembly in nerve growth cones

We identify an actin-based protrusive structure in growth cones termed "intrapodium." Unlike filopodia, intrapodia are initiated exclusively within lamellipodia and elongate in a continuous (nonsaltatory) manner parallel to the plane of the dorsal plasma membrane causing a ridge-like protrusion. Intrapodia resemble the actin-rich structures induced by intracellular pathogens (e.g., Listeria) or by extracellular beads. Cytochalasin B inhibits intrapodial elongation and removal of cytochalasin B produced a burst of intrapodial activity. Electron microscopic studies revealed that lamellipodial intrapodia contain both short and long actin filaments oriented with their barbed ends toward the membrane surface or advancing end. Our data suggest an interaction between microtubule endings and intrapodia formation. Disruption of microtubules by acute nocodazole treatment decreased intrapodia frequency, and washout of nocodazole or addition of the microtubule-stabilizing drug Taxol caused a burst of intrapodia formation. Furthermore, individual microtubule ends were found near intrapodia initiation sites. Thus, microtubule ends or associated structures may regulate these actin-dependent structures. We propose that intrapodia are the consequence of an early step in a cascade of events that leads to the development of F-actin-associated plasma membrane specializations.

Direct observation of microtubule-f-actin interaction in cell free lysates

Coordinated interplay of the microtubule and actin cytoskeletons has long been known to be crucial for many cellular processes including cell migration and cytokinesis. However, interactions between these two systems have been difficult to document by conventional approaches, for a variety of technical reasons. Here the distribution of f-actin and microtubules were analyzed in the absence of fixation using Xenopus egg extracts as an in vitro source of microtubules and f-actin, demembranated Xenopus sperm to nucleate microtubule asters, fluorescent phalloidin as a probe for f-actin, and fluorescent tubulin as a probe for microtubules. F-actin consistently colocalized in a lengthwise manner with microtubules of asters subjected to extensive washing in flow chambers. F-actin-microtubule association was heterogenous within a given aster, such that f-actin is most abundant toward the distal (plus) ends of microtubules, and microtubules heavily labeled with f-actin are found in close proximity to microtubules devoid of f-actin. However, this distribution changed over time, in that 5 minute asters had more f-actin in their interiors than did 15 minute asters. Microtubule association with f-actin was correlated with microtubule bending and kinking, while elimination of f-actin resulted in straighter microtubules, indicating that the in vitro interaction between f-actin and microtubules is functionally significant. F-actin was also found to associate in a lengthwise fashion with microtubules in asters centrifuged through 30% sucrose, and microtubules alone (i.e. microtubules not seeded from demembranated sperm) centrifuged through sucrose, indicating that the association cannot be explained by flow-induced trapping and alignment of f-actin by aster microtubules. Further, cosedimentation analysis revealed that microtubule-f-actin association could be reconstituted from microtubules assembled from purified brain tubulin and f-actin assembled from purified muscle actin in the presence, but not the absence, of Xenopus oocyte microtubule binding proteins. The results provide direct evidence for an association between microtubules and f-actin in vitro, indicate that this interaction is mediated by one or more microtubule binding proteins, and suggest that this interaction may be responsible for the mutual regulation of the microtubule and actomyosin cytoskeletons observed in vivo.

Microtubule assembly is regulated by externally applied strain in cultured smooth muscle cells

Mechanical forces clearly regulate the development and phenotype of a variety of tissues and cultured cells. However, it is not clear how mechanical information is transduced intracellularly to alter cellular function. Thermodynamic modeling predicts that mechanical forces influence microtubule assembly, and hence suggest microtubules as one potential cytoskeletal target for mechanical signals. In this study, the assembly of microtubules was analyzed in rat aortic smooth muscle cells cultured on silicon rubber substrates exposed to step increases in applied strain. Cytoskeletal and total cellular protein fractions were extracted from the cells following application of the external strain, and tubulin levels were quantified biochemically via a competitive ELISA and western blotting using bovine brain tubulin as a standard. In the first set of experiments, smooth muscle cells were subjected to a step-increase in strain and the distribution of tubulin between monomeric, polymeric, and total cellular pools was followed with time. Microtubule mass increased rapidly following application of the strain, with a statistically significant increase (P<0.05) in microtubule mass from 373+/-32 pg/cell (t=0) to 514+/-30 pg/cell (t=15 minutes). In parallel, the amount of soluble tubulin decreased approximately fivefold. The microtubule mass decreased after 1 hour to a value of 437+/-24 pg/cell. In the second set of experiments, smooth muscle cells were subjected to increasing doses of externally applied strain using a custom-built strain device. Monomeric, polymeric, and total tubulin fractions were extracted after 15 minutes of applied strain and quantified as for the earlier experiments. Microtubule mass increased with increasing strain while total cellular tubulin levels remained essentially constant at all strain levels. These findings are consistent with a thermodynamic model which predicts that microtubule assembly is promoted as a cell is stretched and compressional loads on the microtubules are presumably relieved. Furthermore, these data suggest microtubules are a potential target for translating changes in externally applied mechanical stimuli to alterations in cellular phenotype.

Cytosolic free calcium and the cytoskeleton in the control of leukocyte chemotaxis

In response to a chemotactic gradient, leukocytes extravasate and chemotax toward the site of pathogen invasion. Although fundamental in the control of many leukocyte functions, the role of cytosolic free Ca2+ in chemotaxis is unclear and has been the subject of debate. Before becoming motile, the cell assumes a polarized morphology, as a result of modulation of the cytoskeleton by G protein and kinase activation. This morphology may be reinforced during chemotaxis by the intracellular redistribution of Ca2+ stores, cytoskeletal constituents, and chemoattractant receptors. Restricted subcellular distributions of signaling molecules, such as Ca2+, Ca2+/calmodulin, diacylglycerol, and protein kinase C, may also play a role in some types of leukocyte. Chemotaxis is an essential function of most cells at some stage during their development, and a deeper understanding of the molecular signaling and structural components involved will enable rational design of therapeutic strategies in a wide variety of diseases.

The cytoskeleton and cell signaling: component localization and mechanical coupling

The three-dimensional intracellular network formed by the filamentous polymers comprising the cytoskeletal affects the way cells sense their extracellular environment and respond to stimuli. Because the cytoskeleton is viscoelastic, it provides a continuous mechanical coupling throughout the cell that changes as the cytoskeleton remodels. Such mechanical effects, based on network formation, can influence ion channel activity at the plasma membrane of cells and may conduct mechanical stresses from the cell membrane to internal organelles. As a result, both rapid responses such as changes in intracellular Ca2+ and slower responses such as gene transcription or the onset of apoptosis can be elicited or modulated by mechanical perturbations. In addition to mechanical features, the cytoskeleton also provides a large negatively charged surface on which many signaling molecules including protein and lipid kinases, phospholipases, and GTPases localize in response to activation of specific transmembrane receptors. The resulting spatial localization and concomitant change in enzymatic activity can alter the magnitude and limit the range of intracellular signaling events.

Astrocytes regulate microglial phagocytosis of senile plaque cores of Alzheimer's disease

We have developed an in vitro model in which isolated senile plaque (SP) cores are presented to rat microglial cells in culture. Microglia rapidly phagocytosed, broke apart, and cleared SP cores. However, when cocultured with astrocytes, microglial phagocytosis was markedly suppressed, allowing the SPs to persist. Suppression of phagocytosis by astrocytes appears to be a general phenomena since microglia in the presence of astrocytes showed reduced capacity to phagocytose latex beads as well. The astrocyte effect on microglia is related in part to a diffusible factor(s) since astrocyte- but not fibroblast-conditioned media also reduced phagocytosis. These results suggest that while microglia have the capacity to phagocytose and remove SPs, astrocytes which lie in close association to microglia may help prevent the efficient clearance of SP material allowing them to persist in Alzheimer's disease.