Arp2/3 complex is required for actin polymerization during platelet shape change

Mechanosensing via a GpIIb/Src/14-3-3ζ axis critically regulates platelet migration in vascular inflammation

Platelets are not only the first responders in thrombosis and hemostasis but also central players in inflammation. Compared with platelets recruited to thrombi, immune-responsive platelets use distinct effector functions including actin-related protein complex 2/3-dependent migration along adhesive substrate gradients (haptotaxis), which prevents inflammatory bleeding and contributes to host defense. How platelet migration in this context is regulated on a cellular level is incompletely understood. Here, we use time-resolved morphodynamic profiling of individual platelets to show that migration, in contrast to clot retraction, requires anisotropic myosin IIa-activity at the platelet rear which is preceded by polarized actin polymerization at the front to initiate and maintain migration. Integrin GPIIb-dependent outside-in signaling via Gα13 coordinates polarization of migrating platelets to trigger tyrosine kinase c-Src/14-3-3ζ-dependent lamellipodium formation and functions independent of soluble agonists or chemotactic signals. Inhibitors of this signaling cascade, including the clinically used ABL/c-Src inhibitor dasatinib, interfere predominantly with the migratory capacity of platelets, without major impairment of classical platelet functions. In murine inflammation models, this translates to reduced migration of platelets visualized by 4D intravital microscopy, resulting in increased inflammation-associated hemorrhage in acute lung injury. Finally, platelets isolated from patients with leukemia treated with dasatinib who are prone to clinically relevant hemorrhage exhibit prominent migration defects, whereas other platelet functions are only partially affected. In summary, we define a distinct signaling pathway essential for migration and provide novel mechanistic insights explaining dasatinib-related platelet dysfunction and bleeding.

Potential Biomarkers of Acute Ischemic Stroke Etiology Revealed by Mass Spectrometry-Based Proteomic Characterization of Formalin-Fixed Paraffin-Embedded Blood Clots

Background and Aims

Besides the crucial role in the treatment of acute ischemic stroke (AIS), mechanical thrombectomy represents a unique opportunity for researchers to study the retrieved clots, with the possibility of unveiling biological patterns linked to stroke pathophysiology and etiology. We aimed to develop a shotgun proteomic approach to study and compare the proteome of formalin-fixed paraffin-embedded (FFPE) cardioembolic and large artery atherosclerotic (LAA) clots.

Methods

We used 16 cardioembolic and 15 LAA FFPE thrombi from 31 AIS patients. The thrombus proteome was analyzed by label-free quantitative liquid chromatography-tandem mass spectrometry (LC-MS/MS). MaxQuant v1.5.2.8 and Perseus v.1.6.15.0 were used for bioinformatics analysis. Protein classes were identified using the PANTHER database and the STRING database was used to predict protein interactions.

Results

We identified 1,581 protein groups as part of the AIS thrombus proteome. Fourteen significantly differentially abundant proteins across the two etiologies were identified. Four proteins involved in the ubiquitin-proteasome pathway, blood coagulation or plasminogen activating cascade were identified as significantly abundant in LAA clots. Ten proteins involved in the ubiquitin proteasome-pathway, cytoskeletal remodeling of platelets, platelet adhesion or blood coagulation were identified as significantly abundant in cardioembolic clots.

Conclusion

Our results outlined a set of 14 proteins for a proof-of-principle characterization of cardioembolic and LAA FFPE clots, advancing the proteome profile of AIS human thrombi and understanding the pathophysiology of ischemic stroke.

Live-cell super-resolved PAINT imaging of piconewton cellular traction forces

Despite the vital role of mechanical forces in biology, it still remains a challenge to image cellular force with sub-100-nm resolution. Here, we present tension points accumulation for imaging in nanoscale topography (tPAINT), integrating molecular tension probes with the DNA points accumulation for imaging in nanoscale topography (DNA-PAINT) technique to map piconewton mechanical events with ~25-nm resolution. To perform live-cell dynamic tension imaging, we engineered reversible probes with a cryptic docking site revealed only when the probe experiences forces exceeding a defined mechanical threshold (~7-21 pN). Additionally, we report a second type of irreversible tPAINT probe that exposes its cryptic docking site permanently and thus integrates force history over time, offering improved spatial resolution in exchange for temporal dynamics. We applied both types of tPAINT probes to map integrin receptor forces in live human platelets and mouse embryonic fibroblasts. Importantly, tPAINT revealed a link between platelet forces at the leading edge of cells and the dynamic actin-rich ring nucleated by the Arp2/3 complex.

New insights into cytoskeletal remodeling during platelet production

The past decade has brought unprecedented advances in our understanding of megakaryocyte (MK) biology and platelet production, processes that are strongly dependent on the cytoskeleton. Facilitated by technological innovations, such as new high-resolution imaging techniques (in vitro and in vivo) and lineage-specific gene knockout and reporter mouse strains, we are now able to visualize and characterize the molecular machinery required for MK development and proplatelet formation in live mice. Whole genome and RNA sequencing analysis of patients with rare platelet disorders, combined with targeted genetic interventions in mice, has led to the identification and characterization of numerous new genes important for MK development. Many of the genes important for proplatelet formation code for proteins that control cytoskeletal dynamics in cells, such as Rho GTPases and their downstream targets. In this review, we discuss how the final stages of MK development are controlled by the cellular cytoskeletons, and we compare changes in MK biology observed in patients and mice with mutations in cytoskeleton regulatory genes.

Assessments of cellular melatonin receptor signaling pathways: β-arrestin recruitment, receptor internalization, and impedance variations

Melatonin receptors belong to the family of G-protein coupled receptors. Agonist-induced receptor activation is terminated with the recruitment of β-arrestin, which leads to receptor internalization. Furthermore, agonist binding induces a shift in cellular shape that translates into a change in the electric impedance of the cell. In the present study, we employed engineered cells to study these internalization-related processes in the context of the two melatonin receptors, MT₁ and MT₂. To assess these three receptor internalization-related functions and validate the results, we employed four classical ligands of melatonin receptors: the natural agonist melatonin; the super-agonist 2-iodo-melatonin and the two antagonists luzindole and 4-phenyl-2-propionamidotetralin. The assessments confirmed the nature of the agonistic ligands but showed that 4-phenyl-2-propionamidotetralin, a described antagonist, is a biased partial agonist at MT₂ with poorer affinity for MT₁. The methods are now available to be applied to any receptor system for which multiple signaling pathways must be evaluated for new molecules.

Sensitized mutagenesis screen in Factor V Leiden mice identifies thrombosis suppressor loci

Factor V Leiden (F5^L ) is a common genetic risk factor for venous thromboembolism in humans. We conducted a sensitized N-ethyl-N-nitrosourea (ENU) mutagenesis screen for dominant thrombosuppressor genes based on perinatal lethal thrombosis in mice homozygous for F5^L (F5^L/L ) and haploinsufficient for tissue factor pathway inhibitor (Tfpi^+/- ). F8 deficiency enhanced the survival of F5^L/LTfpi^+/- mice, demonstrating that F5^L/LTfpi^+/- lethality is genetically suppressible. ENU-mutagenized F5^L/L males and F5^L/+Tfpi^+/- females were crossed to generate 6,729 progeny, with 98 F5^L/LTfpi^+/- offspring surviving until weaning. Sixteen lines, referred to as "modifier of Factor 5 Leiden (MF5L1-16)," exhibited transmission of a putative thrombosuppressor to subsequent generations. Linkage analysis in MF5L6 identified a chromosome 3 locus containing the tissue factor gene (F3). Although no ENU-induced F3 mutation was identified, haploinsufficiency for F3 (F3^+/- ) suppressed F5^L/LTfpi^+/- lethality. Whole-exome sequencing in MF5L12 identified an Actr2 gene point mutation (p.R258G) as the sole candidate. Inheritance of this variant is associated with suppression of F5^L/LTfpi^+/- lethality (P = 1.7 × 10^-6), suggesting that Actr2^p.R258G is thrombosuppressive. CRISPR/Cas9 experiments to generate an independent Actr2 knockin/knockout demonstrated that Actr2 haploinsufficiency is lethal, supporting a hypomorphic or gain-of-function mechanism of action for Actr2^p.R258G Our findings identify F8 and the Tfpi/F3 axis as key regulators in determining thrombosis balance in the setting of F5^L and also suggest a role for Actr2 in this process.

Integrin αIIbβ3 outside-in signaling

Integrin α_IIbβ₃ is a highly abundant heterodimeric platelet receptor that can transmit information bidirectionally across the plasma membrane, and plays a critical role in hemostasis and thrombosis. Upon platelet activation, inside-out signaling pathways increase the affinity of α_IIbβ₃ for fibrinogen and other ligands. Ligand binding and integrin clustering subsequently stimulate outside-in signaling, which initiates and amplifies a range of cellular events driving essential platelet processes such as spreading, thrombus consolidation, and clot retraction. Integrin α_IIbβ₃ has served as an excellent model for the study of integrin biology, and it has become clear that integrin outside-in signaling is highly complex and involves a vast array of enzymes, signaling adaptors, and cytoskeletal components. In this review, we provide a concise but comprehensive overview of α_IIbβ₃ outside-in signaling, focusing on the key players involved, and how they cooperate to orchestrate this critical aspect of platelet biology. We also discuss gaps in the current understanding of α_IIbβ₃ outside-in signaling and highlight avenues for future investigation.

Deletion of the Arp2/3 complex in megakaryocytes leads to microthrombocytopenia in mice

Actin reorganization regulates key processes in platelet activation. Here we examined the role of the Arp2/3 complex, an essential component in actin filament branching, in platelet function. The Arpc2 gene, encoding the p34 subunit of the Arp2/3 complex, was deleted in the megakaryocyte lineage (Arpc2^fl/flPF4-Cre). Deletion of the Arp2/3 complex resulted in marked microthrombocytopenia in mice, caused by premature platelet release into the bone marrow compartment and impaired platelet survival in circulation. Arpc2^fl/flPF4-Cre platelets exhibited alterations in their actin cytoskeleton and their peripheral microtubule coil. Thrombocytopenia was alleviated following clodronate liposome-induced macrophage depletion in Arpc2^fl/flPF4-Cre mice. Arpc2^fl/flPF4-Cre platelets failed to spread and showed a mild defect in integrin activation and aggregation. However, no significant differences in hemostasis or thrombosis were observed between Arpc2^fl/flPF4-Cre and control mice. Thus, Arp2/3 is critical for platelet homeostasis but plays only a minor role for vascular hemostasis.

Balance of microtubule stiffness and cortical tension determines the size of blood cells with marginal band across species

The fast bloodstream of animals is associated with large shear stresses. To withstand these conditions, blood cells have evolved a special morphology and a specific internal architecture to maintain their integrity over several weeks. For instance, nonmammalian red blood cells, mammalian erythroblasts, and platelets have a peripheral ring of microtubules, called the marginal band, that flattens the overall cell morphology by pushing on the cell cortex. In this work, we model how the shape of these cells stems from the balance between marginal band rigidity and cortical tension. We predict that the diameter of the cell scales with the total microtubule polymer and verify the predicted law across a wide range of species. Our analysis also shows that the combination of the marginal band rigidity and cortical tension increases the ability of the cell to withstand forces without deformation. Finally, we model the marginal band coiling that occurs during the disk-to-sphere transition observed, for instance, at the onset of blood platelet activation. We show that when cortical tension increases faster than cross-linkers can unbind, the marginal band will coil, whereas if the tension increases more slowly, the marginal band may shorten as microtubules slide relative to each other.

Pdlim7 Regulates Arf6-Dependent Actin Dynamics and Is Required for Platelet-Mediated Thrombosis in Mice

Upon vessel injury, platelets become activated and rapidly reorganize their actin cytoskeleton to adhere to the site of endothelial damage, triggering the formation of a fibrin-rich plug to prevent further blood loss. Inactivation of Pdlim7 provides the new perspective that regulation of actin cytoskeletal changes in platelets is dependent on the encoded PDZ-LIM protein. Loss-of-function of Pdlim7 triggers hypercoagulopathy and causes significant perinatal lethality in mice. Our in vivo and in vitro studies reveal that Pdlim7 is dynamically distributed along actin fibers, and lack of Pdlim7 leads to a marked inability to rearrange the actin cytoskeleton. Specifically, the absence of Pdlim7 prevents platelets from bundling actin fibers into a concentric ring that defines the round spread shape of activated platelets. Similarly, in mouse embryonic fibroblasts, loss of Pdlim7 abolishes the formation of stress fibers needed to adopt the typical elongated fibroblast shape. In addition to revealing a fundamental cell biological role in actin cytoskeletal organization, we also demonstrate a function of Pdlim7 in regulating the cycling between the GTP/GDP-bound states of Arf6. The small GTPase Arf6 is an essential factor required for actin dynamics, cytoskeletal rearrangements, and platelet activation. Consistent with our findings of significantly elevated initial F-actin ratios and subsequent morphological aberrations, loss of Pdlim7 causes a shift in balance towards an increased Arf6-GTP level in resting platelets. These findings identify a new Pdlim7-Arf6 axis controlling actin dynamics and implicate Pdlim7 as a primary endogenous regulator of platelet-dependent hemostasis.

Cellular Dielectric Spectroscopy: A Label-Free Technology for Drug Discovery

Cellular dielectric spectroscopy (CDS) provides realtime, label-free, universal measurements, enabling comprehensive pharmacological evaluation of cell surface receptors in living cells. The sensitivity of the measurement allows monitoring of ligand-mediated activation of endogenous receptors, therefore generating physiologically relevant data. Activation of receptors results in CDS response profiles that are characteristic of main subsets of G-protein coupled receptors (GPCRs) within a cell line. This allows cluster analysis of response profiles that may be used in several important applications, which include identification of the G-protein coupling of orphan GPCRs and the cataloging of active endogenous receptors in cells. In this study, CDS technology is used in the pharmacological evaluation of multiple receptors in many cell types, including primary cells. Specifically, data is presented demonstrating hit confirmation, receptor selectivity analysis, ligand potency, and Schild analysis of receptor-selective antagonists. CDS results compare favorably to other cell-based assays, and the robustness and reproducibility of CDS assays are reflected by low assay coefficient of variation (CVs) and reliable Z'-scores of the data. Because CDS requires no stable or transiently transfected cells or special reagents, assay development and data acquisition is simple and fast. The ease of use, universality, and label-free nature of the CDS-based platform make it well suited to secondary screening applications in drug discovery.

Non-genomic activities of retinoic acid receptor alpha control actin cytoskeletal events in human platelets

Essentials Platelets employ proteins/signaling pathways traditionally thought reserved for nuclear niche. We determined retinoic-acid-receptor alpha (RARα) expression and function in human platelets. RARα/actin-related protein-2/3 complex (Arp2/3) interact via non-genomic signaling in platelets. RARα regulates Arp2/3-mediated actin cytoskeletal dynamics and platelet spreading.

SUMMARY

Background Platelets utilize proteins and pathways classically reserved for the nuclear niche. Methods We determined whether human platelets express retinoic-acid-receptor family members, traditionally thought of as nuclear transcription factors, and deciphered the function of RARα. Results We found that RARα is robustly expressed in human platelets and megakaryocytes and interacts directly with actin-related protein-2/3 complex (Arp2/3) subunit 5 (Arp2/3s5). Arp2/3s5 co-localized with RARα in situ and regulated platelet cytoskeletal processes. The RARα ligand all-trans retinoic acid (atRA) disrupted RARα-Arp2/3 interactions. When isolated human platelets were treated with atRA, rapid cytoskeletal events (e.g. platelet spreading) were inhibited. In addition, when platelets were cultured for 18 h in the presence of atRA, actin-dependent morphological changes (e.g. extended cell body formation) were similarly inhibited. Using in vitro actin branching assays, RARα and Arp2/3-regulated complex actin branch formation was demonstrated. Consistent with inhibition of cytoskeletal processes in platelets, atRA, when added to this branching assay, resulted in dysregulated actin branching. Conclusion Our findings identify a previously unknown mechanism by which RARα regulates Arp2/3-mediated actin cytoskeletal dynamics through a non-genomic signaling pathway. These findings have broad implications in both nucleated and anucleate cells, where actin cytoskeletal events regulate cell morphology, movement and division.

Evanescent Light-Scattering Microscopy for Label-Free Interfacial Imaging: From Single Sub-100 nm Vesicles to Live Cells

Advancement in the understanding of biomolecular interactions has benefited greatly from the development of surface-sensitive bioanalytical sensors. To further increase their broad impact, significant efforts are presently being made to enable label-free and specific biomolecule detection with high sensitivity, allowing for quantitative interpretation and general applicability at low cost. In this work, we have addressed this challenge by developing a waveguide chip consisting of a flat silica core embedded in a symmetric organic cladding with a refractive index matching that of water. This is shown to reduce stray light (background) scattering and thereby allow for label-free detection of faint objects, such as individual sub-20 nm gold nanoparticles as well as sub-100 nm lipid vesicles. Measurements and theoretical analysis revealed that light-scattering signals originating from single surface-bound lipid vesicles enable characterization of their sizes without employing fluorescent lipids as labels. The concept is also demonstrated for label-free measurements of protein binding to and enzymatic (phospholipase A2) digestion of individual lipid vesicles, enabling an analysis of the influence on the measured kinetics of the dye-labeling of lipids required in previous assays. Further, diffraction-limited imaging of cells (platelets) binding to a silica surface showed that distinct subcellular features could be visualized and temporally resolved during attachment, activation, and spreading. Taken together, these results underscore the versatility and general applicability of the method, which due to its simplicity and compatibility with conventional microscopy setups may reach a widespread in life science and beyond.

PKCθ-regulated signalling in health and disease

Protein kinase Cθ (PKCθ) is a key enzyme in T-lymphocytes where it plays an important role in signal transduction downstream of the activated T-cell receptor (TCR) and the CD28 co-stimulatory receptor. Antigenic stimulation of T-cells triggers PKCθ translocation to the centre of the immunological synapse (IS) at the contact site between antigen-specific T-cells and antigen-presenting cells (APCs). The IS-residing PKCθ phosphorylates and activates effector molecules that transduce signals into distinct subcellular compartments and activate the transcription factors, nuclear factor κB (NF-κB), nuclear factor of activated T-cells (NFAT) and activating protein 1 (AP-1), which are essential for the induction of T-cell-mediated responses. Besides its major biological role in T-cells, PKCθ is expressed in several additional cell types and is involved in a variety of distinct physiological and pathological phenomena. For example, PKCθ is expressed at high levels in platelets where it regulates signal transduction from distinct surface receptors, and is required for optimal platelet activation and aggregation, as well as haemostasis. In addition, PKCθ is involved in physiological processes regulating insulin resistance and susceptibility to obesity, and is expressed at high levels in gastrointestinal stromal tumours (GISTs), although the functional importance of PKCθ in these processes and cell types is not fully clear. The present article briefly reviews selected topics relevant to the biological roles of PKCθ in health and disease.

VAMP-7 links granule exocytosis to actin reorganization during platelet activation

Platelet activation results in profound morphologic changes accompanied by release of granule contents. Recent evidence indicates that fusion of granules with the plasma membrane during activation provides auxiliary membrane to cover growing actin structures. Yet little is known about how membrane fusion is coupled with actin reorganization. Vesicle-associated membrane protein (VAMP)-7 is found on platelet vesicles and possesses an N-terminal longin domain capable of linking exocytosis to cytoskeletal remodeling. We have evaluated platelets from VAMP-7(-/-) mice to determine whether this VAMP isoform contributes to granule release and platelet spreading. VAMP-7(-/-) platelets demonstrated a partial defect in dense granule exocytosis and impaired aggregation. α Granule exocytosis from VAMP-7(-/-) platelets was diminished both in vitro and in vivo during thrombus formation. Consistent with a role of VAMP-7 in cytoskeletal remodeling, spreading on matrices was decreased in VAMP-7(-/-) platelets compared to wild-type controls. Immunoprecipitation of VAMP-7 revealed an association with VPS9-domain ankyrin repeat protein (VARP), an adaptor protein that interacts with both membrane-bound and cytoskeleton proteins and with Arp2/3. VAMP-7, VARP, and Arp2/3 localized to the platelet periphery during spreading. These studies demonstrate that VAMP-7 participates in both platelet granule secretion and spreading and suggest a mechanism whereby VAMP-7 links granule exocytosis with actin reorganization.

Platelet Rho GTPases-a focus on novel players, roles and relationships

Rho GTPases are critical for platelet function. Although the roles of RhoA, Rac and Cdc42 are characterized, platelets express other Rho GTPases, whose activities are less well understood. This review summarizes our understanding of the roles of platelet Rho GTPases and focuses particularly on the functions of Rif and RhoG. In human platelets, Rif interacts with cytoskeleton regulators including formins mDia1 and mDia3, whereas RhoG binds SNARE-complex proteins and cytoskeletal regulators ELMO and DOCK1. Knockout mouse studies suggest that Rif plays no critical functions in platelets, likely due to functional overlap with other Rho GTPases. In contrast, RhoG is essential for normal granule secretion downstream of the collagen receptor GPVI. The central defect in RhoG-/- platelets is reduced dense granule secretion, which impedes integrin activation and aggregation and limits platelet recruitment to growing thrombi under shear, translating into reduced thrombus formation in vivo. Potential avenues for future work on Rho GTPases in platelets are also highlighted, including identification of the key regulator for platelet filopodia formation and investigation of the role of the many Rho GTPase regulators in platelet function in both health and disease.

Proteomic analysis of differential protein expression in platelets of septic patients

Sepsis is one of the major health problems all over the world. Early diagnostic of sepsis is an attractive strategy to decrease the mortality of septic patients. However, an effective biomarker that fulfills all the necessary requirements for the accurate characterization of sepsis is still unavailable until now. In this study, the 2-DE technique followed by mass spectrometry and a database search was used for searching and identifying the differential expressed proteins in platelets between septic patients and paired healthy controls. Platelet 2-DE profiles of septic patients and paired healthy controls with high resolution and reproducibility were obtained. Differential platelet 2-DE profiles between septic patients and paired healthy controls were established. Differential protein spots between normal healthy volunteers and septic patients from platelet 2-DE profiles were identified by 2-DE followed with mass spectrometry and a database search. Five proteins with increased expression were identified between septic patients and healthy controls from platelet samples. These up-expressed proteins were EF-hand calcium-binding domain-containing protein 7, actin, interleukin-1β, glycoprotein IX, and glycoprotein IIB. Sepsis induces a complex regulation of platelet protein changes. Our study highlights the important role of these differential expressed proteins in sepsis, which deserve further research as potential candidates for therapeutic strategies. Furthermore, our research is beneficial for the future developments of sepsis diagnosis and therapy.

Precise patterning of the SEBS surface by UV lithography to evaluate the platelet function through single platelet adhesion

The micropatterned surface which was createdviaUV lithography with a photomask, was demonstrated to control single platelet adhesion, render platelets sensitive to proteins and enable assessment of the platelet function in the presence of antiplatelet agents through single platelet adhesion.

p21 activated kinase signaling coordinates glycoprotein receptor VI-mediated platelet aggregation, lamellipodia formation, and aggregate stability under shear

OBJECTIVE

Rho GTPase proteins play a central role in regulating the dynamics of the platelet actin cytoskeleton. Yet, little is known regarding how Rho GTPase activation coordinates platelet activation and function. In this study, we aimed to characterize the role of the Rho GTPase effector, p21 activated kinase (PAK), in platelet activation, lamellipodia formation, and aggregate formation under shear.

APPROACH AND RESULTS

Stimulation of platelets with the glycoprotein receptor VI agonist, collagen-related peptide, rapidly activated PAK in a time course preceding phosphorylation of PAK substrates, LIM domain kinase LIMK1 and the MAPK/ERK kinase MEK, and the subsequent activation of MAPKs and Akt. Pharmacological inhibitors of PAK blocked signaling events downstream of PAK and prevented platelet secretion as well as platelet aggregation in response to collagen-related peptide. PAK inhibitors also prevented PAK activation and platelet spreading on collagen surfaces. PAK was also required for the formation of platelet aggregates and to maintain aggregate stability under physiological shear flow conditions.

CONCLUSIONS

These results suggest that PAK serves as an orchestrator of platelet functional responses after activation downstream of the platelet collagen receptor, glycoprotein receptor VI.

The abluminal endothelial membrane in neurovascular remodeling in health and disease

After brain injury, blood-brain barrier (BBB) integrity can be compromised as a consequence of the breakdown of cell-cell interactions in the neurovascular unit, resulting in the loss of the characteristic endothelial luminal-to-abluminal structure. During the process of restoration of the BBB and vascularization, the endothelial cells are continuously reshaped, with both the luminal and abluminal membranes serving as sites of signaling. Here, we focus on the bidirectional signaling processes that are rapidly initiated between endothelial and perivascular cells and occur in certain brain diseases or in response to injury. The goal of these processes is (i) the reemergence of endothelial cell polarity, (ii) the remodeling of extracellular matrix interactions, (iii) the realignment of pericytes and astrocytic endfeet with endothelial cells, and (iv) the restitution of a well-organized and stable BBB. This abluminal membrane exemplifies how the brain vasculature responds to stressors and may represent promising targets for therapeutic interventions of brain diseases.

Proteome of platelets in patients with coronary artery disease

OBJECTIVE

This study aimed at investigating the protein patterns of platelets from patients with stable or acute coronary atherosclerosis (CAD), in which platelets play a key role.

MATERIALS AND METHODS

A proteomic approach was adopted to investigate specific protein patterns in platelets of patients with non-ST elevation acute coronary syndrome, stable angina, or of subjects with no history of CAD.

RESULTS

Six differentially expressed proteins were identified: two involved in energy metabolism (2-oxoglutarate dehydrogenase [OGDH], and lactate dehydrogenase [LDH]); three were associated with cytoskeleton-based processes (gamma-actin, coronin 1B, and pleckstrin); and one involved in protein degradation (proteasome subunit type 8). Expression levels of OGDH and a cleaved form of gamma-actin were significantly higher in the platelets of patients than in controls, whereas that of LDH was higher only in the platelets of patients with acute coronary disease. The increases in protein expression of OGDH and LDH are paralleled by changes in their functional activities. Coronin and proteasome subunit type 8 were less expressed in the platelets of patients, as were the basic isoforms of pleckstrin.

CONCLUSION

The platelet proteome is altered in CAD patients with stable or acute coronary syndrome possibly because of the ongoing atherosclerotic process. The identified protein changes not previously connected with CAD were an increase in the energy metabolism enzymes and alterations in the proteins associated with cytoskeleton-based processes, both of which indicate platelet activation.

Studies on the actin-binding protein HS1 in platelets

BACKGROUND

The platelet cytoskeleton mediates the dramatic change in platelet morphology that takes place upon activation and stabilizes thrombus formation. The Arp2/3 complex plays a vital role in these processes, providing the protrusive force for lamellipodia formation. The Arp2/3 complex is highly regulated by a number of actin-binding proteins including the haematopoietic-specific protein HS1 and its homologue cortactin. The present study investigates the role of HS1 in platelets using HS1-/- mice.

RESULTS

The present results demonstrate that HS1 is not required for platelet activation, shape change or aggregation. Platelets from HS1-/- mice spread normally on a variety of adhesion proteins and have normal F-actin and Arp2/3 complex distributions. Clot retraction, an actin-dependent process, is also normal in these mice. Platelet aggregation and secretion is indistinguishable between knock out and littermates and there is no increase in bleeding using the tail bleeding assay.

CONCLUSION

This study concludes that HS1 does not play a major role in platelet function. It is possible that a role for HS1 is masked by the presence of cortactin.

Cellular Dielectric Spectroscopy: A Label-Free Comprehensive Platform for Functional Evaluation of Endogenous Receptors

The CellKey (MDS Sciex, South San Francisco, CA) system enables comprehensive pharmacological evaluation of cell surface receptors, including G-protein coupled receptors (GPCRs) and tyrosine kinase receptors, using adherent and suspension cell lines and primary cells. A unique application enabled by the ability of the CellKey system to reliably quantify activation of endogenous receptors is receptor panning. This application allows investigators to easily screen disease-relevant cell types for functionally active target receptors by treating cells with a panel of receptor-specific ligands. Receptor panning of multiple cell types including Chinese hamster ovary, human embryonic kidney 293, HeLa, U-937, U-2 OS, and TE671 cells resulted in the identification of many functionally active, differently coupled endogenous GPCRs, some of which have not been previously documented in the literature. Upon detecting GPCR activation in live cells, unique cellular dielectric spectroscopy (CDS) response profiles are generated within minutes that reflect the signaling pathways utilized and have been shown to be characteristic of Gs, Gq, and Gi GPCRs. The fact that the CDS response profiles are predictive of the G-protein coupling mechanism of the receptor was demonstrated by using examples of subtype-selective agonists/antagonists to identify the subtypes of the endogenous histamine and beta-adrenergic receptors expressed in U-2 OS cells. A direct correlation is shown between receptor subtype G-protein coupling and CDS response profile. In addition, complex pharmacology, including detection of partial agonism and Schild analysis for endogenous receptors, is presented. The CellKey system allows investigators to conduct studies using endogenously expressed receptors to generate data that are physiologically relevant and in disease context.

Chemical enhancers of cytokine signaling that suppress microfilament turnover and tumor cell growth

The transforming growth factor-beta (TGF-beta) family of cytokines regulates cell proliferation, morphogenesis, and specialized cell functions in metazoans. Herein, we screened a compound library for modifiers of TGF-beta signaling in NMuMG epithelial cells using a cell-based assay to measure Smad2/3 nuclear translocation. We identified five enhancers of TGF-beta signaling that share a core structure of diethyl 2-(anilinomethylene)malonate (DAM), and D(50) values of 1 to 4 micromol/L. Taking advantage of the Mgat5 mutant phenotype of accelerated receptor loss to endocytosis, we determined that DAM-1976 restored the sensitivity of Mgat5(-/-) carcinoma cells to both TGF-beta and epidermal growth factor (EGF). In Mgat5 mutant and wild-type carcinoma cells, DAM-1976 enhanced and prolonged TGF-beta- and EGF-dependent Smad2/3 and Erk activation, respectively. DAM-1976 reduced ligand-dependent EGF receptor endocytosis, actin microfilament turnover, and cell spreading, suggesting that the compound attenuates vesicular trafficking. Hyperactivation of intracellular signaling has the potential to suppress tumor cell growth and, in this regard, DAM-1976 represents a new pharmacophore that increases basal activation of Smad2/3 and Erk, inhibits microfilament remodeling, and suppresses carcinoma cell growth.

A role for PKCtheta in outside-in alpha(IIb)beta3 signaling

Fibrinogen binding to platelets triggers alpha(IIb)beta3-dependent outside-in signals that promote actin rearrangements and cell spreading. Studies with chemical inhibitors or activators have implicated protein kinase C (PKC) in alpha(IIb)beta3 function. However, the role of individual PKC isoforms is poorly understood. Biochemical and genetic approaches were used to determine whether PKCtheta is involved in alpha(IIb)beta3 signaling. PKCtheta was constitutively associated with alpha(IIb)beta3 in human and murine platelets. Fibrinogen binding to alpha(IIb)beta3 stimulated the association of PKCtheta with tyrosine kinases Btk and Syk, and tyrosine phosphorylation of PKCtheta, Btk and the actin regulator, Wiskott-Aldrich syndrome protein (WASP). Mouse platelets deficient in PKCtheta or Btk failed to spread on fibrinogen. Furthermore, PKCtheta was required for phosphorylation of WASP-interacting protein on Ser-488, an event that has been linked to WASP activation of the Arp2/3 complex and actin polymerization in lymphocytes. Neither PKCtheta nor Btk were required for agonist-induced inside-out signaling and fibrinogen binding to alpha(IIb)beta3. Thus, PKCtheta is a newly identified, essential member of a dynamic outside-in signaling complex that includes Btk and that couples alpha(IIb)beta3 to the actin cytoskeleton.

Rac1 is essential for platelet lamellipodia formation and aggregate stability under flow

The role of Rac family proteins in platelet spreading on matrix proteins under static and flow conditions has been investigated by using Rac-deficient platelets. Murine platelets form filopodia and undergo limited spreading on fibrinogen independent of Rac1 and Rac2. In the presence of thrombin, marked lamellipodia formation is observed on fibrinogen, which is abrogated in the absence of Rac1. However, Rac1 is not required for thrombin-induced aggregation or elevation of F-actin levels. Formation of lamellipodia on collagen and laminin is also Rac1-dependent. Analysis of platelet adhesion dynamics on collagen under flow conditions in vitro revealed that Rac1 is required for platelet aggregate stability at arterial rates of shear, as evidenced by a dramatic increase in platelet embolization. Furthermore, studies employing intravital microscopy demonstrated that Rac1 plays a critical role in the development of stable thrombi at sites of vascular injury in vivo. Thus, our data demonstrated that Rac1 is essential for lamellipodia formation in platelets and indicated that Rac1 is required for aggregate integrity leading to thrombus formation under physiologically relevant levels of shear both in vitro and in vivo.

The integrin alpha6beta1 modulation of PI3K and Cdc42 activities induces dynamic filopodium formation in human platelets

Platelets are an ideal model for studying a rapid morphological change in response to various signal transduction systems. Morphological changes via the activation of integrin alphaIIbbeta3 in platelets have been investigated intensively. In contrast, activation via integrin alpha6beta1 is less well studied. Here, we provide the first biochemical evidence that integrins alpha6beta1 and alphaIIbbeta3 of platelets are associated with different membrane proteins. We also demonstrate that platelets activated by integrin alpha6beta1 show dynamic change by actively forming filopodia and never fully spreading over a period of more than an hour. In addition, platelets activated by integrin alpha6beta1 are different from those activated by integrin alphaIIbbeta3 in terms of cell-substrate contact and in their distribution pattern of actin, Arp2/3 and various phosphotyrosine proteins. The morphological appearance of platelets produced through integrin alpha6beta1 activation is highly dependent on PI3 kinase (PI3K) but less dependent on Src kinase. Suppression of PI3K activity in integrin alpha6beta1 activated platelets induces an increase in Cdc42 activity and more filopodium formation. However, both Cdc42 and PI3K activity are higher in platelets activated by integrin alpha6beta1 than in those activated by integrin alphaIIbbeta3. Taken together, this study demonstrates that the signals induced by integrin alpha6beta1 modulate at the level of PI3K and Cdc42 activity to allow platelets to actively form filopodia.

Regulation of LIM-kinase 1 and cofilin in thrombin-stimulated platelets

Cofilin is a regulator of actin filament dynamics. We studied whether during platelet activation Rho kinase stimulates LIM kinase (LIMK) leading to subsequent phosphorylation and inactivation of cofilin. Platelet shape change and aggregation/secretion were induced by low and high concentrations of thrombin, respectively. We found that during these platelet responses Rho kinase activation was responsible for mediating rapid Thr508 phosphorylation and activation of LIMK-1 and for the F-actin increase during shape change and, in part, during secretion. Surprisingly, during shape change cofilin phosphorylation was unaltered, and during aggregation/secretion cofilin was first rapidly dephosphorylated by an okadaic acid-insensitive phosphatase and then slowly rephosphorylated by LIMK-1. LIMK-1 phosphorylation and cofilin dephosphorylation and rephosphorylation during aggregation were independent of integrin alpha(IIb)beta(3) engagement. Cofilin phosphorylation did not regulate cofilin association with F-actin and was unrelated to the F-actin increase in thrombin-activated platelets. Our study identifies LIMK-1 as being activated by Rho kinase in thrombin-stimulated platelets. Two counteracting pathways, a cofilin phosphatase and LIMK-1, are activated during platelet aggregation/secretion regulating cofilin phosphorylation sequentially and independently of integrin alpha(IIb)beta(3) engagement. Rho kinase-mediated F-actin increase during platelet shape change and secretion involves a mechanism other than LIMK-1-mediated cofilin phosphorylation, raising the possibility of another LIMK substrate regulating platelet actin assembly.

Two-dimensional Blue Native/sodium dodecyl sulfate gel electrophoresis for analysis of multimeric proteins in platelets

Two-dimensional (2-D) Blue Native/SDS gel electrophoresis combines a first-dimensional separation of monomeric and multimeric proteins in their native state with a second denaturing dimension. These high-resolution 2-D gels aim at identifying multiprotein complexes with respect to their subunit composition. We applied this method for the first time to analyze two human platelet subproteomes: the cytosolic and the microsomal membrane protein fraction. Solubilization of platelet membrane proteins was achieved with the nondenaturing detergent n-dodecyl-beta-D-maltoside. To validate native solubilization conditions, we demonstrated the correct assembly of the Na,K-ATPase, a functional multimeric transmembrane protein, when expressed in Xenopus oocytes. We identified 63 platelet proteins after in-gel tryptic digestion of 58 selected protein spots and liquid chromatography-coupled tandem mass spectrometry. Nine proteins were detected for the first time in platelets by a proteomic approach. We also show that this technology efficiently resolves several known membrane and cytosolic multiprotein complexes. Blue Native/SDS gel electrophoresis is thus a valuable procedure to analyze specific platelet subproteomes, like the membrane(-bound) protein fraction, by mass spectrometry and immunoblotting and could be relevant for the study of protein-protein interactions generated following platelet activation.

The formin homology 1 domain modulates the actin nucleation and bundling activity of Arabidopsis FORMIN1

The organization of actin filaments into large ordered structures is a tightly controlled feature of many cellular processes. However, the mechanisms by which actin filament polymerization is initiated from the available pool of profilin-bound actin monomers remain unknown in plants. Because the spontaneous polymerization of actin monomers bound to profilin is inhibited, the intervention of an actin promoting factor is required for efficient actin polymerization. Two such factors have been characterized from yeasts and metazoans: the Arp2/3 complex, a complex of seven highly conserved subunits including two actin-related proteins (ARP2 and ARP3), and the FORMIN family of proteins. The recent finding that Arabidopsis thaliana plants lacking a functional Arp2/3 complex exhibit rather modest morphological defects leads us to consider whether the large FORMIN family plays a central role in the regulation of actin polymerization. Here, we have characterized the mechanism of action of Arabidopsis FORMIN1 (AFH1). Overexpression of AFH1 in pollen tubes has been shown previously to induce abnormal actin cable formation. We demonstrate that AFH1 has a unique behavior when compared with nonplant formins. The activity of the formin homology domain 2 (FH2), containing the actin binding activity, is modulated by the formin homology domain 1 (FH1). Indeed, the presence of the FH1 domain switches the FH2 domain from a tight capper (Kd approximately 3.7 nM) able to nucleate actin filaments that grow only in the pointed-end direction to a leaky capper that allows barbed-end elongation and efficient nucleation of actin filaments from actin monomers bound to profilin. Another exciting feature of AFH1 is its ability to bind to the side and bundle actin filaments. We have identified an actin nucleator that is able to organize actin filaments directly into unbranched actin filament bundles. We suggest that AFH1 plays a central role in the initiation and organization of actin cables from the pool of actin monomers bound to profilin.

Expression and subcellular localization of WAVE isoforms in the megakaryocyte/platelet lineage

WAVE isoforms, which consist of WAVE-1, WAVE-2 and WAVE-3, are members of the Wiskott-Aldrich syndrome protein (WASP) family. They are implicated in the regulation of actin-cytoskeletal reorganization downsteam of the small GTPase, Rac. Since platelet attachment to extracellular matrices leads to filopodial and lamellipodial extension, we examined the expression and subcellular localization of WAVEs in platelets. Employing primary megakaryocytic cells derived from cord blood as well as megakaryocytic cell lines, we also examined their expression during megakaryocytic differentiation. Immunoblotting and immunohistochemical analysis revealed that platelets expressed WAVE-1 and WAVE-2, whereas WAVE-3 expression was hardly to be detected. WAVE-1 expression was associated with megakaryocytic differentiation, whereas WAVE-2 and WAVE-3 expression was not changed by the differentiation. In adhered platelets both WAVE-1 and WAVE-2 were localized at the edge of the lamellipodia and at the tips of filopodia. In WASP-deficient platelets we found normal lamellipodial formation and localization of WAVE-1 and WAVE-2 at the edges of lamellipodia. Furthermore, we demonstrated that WAVE-1 and WAVE-2 moved from a detergent-soluble cytosolic fraction to insoluble cytoskeleton fraction after platelet aggregation. These results suggest that WAVE-1 and WAVE-2 regulate actin reorganization during platelet spreading and aggregate formation.

Advances in antiplatelet therapy

Cardiovascular diseases (CVDs) are the leading cause of morbidity and mortality in the western world. These disorders share a common pathophysiology -- atherosclerosis, which affects various arterial beds, leading to protean manifestations (coronary artery disease [CAD], stroke, peripheral arterial disease [PAD]). The platelet plays a pivotal role in the perpetuation and clinical expression of these disorders. The platelet, once believed to have a role confined to modulation of thrombosis and haemostasis, also plays an active role in vascular inflammation. Antiplatelet agents have become first-line therapy for CVD, and their unequivocal benefits are demonstrated in various basic and experimental models and supported by overwhelming evidence from clinical trials. Search is underway for more effective and safer antiplatelet therapy. Novel therapies are emerging to target the redundant pathways of platelet adhesion, activation and aggregation. Efforts are also ongoing to enhance implementation of existent therapy, target therapy selectively to high-risk patients and to those likely to respond (pharmacogenomics), and study the incremental benefits and safety of various antiplatelet combinations and their interaction with other medications in patients with CVD treated with polypharmacy.

Signalling to actin assembly via the WASP (Wiskott-Aldrich syndrome protein)-family proteins and the Arp2/3 complex

The assembly of a branched network of actin filaments provides the mechanical propulsion that drives a range of dynamic cellular processes, including cell motility. The Arp2/3 complex is a crucial component of such filament networks. Arp2/3 nucleates new actin filaments while bound to existing filaments, thus creating a branched network. In recent years, a number of proteins that activate the filament nucleation activity of Arp2/3 have been identified, most notably the WASP (Wiskott-Aldrich syndrome protein) family. WASP-family proteins activate the Arp2/3 complex, and consequently stimulate actin assembly, in response to extracellular signals. Structural studies have provided a significant refinement in our understanding of the molecular detail of how the Arp2/3 complex nucleates actin filaments. There has also been much progress towards an understanding of the complicated signalling processes that regulate WASP-family proteins. In addition, the use of gene disruption in a number of organisms has led to new insights into the specific functions of individual WASP-family members. The present review will discuss the Arp2/3 complex and its regulators, in particular the WASP-family proteins. Emphasis will be placed on recent developments in the field that have furthered our understanding of actin dynamics and cell motility.

The Tumor Suppressor hSNF5/INI1 Modulates Cell Growth and Actin Cytoskeleton Organization

hSNF5/INI1, which encodes a component of the ATP-dependent chromatin remodeling hSWI-SNF complex, is a tumor suppressor gene mutated in malignant rhabdoid tumors. We have developed a tetracycline-based hSNF5/INI1-inducible system in a hSNF5/INI1-deficient malignant rhabdoid tumor cell line and studied time course variation of 22,000 genes/expressed sequence tags upon hSNF5/INI1 induction. A total of 482 responsive genes were identified and further clustered into 9 groups of coregulated genes. Among genes with early and strong inductions, the use of a fusion protein with the hormone-binding domain of the estrogen receptor enabled the identification of a subset of direct targets regulated independently of de novo protein synthesis. We show that the G(1) arrest induced by hSNF5/INI1 is reversible and associated with the down-regulation of components of the DNA replication complex. We also identify an unsuspected role of hSNF5/INI1 in cytoskeleton organization. Indeed, induction of hSNF5/INI1 induces dramatic modifications of the cell shape including complete disruption of the actin stress fiber network and disappearance of focal adhesions associated with up-regulation of genes involved in the organization of the actin cytoskeleton. We document a strong decrease of Rho activity upon hSNF5/INI1 expression, suggesting that the regulation of this activity constitutes a crucial step of the hSNF5/INI1-induced reorganization of the actin network. This study identifies hSNF5/INI1 target genes and provides evidence that hSNF5/INI1 may modulate the cell cycle control and cytoskeleton organization through the regulation of the retinoblastoma protein-E2F and Rho pathways.

Association of CIB with GPIIb/IIIa during outside-in signaling is required for platelet spreading on fibrinogen

Platelet spreading on immobilized fibrinogen (Fg) involves progression through a number of morphologic stages that, although distinctive, are not well understood mechanistically. Here we demonstrate that an association between GPIIb/IIIa and calcium- and integrin-binding protein (CIB) is required for the process of platelet spreading. Upon platelet adhesion to immobilized Fg, CIB localizes to the transiently formed filopodia and then redistributes diffusely along the membrane periphery of spread platelets. Immunoprecipitation analyses indicate that CIB and glycoprotein IIb/IIIa (GPIIb/IIIa) interact with each other as platelets adhere to immobilized Fg, and together they associate with the platelet cytoskeleton. Introduction of anti-CIB antibody or GPIIb cytoplasmic peptide into platelets blocks lamellipodia but not filopodia formation. GPIIb peptide-induced inhibition of platelet spreading is recovered by the incorporation of recombinant CIB protein, suggesting that interaction between CIB and GPIIb/IIIa is required for progression from filopodial to spread morphologies. Further, anti-CIB- or GPIIb peptide-induced inhibition of platelet spreading can be overcome by the addition of exogenous adenosine diphosphate (ADP). These data suggest that formation of the CIB-GPIIb/IIIa complex may be necessary for initiation of downstream signaling events, such as ADP secretion, that lead to platelet spreading.

Formation of filopodia-like bundles in vitro from a dendritic network

We report the development and characterization of an in vitro system for the formation of filopodia-like bundles. Beads coated with actin-related protein 2/3 (Arp2/3)-activating proteins can induce two distinct types of actin organization in cytoplasmic extracts: (1) comet tails or clouds displaying a dendritic array of actin filaments and (2) stars with filament bundles radiating from the bead. Actin filaments in these bundles, like those in filopodia, are long, unbranched, aligned, uniformly polar, and grow at the barbed end. Like filopodia, star bundles are enriched in fascin and lack Arp2/3 complex and capping protein. Transition from dendritic to bundled organization was induced by depletion of capping protein, and add-back of this protein restored the dendritic mode. Depletion experiments demonstrated that star formation is dependent on Arp2/3 complex. This poses the paradox of how Arp2/3 complex can be involved in the formation of both branched (lamellipodia-like) and unbranched (filopodia-like) actin structures. Using purified proteins, we showed that a small number of components are sufficient for the assembly of filopodia-like bundles: Wiskott-Aldrich syndrome protein (WASP)-coated beads, actin, Arp2/3 complex, and fascin. We propose a model for filopodial formation in which actin filaments of a preexisting dendritic network are elongated by inhibition of capping and subsequently cross-linked into bundles by fascin.

Mechanism of filopodia initiation by reorganization of a dendritic network

Afilopodium protrudes by elongation of bundled actin filaments in its core. However, the mechanism of filopodia initiation remains unknown. Using live-cell imaging with GFP-tagged proteins and correlative electron microscopy, we performed a kinetic-structural analysis of filopodial initiation in B16F1 melanoma cells. Filopodial bundles arose not by a specific nucleation event, but by reorganization of the lamellipodial dendritic network analogous to fusion of established filopodia but occurring at the level of individual filaments. Subsets of independently nucleated lamellipodial filaments elongated and gradually associated with each other at their barbed ends, leading to formation of cone-shaped structures that we term Lambda-precursors. An early marker of initiation was the gradual coalescence of GFP-vasodilator-stimulated phosphoprotein (GFP-VASP) fluorescence at the leading edge into discrete foci. The GFP-VASP foci were associated with Lambda-precursors, whereas Arp2/3 was not. Subsequent recruitment of fascin to the clustered barbed ends of Lambda-precursors initiated filament bundling and completed formation of the nascent filopodium. We propose a convergent elongation model of filopodia initiation, stipulating that filaments within the lamellipodial dendritic network acquire privileged status by binding a set of molecules (including VASP) to their barbed ends, which protect them from capping and mediate association of barbed ends with each other.

Scientific and therapeutic advances in antiplatelet therapy

Over the past decade, the platelet has emerged as a pivotal entity in cardiovascular diseases. Indeed, the 'preeminence of the platelet' has spawned a variety of drugs that have been shown in large-scale randomized trials to improve patient outcomes in acute coronary syndromes and percutaneous revascularization procedures. Although the platelet was initially viewed only as a bystander in haemostasis, it is now evident that the platelet is in fact a key mediator of thrombosis as well as of inflammation. New insights at the cellular and genomic levels will probably generate novel drugs to inhibit platelet function more effectively and safely than previously possible.

The role of the cytoskeleton in the life cycle of viruses and intracellular bacteria: tracks, motors, and polymerization machines

Recent advances in microbiology implicate the cytoskeleton in the life cycle of some pathogens, such as intracellular bacteria, Rickettsia and viruses. The cellular cytoskeleton provides the basis for intracellular movements such as those that transport the pathogen to and from the cell surface to the nuclear region, or those that produce cortical protrusions that project the pathogen outwards from the cell surface towards an adjacent cell. Transport in both directions within the neuron is required for pathogens such as the herpesviruses to travel to and from the nucleus and perinuclear region where replication takes place. This trafficking is likely to depend on cellular motors moving on a combination of microtubule and actin filament tracks. Recently, Bearer et al. reconstituted retrograde transport of herpes simplex virus (HSV) in the giant axon of the squid. These studies identified the tegument proteins as the viral proteins most likely to recruit retrograde motors for the transport of HSV to the neuronal nucleus. Similar microtubule-based intracellular movements are part of the biological behavior of vaccinia, a poxvirus, and of adenovirus. Pathogen-induced surface projections and motility within the cortical cytoplasm also play a role in the life cycle of intracellular pathogens. Such motility is driven by pathogen-mediated actin polymerization. Virulence depends on this actin-based motility, since virulence is reduced in Listeria ActA mutants that lack the ability to recruit Arp2/3 and polymerize actin and in vaccinia virus mutants that cannot stimulate actin polymerization. Inhibition of intracellular movements provides a potential strategy to limit pathogenicity. The host cell motors and tracks, as well as the pathogen factors that interact with them, are potential targets for novel antimicrobial therapy.

Actin dynamics in platelets

The human blood platelet circulates in the blood as a non-adherent disk. Upon receiving signals of blood vessel damage, the platelet reorganizes its actin cytoskeleton which transforms it into a spiky dynamic adherent glue. This transformation involves a temporal sequence of four morphologically distinct steps which is reproducible in vitro. The actin dynamics underlying these shape changes depend on a large number of actin-binding proteins. Maintenance of the discoid shape requires actin-binding proteins that inhibit these reorganizations, whereas transformation involves other proteins, some to disassemble old filaments and others to polymerize new ones. F-Actin-affinity chromatography identified a large set of actin-binding proteins including VASP, Arp2 and 2E4/kaptin. Recent discoveries show that VASP inhibits filament disassembly and Arp2/3 is required to polymerize new filaments. Morphological analysis of the distribution of these actin-binding proteins in spread platelets together with biochemical measurements of their interactions with actin lead to a model of interactions with actin that mediate shape change.