The Wiskott-Aldrich syndrome protein directs actin-based motility by stimulating actin nucleation with the Arp2/3 complex

Membrane curvature catalyzes actin nucleation through nano-scale condensation of N-WASP-FBP17

Actin remodeling is spatiotemporally regulated by surface topographical cues on the membrane for signaling across diverse biological processes. Yet, the mechanism dynamic membrane curvature prompts quick actin cytoskeletal changes in signaling remain elusive. Leveraging the precision of nanolithography to control membrane curvature, we reconstructed catalytic reactions from the detection of nano-scale curvature by sensing molecules to the initiation of actin polymerization, which is challenging to study quantitatively in living cells. We show that this process occurs via topographical signal-triggered condensation and activation of the actin nucleation-promoting factor (NPF), Neuronal Wiskott-Aldrich Syndrome protein (N-WASP), which is orchestrated by curvature-sensing BAR-domain protein FBP17. Such N-WASP activation is fine-tuned by optimizing FBP17 to N-WASP stoichiometry over different curvature radii, allowing a curvature-guided macromolecular assembly pattern for polymerizing actin network locally. Our findings shed light on the intricate relationship between changes in curvature and actin remodeling via spatiotemporal regulation of NPF/BAR complex condensation.

nWASP Inhibition Increases Wound Healing via TrKb/PLCγ Signalling

(1) Background: Chronic wounds represent a major burden to patients and healthcare systems and identifying new therapeutic targets to encourage wound healing is a significant challenge. This study evaluated nWASP as a new therapeutic target in human wound healing and determined how this can be regulated. (2) Methods: Clinical cohorts from patients with chronic wounds were tested for the expression of nWASP and cell models were employed to evaluate the influence of nWASP on cellular functions that are key to the healing process following knockdown and/or the use of nWASP-specific inhibitors. (3) Results: nWASP was significantly elevated at transcript levels in human non-healing chronic wounds versus healing tissues. nWASP inhibitors, wiskostatin and 187-1, along with the knockdown of nWASP, modified both HaCaT and HECV cell behaviour. We then identified two signalling pathways affected by nWASP inhibition: TrkB signalling and downstream PLCγ1 phosphorylation were impaired by nWASP inhibition in HaCaT cells. The healing of wounds in a diabetic murine model was significantly improved with an nWASP inhibitor treatment. (4) Conclusions: This study showed that nWASP activity was related to the non-healing behaviour of chronic wounds and together with the findings in the in vivo models, it strongly suggested nWASP as a therapeutic target in non-healing wounds that are regulated via TrkB and PLCγ1 signalling.

The pathobiology of platelet and megakaryocyte extracellular vesicles: A (c)lot has changed

Platelet-derived extracellular vesicles (PEVs) were originally studied for their potential as regulators of coagulation, a function redundant with that of their parent cells. However, as the understanding of the diverse roles of platelets in hemostasis and disease has developed, so has the understanding of PEVs. In addition, the more recent revelation of constitutively released megakaryocyte-derived extracellular vesicles (MKEVs) in circulation provides an interesting counterpoint and avenue for investigation. In this review, we highlight the historical link of PEVs to thrombosis and hemostasis and provide critical updates. We also expand our discussion to encompass the roles that distinguish PEVs and MKEVs from their parent cells. Furthermore, the role of extracellular vesicles in disease pathology, both as biomarkers and as exacerbators, has been of great interest in recent years. We highlight some of the key roles that PEVs and MKEVs play in autoimmune blood cell disorders, liver pathology, and cardiovascular disease. We then look at the future of PEVs and MKEVs as candidates for novel therapeutics.

Centronuclear Myopathy Caused by Defective Membrane Remodelling of Dynamin 2 and BIN1 Variants

Centronuclear myopathy (CNM) is a congenital myopathy characterised by centralised nuclei in skeletal myofibers. T-tubules, sarcolemmal invaginations required for excitation-contraction coupling, are disorganised in the skeletal muscles of CNM patients. Previous studies showed that various endocytic proteins are involved in T-tubule biogenesis and their dysfunction is tightly associated with CNM pathogenesis. DNM2 and BIN1 are two causative genes for CNM that encode essential membrane remodelling proteins in endocytosis, dynamin 2 and BIN1, respectively. In this review, we overview the functions of dynamin 2 and BIN1 in T-tubule biogenesis and discuss how their dysfunction in membrane remodelling leads to CNM pathogenesis.

Ube2N is present and functions within listeria Actin-rich structures and lamellipodia: A localization and pharmacological inhibition study

The actin cytoskeleton forms much of the structure needed for the intracellular motility of an assortment of microbes as well as entire cells. The co-factor to the ubiquitin conjugating enzyme Ube2N (Ube2V1) has been implicated in both cancer cell metastasis and lysine-63 ubiquitylation of β actin. As this protein complexes with Ube2N, we sought to investigate whether Ube2N itself was involved in actin-based events occurring during the Listeria monocytogenes infections as well as within motile whole cells. Through examination of L. monocytogenes actin clouds, comet tails and membrane protrusions as well as lamellipodia in migrating cells, we show that Ube2N is recruited to actin-rich structures. When pharmacologically inhibited we demonstrate that Ube2N is crucial for the function of actin-rich structures when associated with the plasma membrane.

Rac-maninoff and Rho-vel: The symphony of Rho-GTPase signaling at excitatory synapses

Synaptic connections between neurons are essential for every facet of human cognition and are thus regulated with extreme precision. Rho-family GTPases, molecular switches that cycle between an active GTP-bound state and an inactive GDP-bound state, comprise a critical feature of synaptic regulation. Rho-GTPases are exquisitely controlled by an extensive suite of activators (GEFs) and inhibitors (GAPs and GDIs) and interact with many different signalling pathways to fulfill their roles in orchestrating the development, maintenance, and plasticity of excitatory synapses of the central nervous system. Among the mechanisms that control Rho-GTPase activity and signalling are cell surface receptors, GEF/GAP complexes that tightly regulate single Rho-GTPase dynamics, GEF/GAP and GEF/GEF functional complexes that coordinate multiple Rho-family GTPase activities, effector positive feedback loops, and mutual antagonism of opposing Rho-GTPase pathways. These complex regulatory mechanisms are employed by the cells of the nervous system in almost every step of development, and prominently figure into the processes of synaptic plasticity that underlie learning and memory. Finally, misregulation of Rho-GTPases plays critical roles in responses to neuronal injury, such as traumatic brain injury and neuropathic pain, and in neurodevelopmental and neurodegenerative disorders, including intellectual disability, autism spectrum disorder, schizophrenia, and Alzheimer's Disease. Thus, decoding the mechanisms of Rho-GTPase regulation and function at excitatory synapses has great potential for combatting many of the biggest current challenges in mental health.

Identification of A functional region in Bombyx mori nucleopolyhedrovirus VP39 that is essential for nuclear actin polymerization

Nuclear actin polymerization plays an indispensable role in the nuclear assembly of baculovirus nucleocapsid, but the underlying viral infection-mediated mechanism remains unclear. VP39 is the major protein in baculovirus capsid, which builds the skeleton of the capsid tubular structure. VP39 is suggested in previous studies to interact with cellular actin and mediate actin polymerization. However, it is unclear about the role of VP39 in mediating nuclear actin polymerization. Results in this study indicated that vp39 deletion abolished nuclear actin polymerization, which was recovered after vp39 repair, revealing the essential part of VP39 in nuclear actin polymerization. Furthermore, a series of mutants with vp39 deletions were constructed to analyze the important region responsible for nuclear actin polymerization. In addition, intracellular localization analysis demonstrated that the amino acids 192-286 in VP39 C-terminal are responsible for nuclear actin polymerization.

Mimicry of Short Linear Motifs by Bacterial Pathogens: A Drugging Opportunity

Bacterial pathogens have developed complex strategies to successfully survive and proliferate within their hosts. Throughout the infection cycle, direct interaction with host cells occurs. Many bacteria have been found to secrete proteins, such as effectors and toxins, directly into the host cell with the potential to interfere with cell regulatory processes, either enzymatically or through protein-protein interactions (PPIs). Short linear motifs (SLiMs) are abundant peptide modules in cell signaling proteins. Here, we cover the reported examples of eukaryotic-like SLiM mimicry being used by pathogenic bacteria to hijack host cell machinery and discuss how drugs targeting SLiM-regulated cell signaling networks are being evaluated for interference with bacterial infections. This emerging anti-infective opportunity may become an essential contributor to antibiotic replacement strategies.

Cell Motility and Cytokinesis: From Mysteries to Molecular Mechanisms in Five Decades

This is the story of someone who has been fortunate to work in a field of research where essentially nothing was known at the outset but that blossomed with the discovery of profound insights about two basic biological processes: cell motility and cytokinesis. The field started with no molecules, just a few people, and primitive methods. Over time, technological advances in biophysics, biochemistry, and microscopy allowed the combined efforts of scientists in hundreds of laboratories to explain mysterious processes with molecular mechanisms that can be embodied in mathematical equations and simulated by computers. The success of this field is a tribute to the power of the reductionist strategy for understanding biology.

Assembling nuclear domains: Lessons from DNA repair

Schrank and Gautier discuss the generation and function of nuclear domains during DNA repair with a special focus on nuclear actin polymerization.

Eukaryotic nuclei are organized into nuclear domains that unite loci sharing a common function. These domains are essential for diverse processes including (1) the formation of topologically associated domains (TADs) that coordinate replication and transcription, (2) the formation of specialized transcription and splicing factories, and (3) the clustering of DNA double-strand breaks (DSBs), which concentrates damaged DNA for repair. The generation of nuclear domains requires forces that are beginning to be identified. In the case of DNA DSBs, DNA movement and clustering are driven by actin filament nucleators. Furthermore, RNAs and low-complexity protein domains such as RNA-binding proteins also accumulate around sites of transcription and repair. The link between liquid–liquid phase separation and actin nucleation in the formation of nuclear domains is still unknown. This review discusses DSB repair domain formation as a model for functional nuclear domains in other genomic contexts.

The role of WASp in T cells and B cells

Wiskott-Aldrich syndrome (WAS) is a form of primary immunodeficiency (PIDs) resulting from mutations of the gene that encodes Wiskott-Aldrich syndrome protein (WASp). WASp is the first identified and most widely studied protein belonging to the actin nucleation-promoting factor family and plays significant role in integrating and transforming signals from critical receptors on the cell surface to actin remodeling. WASp functions in immune defense and homeostasis through the regulation of actin cytoskeleton-dependent cellular processes as well as processes uncoupled with actin polymerization like nuclear transcription programs. In this article, we review the mechanisms of WASp activation through an understanding of its structure. We further discuss the role of WASp in adaptive immunity, paying special attention to some recent findings on the crucial role of WASp in the formation of immunological synapse, the regulation of T follicular helper (Tfh) cells and in the prevention of autoimmunity.

A fruitful tree: developing the dendritic nucleation model of actin-based cell motility

A fundamental question in cell biology concerns how cells move, and this has been the subject of intense research for decades. In the 1990s, a major leap forward was made in our understanding of cell motility, with the proposal of the dendritic nucleation model. This essay describes the events leading to the development of the model, including findings from many laboratories and scientific disciplines. The story is an excellent example of the scientific process in action, with the combination of multiple perspectives leading to robust conclusions.

Dissecting cellular mechanics: Implications for aging, cancer, and immunity

Cells are dynamic structures that must respond to complex physical and chemical signals from their surrounding environment. The cytoskeleton is a key mediator of a cell's response to the signals of both the extracellular matrix and other cells present in the local microenvironment and allows it to tune its own mechanical properties in response to these cues. A growing body of evidence suggests that altered cellular viscoelasticity is a strong indicator of disease state; including cancer, laminopathy (genetic disorders of the nuclear lamina), infection, and aging. Here, we review recent work on the characterization of cell mechanics in disease and discuss the implications of altered viscoelasticity in regulation of immune responses. Finally, we provide an overview of techniques for measuring the mechanical properties of cells deeply embedded within tissues.

Phosphorylation of Arp2 is not essential for Arp2/3 complex activity in fission yeast

LeClaire et al presented evidence that phosphorylation of three sites on the Arp2 subunit activates the Arp2/3 complex to nucleate actin filaments. We mutated the homologous residues of Arp2 (Y198, T233, and T234) in the fission yeast genome to amino acids that preclude or mimic phosphorylation. Arp2/3 complex is essential for the viability of fission yeast, yet strains unable to phosphorylate these sites grew normally. Y198F/T233A/T234A Arp2 was only nonfunctional if GFP-tagged, as observed by LeClaire et al in Drosophila cells. Replacing both T233 and T234 with aspartic acid was lethal, suggesting that phosphorylation might be inhibitory. Nevertheless, blocking phosphorylation at these sites had the same effect as mimicking it: slowing assembly of endocytic actin patches. Mass spectrometry revealed phosphorylation at a fourth conserved Arp2 residue, Y218, but both blocking and mimicking phosphorylation of Y218 only slowed actin patch assembly slightly. Therefore, phosphorylation of Y198, T233, T234, and Y218 is not required for the activity of fission yeast Arp2/3 complex.

Conformational changes in Arp2/3 complex induced by ATP, WASp-VCA, and actin filaments

We used fluorescence spectroscopy and EM to determine how binding of ATP, nucleation-promoting factors, actin monomers, and actin filaments changes the conformation of Arp2/3 complex during the process that nucleates an actin filament branch. We mutated subunits of Schizosaccharomyces pombe Arp2/3 complex for labeling with fluorescent dyes at either the C termini of Arp2 and Arp3 or ArpC1 and ArpC3. We measured Förster resonance energy transfer (FRET) efficiency (ET_eff) between the dyes in the presence of the various ligands. We also computed class averages from electron micrographs of negatively stained specimens. ATP binding made small conformational changes of the nucleotide-binding cleft of the Arp2 subunit. WASp-VCA, WASp-CA, and WASp-actin-VCA changed the ET_eff between the dyes on the Arp2 and Arp3 subunits much more than between dyes on ArpC1 and ArpC3. Ensemble FRET detected an additional structural change that brought ArpC1 and ArpC3 closer together when Arp2/3 complex bound actin filaments. VCA binding to Arp2/3 complex causes a conformational change that favors binding to the side of an actin filament, which allows further changes required to nucleate a daughter filament.

Phospho-regulation of intrinsically disordered proteins for actin assembly and endocytosis

Actin filament assembly contributes to the endocytic pathway pleiotropically, with active roles in clathrin-dependent and clathrin-independent endocytosis as well as subsequent endosomal trafficking. Endocytosis comprises a series of dynamic events, including the initiation of membrane curvature, bud invagination, vesicle abscission and subsequent vesicular transport. The ultimate success of endocytosis requires the coordinated activities of proteins that trigger actin polymerization, recruit actin-binding proteins (ABPs) and organize endocytic proteins (EPs) that promote membrane curvature through molecular crowding or scaffolding mechanisms. A particularly interesting phenomenon is that multiple EPs and ABPs contain a substantial percentage of intrinsically disordered regions (IDRs), which can contribute to protein coacervation and phase separation. In addition, intrinsically disordered proteins (IDPs) frequently contain sites for post-translational modifications (PTMs) such as phosphorylation, and these modifications exhibit a high preference for IDR residues [Groban ES et al. (2006) PLoS Comput Biol 2, e32]. PTMs are implicated in regulating protein function by modulating the protein conformation, protein-protein interactions and the transition between order and disorder states of IDPs. The molecular mechanisms by which IDRs of ABPs and EPs fine-tune actin assembly and endocytosis remain mostly unexplored and elusive. In this review, we analyze protein sequences of budding yeast EPs and ABPs, and discuss the potential underlying mechanisms for regulating endocytosis and actin assembly through the emerging concept of IDR-mediated protein multivalency, coacervation, and phase transition, with an emphasis on the phospho-regulation of IDRs. Finally, we summarize the current understanding of how these mechanisms coordinate actin cytoskeleton assembly and membrane curvature formation during endocytosis in budding yeast.

The Arp2/3 complex binding protein HS1 is required for efficient dendritic cell random migration and force generation

Dendritic cell migration to the T-cell-rich areas of the lymph node is essential for their ability to initiate the adaptive immune response. While it has been shown that the actin cytoskeleton is required for normal DC migration, the role of many of the individual cytoskeletal molecules is poorly understood. In this study, we investigated the contribution of the Arp2/3 complex binding protein, haematopoietic lineage cell-specific protein 1 (HS1), to DC migration and force generation. We quantified the random migration of HS1^-/- DCs on 2D micro-contact printed surfaces and found that in the absence of HS1, DCs have greatly reduced motility and speed. This same reduction in motility was recapitulated when adding Arp2/3 complex inhibitor to WT DCs or using DCs deficient in WASP, an activator of Arp2/3 complex-dependent actin polymerization. We further investigated the importance of HS1 by measuring the traction forces of HS1^-/- DCs on micropost array detectors (mPADs). In HS1 deficient DCs, there was a significant reduction in force generation (3.96 ± 0.40 nN per cell) compared to WT DCs (13.76 ± 0.84 nN per cell). Interestingly, the forces generated in DCs lacking WASP were only slightly reduced compared to WT DCs. Taken together, these findings show that HS1 and Arp2/3 complex-mediated actin polymerization are essential for the most efficient DC random migration and force generation.

Glycerol monolaurate induces filopodia formation by disrupting the association between LAT and SLP-76 microclusters

Glycerol monolaurate (GML) is a monoglyceride with potent antimicrobial properties that suppresses T cell receptor (TCR)-induced signaling and T cell effector function. Actin rearrangement is needed for the interaction of T cells with antigen-presenting cells and for migration to sites of infection. Because of the critical role actin rearrangement plays in T cell effector function, we analyzed the effect of GML on the rearrangement of the actin cytoskeleton after TCR activation. We found that GML-treated human T cells were less adherent than untreated T cells and did not form actin ring structures but instead developed numerous inappropriate actin-mediated filopodia. The formation of these filopodia was not due to disruption of TCR-proximal regulators of actin or microtubule polymerization. Instead, total internal reflection fluorescence microscopy demonstrated mislocalization of actin nucleation protein Arp2 microclusters, but not those containing the adaptor proteins SLP-76 and WASp, or the actin nucleation protein ARPC3, which are necessary for TCR-induced actin rearrangement. Additionally, SLP-76 microclusters colocalized with WASp and WAVE microclusters but not with LAT. Together, our data suggest that GML alters actin cytoskeletal rearrangements and identify diverse functions for GML as a T cell-suppressive agent.

The Arp2/3 Regulatory System and Its Deregulation in Cancer

The Arp2/3 complex is an evolutionary conserved molecular machine that generates branched actin networks. When activated, the Arp2/3 complex contributes the actin branched junction and thus cross-links the polymerizing actin filaments in a network that exerts a pushing force. The different activators initiate branched actin networks at the cytosolic surface of different cellular membranes to promote their protrusion, movement, or scission in cell migration and membrane traffic. Here we review the structure, function, and regulation of all the direct regulators of the Arp2/3 complex that induce or inhibit the initiation of a branched actin network and that controls the stability of its branched junctions. Our goal is to present recent findings concerning novel inhibitory proteins or the regulation of the actin branched junction and place these in the context of what was previously known to provide a global overview of how the Arp2/3 complex is regulated in human cells. We focus on the human set of Arp2/3 regulators to compare normal Arp2/3 regulation in untransformed cells to the deregulation of the Arp2/3 system observed in patients affected by various cancers. In many cases, these deregulations promote cancer progression and have a direct impact on patient survival.

Whole Wiskott‑Aldrich syndrome protein gene deletion identified by high throughput sequencing

Wiskott‑Aldrich syndrome (WAS) is a rare X‑linked recessive immunodeficiency disorder, characterized by thrombocytopenia, small platelets, eczema and recurrent infections associated with increased risk of autoimmunity and malignancy disorders. Mutations in the WAS protein (WASP) gene are responsible for WAS. To date, WASP mutations, including missense/nonsense, splicing, small deletions, small insertions, gross deletions, and gross insertions have been identified in patients with WAS. In addition, WASP‑interacting proteins are suspected in patients with clinical features of WAS, in whom the WASP gene sequence and mRNA levels are normal. The present study aimed to investigate the application of next generation sequencing in definitive diagnosis and clinical therapy for WAS. A 5 month‑old child with WAS who displayed symptoms of thrombocytopenia was examined. Whole exome sequence analysis of genomic DNA showed that the coverage and depth of WASP were extremely low. Quantitative polymerase chain reaction indicated total WASP gene deletion in the proband. In conclusion, high throughput sequencing is useful for the verification of WAS on the genetic profile, and has implications for family planning guidance and establishment of clinical programs.

Myofibril contraction and crosslinking drive nuclear movement to the periphery of skeletal muscle

Nuclear movements are important for multiple cellular functions, and are driven by polarized forces generated by motor proteins and the cytoskeleton. During skeletal myofibre formation or regeneration, nuclei move from the centre to the periphery of the myofibre for proper muscle function. Centrally located nuclei are also found in different muscle disorders. Using theoretical and experimental approaches, we demonstrate that nuclear movement to the periphery of myofibres is mediated by centripetal forces around the nucleus. These forces arise from myofibril contraction and crosslinking that 'zip' around the nucleus in combination with tight regulation of nuclear stiffness by lamin A/C. In addition, an Arp2/3 complex containing Arpc5L together with γ-actin is required to organize desmin to crosslink myofibrils for nuclear movement. Our work reveals that centripetal forces exerted by myofibrils squeeze the nucleus to the periphery of myofibres.

CRMP-1 enhances EVL-mediated actin elongation to build lamellipodia and the actin cortex

CRMP proteins regulate the cytoskeleton, but the underlying mechanisms are poorly understood. Yu-Kemp et al. show that CRMP-1 helps Ena/VASP proteins elongate actin filaments to assemble actin networks that are necessary for the integrity of epithelial sheets.

Cells can control actin polymerization by nucleating new filaments or elongating existing ones. We recently identified CRMP-1 as a factor that stimulates the formation of Listeria monocytogenes actin comet tails, thereby implicating it in actin assembly. We now show that CRMP-1 is a major contributor to actin assembly in epithelial cells, where it works with the Ena/VASP family member EVL to assemble the actin cytoskeleton in the apical cortex and in protruding lamellipodia. CRMP-1 and EVL bind to one another and together accelerate actin filament barbed-end elongation. CRMP-1 also stimulates actin assembly in the presence of VASP and Mena in vitro, but CRMP-1–dependent actin assembly in MDCK cells is EVL specific. Our results identify CRMP-1 as a novel regulator of actin filament elongation and reveal a surprisingly important role for CRMP-1, EVL, and actin polymerization in maintaining the structural integrity of epithelial sheets.

Role of Rac1/WAVE2 Signaling in Mediating the Inhibitory Effects of γ-Tocotrienol on Mammary Cancer Cell Migration and Invasion

The majority of breast cancer deaths result from the progression of this disease to a metastatic phenotype. Rac1 and Cdc42 are Rho family members that together with their downstream effectors, Wiskott-Aldrich Syndrome protein-family verprolin-homologous protein 2 (WAVE2) and Arp2/3, play an important role in cytoskeletal reorganization and the formation of membrane protrusions that promote cancer cell migration and invasion. γ-Tocotrienol, is a natural isoform within the vitamin E family of compounds that inhibits breast cancer cell growth and progression by suppressing various signaling pathways involved in mitogenic signaling and metastatic progression. Studies were conducted to examine the effects of γ-tocotrienol on Rac1/WAVE2 signaling dependent migration and invasion in highly metastatic mouse +SA and human MDA-MB-231 mammary cancer cells. Exposure to γ-tocotrienol resulted in a dose-responsive decrease in Rac1/WAVE2 signaling as characterized by a suppression in the levels of Rac1/Cdc42, phospho-Rac1/Cdc42, WAVE2, Arp2, and Arp3 expression. Additional studies also demonstrated that similar treatment with γ-tocotrienol resulted in a significant reduction in tumor cell migration and invasion. Taken together, these findings indicate that γ-tocotrienol treatment effectively inhibits Rac1/WAVE2 signaling and reduces metastatic phenotypic expression in mammary cancer cells, suggesting that γ-tocotrienol may provide some benefit as a novel therapeutic approach in the treatment of metastatic breast cancer.

Local Arp2/3-dependent actin assembly modulates applied traction force during apCAM adhesion site maturation

In growth cones, local Arp 2/3-dependent actin assembly mechanically buffers apCAM adhesions from retrograde flow–associated traction forces. The resulting propulsive forces drive the exploratory motility of inductopodia. Increasing the stiffness of apCAM targets induces an extensive 3D actin cup to form at the adhesion during evoked growth responses.

Homophilic binding of immunoglobulin superfamily molecules such as the Aplysia cell adhesion molecule (apCAM) leads to actin filament assembly near nascent adhesion sites. Such actin assembly can generate significant localized forces that have not been characterized in the larger context of axon growth and guidance. We used apCAM-coated bead substrates applied to the surface of neuronal growth cones to characterize the development of forces evoked by varying stiffness of mechanical restraint. Unrestrained bead propulsion matched or exceeded rates of retrograde network flow and was dependent on Arp2/3 complex activity. Analysis of growth cone forces applied to beads at low stiffness of restraint revealed switching between two states: frictional coupling to retrograde flow and Arp2/3-dependent propulsion. Stiff mechanical restraint led to formation of an extensive actin cup matching the geometric profile of the bead target and forward growth cone translocation; pharmacological inhibition of the Arp2/3 complex or Rac attenuated F-actin assembly near bead binding sites, decreased the efficacy of growth responses, and blocked accumulation of signaling molecules associated with nascent adhesions. These studies introduce a new model for regulation of traction force in which local actin assembly forces buffer nascent adhesion sites from the mechanical effects of retrograde flow.

The Diverse Family of Arp2/3 Complexes

The Arp2/3 complex has so far been considered to be a single seven-subunit protein complex required for actin nucleation and actin filament polymerization in diverse critical cellular functions including phagocytosis, vesicular trafficking and lamellipodia extension. The Arp2/3 complex is also exploited by bacterial pathogens and viruses during cellular infectious processes. Recent studies suggest that some subunits of the complex are dispensable in specific cellular contexts, pointing to the existence of alternative 'hybrid Arp2/3 complexes' containing other components such as vinculin or α-actinin, as well as different isoforms or phosphorylation variants of canonical Arp2/3 subunits. Therefore, this diversity should be now considered when assigning specific Arp2/3 assemblies to different actin-dependent cellular processes.

Reconstituting the actin cytoskeleton at or near surfaces in vitro

Actin filament dynamics have been studied for decades in pure protein solutions or in cell extracts, but a breakthrough in the field occurred at the turn of the century when it became possible to reconstitute networks of actin filaments, growing in a controlled but physiological manner on surfaces, mimicking the actin assembly that occurs at the plasma membrane during cell protrusion and cell shape changes. The story begins with the bacteria Listeria monocytogenes, the study of which led to the reconstitution of cellular actin polymerization on a variety of supports including plastic beads. These studies made possible the development of liposome-type substrates for filament assembly and micropatterning of actin polymerization nucleation. Based on the accumulated expertise of the last 15 years, many exciting approaches are being developed, including the addition of myosin to biomimetic actin networks to study the interplay between actin structure and contractility. The field is now poised to make artificial cells with a physiological and dynamic actin cytoskeleton, and subsequently to put these cells together to make in vitro tissues. This article is part of a Special Issue entitled: Mechanobiology.

Role of the WASP and WAVE family proteins in breast cancer invasion and metastasis

The Wiskott–Aldrich syndrome protein (WASP) and WASP family verprolin-homologous protein (WAVE) family are a group of molecules that form a key link between GTPases and the actin cytoskeleton. The role of WASP/WAVE family proteins in the control of actin polymerization through activation of the actin-related protein 2/3 complex is critical in the formation of the actin-based membrane protrusions seen in cell migration and invasion. For this reason, the activity of the WASP/WAVE family in cancer cell invasion and migration has been of great interest in recent years. Many reports have highlighted the potential of targeting the WASP/WAVE family as a therapy for the prevention of cancer progression, in particular breast cancer. This review focuses on the role of the WASP/WAVE family in breast cancer cell invasion and migration and how this relates to the molecular mechanisms of WASP/WAVE activity, their exact contributions to the stages of cancer progression, and how this can lead to the development of anticancer drugs that target the WASP/WAVE family and related pathways.

Spontaneous polarization in an interfacial growth model for actin filament networks with a rigorous mechanochemical coupling

Many processes in eukaryotic cells, including cell motility, rely on the growth of branched actin networks from surfaces. Despite its central role the mechanochemical coupling mechanisms that guide the growth process are poorly understood, and a general continuum description combining growth and mechanics is lacking. We develop a theory that bridges the gap between mesoscale and continuum limit and propose a general framework providing the evolution law of actin networks growing under stress. This formulation opens an area for the systematic study of actin dynamics in arbitrary geometries. Our framework predicts a morphological instability of actin growth on a rigid sphere, leading to a spontaneous polarization of the network with a mode selection corresponding to a comet, as reported experimentally. We show that the mechanics of the contact between the network and the surface plays a crucial role, in that it determines directly the existence of the instability. We extract scaling laws relating growth dynamics and network properties offering basic perspectives for new experiments on growing actin networks.

Arp2/3 complex function in the epidermis

An exciting frontier in biology is understanding the functions of basic cell biological machinery in complex tissues. This approach is expected to uncover novel modes of regulation as well as reveal how core machinery is repurposed by different tissues to accomplish different physiological outputs. F-actin plays roles in cell shape, adhesion, migration and signaling – diverse functions that require a specific organization established by a myriad of regulators. Here, we discuss the role of the actin nucleating Arp2/3 complex and the unexpected roles that it plays in a stratified epithelial tissue, the epidermis. While many expected phenotypes such as defects in architecture and cell adhesion were lacking, loss of the Arp2/3 complex activity resulted in epidermal barrier and differentiation defects. This teaches us that, while informative, cell culture approaches are limiting and that studies of the Arp2/3 complex in diverse tissues are expected to yield many more surprises.

Control of actin-based motility through localized actin binding

A wide variety of cell biological and biomimetic systems use actin polymerization to drive motility. It has been suggested that an object such as a bacterium can propel itself by self-assembling a high concentration of actin behind it, if it is repelled by actin. However, it is also known that it is essential for the moving object to bind actin. Therefore, a key question is how the actin tail can propel an object when it both binds and repels the object. We present a physically consistent Brownian dynamics model for actin-based motility that includes the minimal components of the dendritic nucleation model and allows for both attractive and repulsive interactions between actin and a moveable disc. We find that the concentration gradient of filamentous actin generated by polymerization is sufficient to propel the object, even with moderately strong binding interactions. Additionally, actin binding can act as a biophysical cap, and may directly control motility through modulation of network growth. Overall, this mechanism is robust in that it can drive motility against a load up to a stall pressure that depends on the Young's modulus of the actin network and can explain several aspects of actin-based motility.

Cell-cycle regulation of formin-mediated actin cable assembly

Assembly of appropriately oriented actin cables nucleated by formin proteins is necessary for many biological processes in diverse eukaryotes. However, compared with knowledge of how nucleation of dendritic actin filament arrays by the actin-related protein-2/3 complex is regulated, the in vivo regulatory mechanisms for actin cable formation are less clear. To gain insights into mechanisms for regulating actin cable assembly, we reconstituted the assembly process in vitro by introducing microspheres functionalized with the C terminus of the budding yeast formin Bni1 into extracts prepared from yeast cells at different cell-cycle stages. EM studies showed that unbranched actin filament bundles were reconstituted successfully in the yeast extracts. Only extracts enriched in the mitotic cyclin Clb2 were competent for actin cable assembly, and cyclin-dependent kinase 1 activity was indispensible. Cyclin-dependent kinase 1 activity also was found to regulate cable assembly in vivo. Here we present evidence that formin cell-cycle regulation is conserved in vertebrates. The use of the cable-reconstitution system to test roles for the key actin-binding proteins tropomyosin, capping protein, and cofilin provided important insights into assembly regulation. Furthermore, using mass spectrometry, we identified components of the actin cables formed in yeast extracts, providing the basis for comprehensive understanding of cable assembly and regulation.

Arp2/3-mediated actin-based motility: a tail of pathogen abuse

Intracellular pathogens have developed elaborate mechanisms to exploit the different cellular systems of their unwilling hosts to facilitate their entry, replication, and survival. In particular, a diverse range of bacteria and viruses have evolved unique strategies to harness the power of Arp2/3-mediated actin polymerization to enhance their cell-to-cell spread. In this review, we discuss how studying these pathogens has revolutionized our molecular understanding of Arp2/3-dependent actin assembly and revealed key signaling pathways regulating actin assembly in cells. Future analyses of microbe-host interactions are likely to continue uncovering new mechanisms regulating actin assembly and dynamics, as well as unexpected cellular functions for actin. Further, studies with known and newly emerging pathogens will also undoubtedly continue to enhance our understanding of the role of the actin cytoskeleton during pathogenesis and potentially highlight future therapeutic approaches.

Molecular mechanisms of cell-cell spread of intracellular bacterial pathogens

Several bacterial pathogens, including Listeria monocytogenes, Shigella flexneri and Rickettsia spp., have evolved mechanisms to actively spread within human tissues. Spreading is initiated by the pathogen-induced recruitment of host filamentous (F)-actin. F-actin forms a tail behind the microbe, propelling it through the cytoplasm. The motile pathogen then encounters the host plasma membrane, forming a bacterium-containing protrusion that is engulfed by an adjacent cell. Over the past two decades, much progress has been made in elucidating mechanisms of F-actin tail formation. Listeria and Shigella produce tails of branched actin filaments by subverting the host Arp2/3 complex. By contrast, Rickettsia forms tails with linear actin filaments through a bacterial mimic of eukaryotic formins. Compared with F-actin tail formation, mechanisms controlling bacterial protrusions are less well understood. However, recent findings have highlighted the importance of pathogen manipulation of host cell–cell junctions in spread. Listeria produces a soluble protein that enhances bacterial protrusions by perturbing tight junctions. Shigella protrusions are engulfed through a clathrin-mediated pathway at ‘tricellular junctions’—specialized membrane regions at the intersection of three epithelial cells. This review summarizes key past findings in pathogen spread, and focuses on recent developments in actin-based motility and the formation and internalization of bacterial protrusions.

Identification of Shigella flexneri IcsA residues affecting interaction with N-WASP, and evidence for IcsA-IcsA co-operative interaction

The Shigella flexneri IcsA (VirG) protein is a polarly distributed outer membrane protein that is a fundamental virulence factor which interacts with neural Wiskott-Aldrich syndrome protein (N-WASP). The activated N-WASP then activates the Arp2/3 complex which initiates de novo actin nucleation and polymerisation to form F-actin comet tails and allows bacterial cell-to-cell spreading. In a previous study, IcsA was found to have three N-WASP interacting regions (IRs): IR I (aa 185-312), IR II (aa 330-382) and IR III (aa 508-730). The aim of this study was to more clearly define N-WASP interacting regions II and III by site-directed mutagenesis of specific amino acids. Mutant IcsA proteins were expressed in both smooth lipopolysaccharide (S-LPS) and rough LPS (R-LPS) S. flexneri strains and characterised for IcsA production level, N-WASP recruitment and F-actin comet tail formation. We have successfully identified new amino acids involved in N-WASP recruitment within different N-WASP interacting regions, and report for the first time using co-expression of mutant IcsA proteins, that N-WASP activation involves interactions with different regions on different IcsA molecules as shown by Arp3 recruitment. In addition, our findings suggest that autochaperone (AC) mutant protein production was not rescued by another AC region provided in trans, differing to that reported for two other autotransporters, PrtS and BrkA autotransporters.

ARP2/3-dependent growth in the plant kingdom: SCARs for life

In the human experience SCARs (suppressor of cAMP receptors) are permanent reminders of past events, not always based on bad decisions, but always those in which an interplay of opposing forces leaves behind a clear record in the form of some permanent watery mark. During plant morphogenesis, SCARs are important proteins that reflect an unusual evolutionary outcome, in which the plant kingdom relies heavily on this single class of actin-related protein (ARP) 2/3 complex activator to dictate the time and place of actin filament nucleation. This unusually simple arrangement may serve as a permanent reminder that cell shape control in plants is fundamentally different from that of crawling cells in mammals that use the power of actin polymerization to define and maintain cell shape. In plant cells, actin filaments indirectly affect cell shape by determining the transport properties of organelles and cargo molecules that modulate the mechanical properties of the wall. It is becoming increasingly clear that polarized bundles of actin filaments operate at whole cell spatial scales to organize the cytoplasm and dictate the patterns of long-distance intracellular transport and secretion. The number of actin-binding proteins and actin filament nucleators that are known to participate in the process of actin network formation are rapidly increasing. In plants, formins and ARP2/3 are two important actin filament nucleators. This review will focus on ARP2/3, and the apparent reliance of most plant species on the SCAR/WAVE (WASP family verprolin homologous) regulatory complex as the sole pathway for ARP2/3 activation.

Emergent complexity of the cytoskeleton: from single filaments to tissue

Despite their overwhelming complexity, living cells display a high degree of internal mechanical and functional organization which can largely be attributed to the intracellular biopolymer scaffold, the cytoskeleton. Being a very complex system far from thermodynamic equilibrium, the cytoskeleton's ability to organize is at the same time challenging and fascinating. The extensive amounts of frequently interacting cellular building blocks and their inherent multifunctionality permits highly adaptive behavior and obstructs a purely reductionist approach. Nevertheless (and despite the field's relative novelty), the physics approach has already proved to be extremely successful in revealing very fundamental concepts of cytoskeleton organization and behavior. This review aims at introducing the physics of the cytoskeleton ranging from single biopolymer filaments to multicellular organisms. Throughout this wide range of phenomena, the focus is set on the intertwined nature of the different physical scales (levels of complexity) that give rise to numerous emergent properties by means of self-organization or self-assembly.

Directed cytoskeleton self-organization

The cytoskeleton architecture supports many cellular functions. Cytoskeleton networks form complex intracellular structures that vary during the cell cycle and between different cell types according to their physiological role. These structures do not emerge spontaneously. They result from the interplay between intrinsic self-organization properties and the conditions imposed by spatial boundaries. Along these boundaries, cytoskeleton filaments are anchored, repulsed, aligned, or reoriented. Such local effects can propagate alterations throughout the network and guide cytoskeleton assembly over relatively large distances. The experimental manipulation of spatial boundaries using microfabrication methods has revealed the underlying physical processes directing cytoskeleton self-organization. Here we review, step-by-step, from molecules to tissues, how the rules that govern assembly have been identified. We describe how complementary approaches, all based on controlling geometric conditions, from in vitro reconstruction to in vivo observation, shed new light on these fundamental organizing principles.

Excess F-actin mechanically impedes mitosis leading to cytokinesis failure in X-linked neutropenia by exceeding Aurora B kinase error correction capacity

The constitutively active mutant of the Wiskott-Aldrich Syndrome protein (CA-WASp) is the cause of X-linked neutropenia and is linked with genomic instability and myelodysplasia. CA-WASp generates abnormally high levels of cytoplasmic F-actin through dysregulated activation of the Arp2/3 complex leading to defects in cell division. As WASp has no reported role in cell division, we hypothesized that alteration of cell mechanics because of increased F-actin may indirectly disrupt dynamic events during mitosis. Inhibition of the Arp2/3 complex revealed that excess cytoplasmic F-actin caused increased cellular viscosity, slowed all phases of mitosis, and perturbed mitotic mechanics. Comparison of chromosome velocity to the cytoplasmic viscosity revealed that cells compensated for increased viscosity by up-regulating force applied to chromosomes and increased the density of microtubules at kinetochores. Mitotic abnormalities were because of overload of the aurora signaling pathway as subcritical inhibition of Aurora in CA-WASp cells caused increased cytokinesis failure, while overexpression reduced defects. These findings demonstrate that changes in cell mechanics can cause significant mitotic abnormalities leading to genomic instability, and highlight the importance of mechanical sensors such as Aurora B in maintaining the fidelity of hematopoietic cell division.

Self-association of the Shigella flexneri IcsA autotransporter protein

The IcsA autotransporter protein is a major virulence factor of the human intracellular pathogen Shigella flexneri. IcsA is distributed at the poles in the outer membrane (OM) of S. flexneri and interacts with components of the host actin-polymerization machinery to facilitate intracellular actin-based motility and subsequent cell-to-cell spreading of the bacterium. We sought to characterize the biochemical properties of IcsA in the bacterial OM. Chemical cross-linking data suggested that IcsA exists in a complex in the OM. Furthermore, reciprocal co-immunoprecipitation of differentially epitope-tagged IcsA proteins indicated that IcsA is able to self-associate. The identification of IcsA linker-insertion mutants that were negatively dominant provided genetic evidence of IcsA-IcsA interactions. From these results, we propose a model whereby IcsA self-association facilitates efficient actin-based motility.

The Arp2/3 complex is required for lamellipodia extension and directional fibroblast cell migration

Embryonic stem cell–derived fibroblasts with genetic disruption of the Arp2/3 complex are unable to form lamellipodia or undergo sustained directional migration.

The Arp2/3 complex nucleates the formation of the dendritic actin network at the leading edge of motile cells, but it is still unclear if the Arp2/3 complex plays a critical role in lamellipodia protrusion and cell motility. Here, we differentiated motile fibroblast cells from isogenic mouse embryonic stem cells with or without disruption of the ARPC3 gene, which encodes the p21 subunit of the Arp2/3 complex. ARPC3^−/− fibroblasts were unable to extend lamellipodia but generated dynamic leading edges composed primarily of filopodia-like protrusions, with formin proteins (mDia1 and mDia2) concentrated near their tips. The speed of cell migration, as well as the rates of leading edge protrusion and retraction, were comparable between genotypes; however, ARPC3^−/− cells exhibited a strong defect in persistent directional migration. This deficiency correlated with a lack of coordination of the protrusive activities at the leading edge of ARPC3^−/− fibroblasts. These results provide insights into the Arp2/3 complex’s critical role in lamellipodia extension and directional fibroblast migration.

Phosphorylation of the Arp2 subunit relieves auto-inhibitory interactions for Arp2/3 complex activation

Actin filament assembly by the actin-related protein (Arp) 2/3 complex is necessary to build many cellular structures, including lamellipodia at the leading edge of motile cells and phagocytic cups, and to move endosomes and intracellular pathogens. The crucial role of the Arp2/3 complex in cellular processes requires precise spatiotemporal regulation of its activity. While binding of nucleation-promoting factors (NPFs) has long been considered essential to Arp2/3 complex activity, we recently showed that phosphorylation of the Arp2 subunit is also necessary for Arp2/3 complex activation. Using molecular dynamics simulations and biochemical assays with recombinant Arp2/3 complex, we now show how phosphorylation of Arp2 induces conformational changes permitting activation. The simulations suggest that phosphorylation causes reorientation of Arp2 relative to Arp3 by destabilizing a network of salt-bridge interactions at the interface of the Arp2, Arp3, and ARPC4 subunits. Simulations also suggest a gain-of-function ARPC4 mutant that we show experimentally to have substantial activity in the absence of NPFs. We propose a model in which a network of auto-inhibitory salt-bridge interactions holds the Arp2 subunit in an inactive orientation. These auto-inhibitory interactions are destabilized upon phosphorylation of Arp2, allowing Arp2 to reorient to an activation-competent state.

The actin nucleation factor JMY is a negative regulator of neuritogenesis

Junction-mediating and regulatory protein (JMY) is a p53 cofactor that was recently shown to nucleate actin assembly by a hybrid mechanism involving tandem actin monomer binding and Arp2/3 complex activation. However, the regulation and function of JMY remain largely uncharacterized. We examined the activity of JMY in vitro and in cells, its subcellular distribution, and its function in fibroblast and neuronal cell lines. We demonstrated that recombinant full-length JMY and its isolated WASP homology 2 domain, connector, and acidic region (WWWCA) have potent actin-nucleating and Arp2/3-activating abilities in vitro. In contrast, the activity of full-length JMY, but not the isolated WWWCA domain, is suppressed in cells. The WWWCA domain is sufficient to promote actin-based bead motility in cytoplasmic extracts, and this activity depends on its ability to activate the Arp2/3 complex. JMY is expressed at high levels in brain tissue, and in various cell lines JMY is predominantly cytoplasmic, with a minor fraction in the nucleus. Of interest, silencing JMY expression in neuronal cells results in a significant enhancement of the ability of these cells to form neurites, suggesting that JMY functions to suppress neurite formation. This function of JMY requires its actin-nucleating activity. These findings highlight a previously unrecognized function for JMY as a modulator of neuritogenesis.

Molecular mechanisms, biological actions, and neuropharmacology of the growth-associated protein GAP-43

GAP-43 is an intracellular growth-associated protein that appears to assist neuronal pathfinding and branching during development and regeneration, and may contribute to presynaptic membrane changes in the adult, leading to the phenomena of neurotransmitter release, endocytosis and synaptic vesicle recycling, long-term potentiation, spatial memory formation, and learning. GAP-43 becomes bound via palmitoylation and the presence of three basic residues to membranes of the early secretory pathway. It is then sorted onto vesicles at the late secretory pathway for fast axonal transport to the growth cone or presynaptic plasma membrane. The palmitate chains do not serve as permanent membrane anchors for GAP-43, because at steady-state most of the GAP-43 in a cell is membrane-bound but is not palmitoylated. Filopodial extension and branching take place when GAP-43 is phosphorylated at Ser-41 by protein kinase C, and this occurs following neurotrophin binding and the activation of numerous small GTPases. GAP-43 has been proposed to cluster the acidic phospholipid phosphatidylinositol 4,5-bisphosphate in plasma membrane rafts. Following GAP-43 phosphorylation, this phospholipid is released to promote local actin filament-membrane attachment. The phosphorylation also releases GAP-43 from calmodulin. The released GAP-43 may then act as a lateral stabilizer of actin filaments. N-terminal fragments of GAP-43, containing 10-20 amino acids, will activate heterotrimeric G proteins, direct GAP-43 to the membrane and lipid rafts, and cause the formation of filopodia, possibly by causing a change in membrane tension. This review will focus on new information regarding GAP-43, including its binding to membranes and its incorporation into lipid rafts, its mechanism of action, and how it affects and is affected by extracellular agents.

Actin filament nucleation and elongation factors--structure-function relationships

The spontaneous and unregulated polymerization of actin filaments is inhibited in cells by actin monomer-binding proteins such as profilin and Tbeta4. Eukaryotic cells and certain pathogens use filament nucleators to stabilize actin polymerization nuclei, whose formation is rate-limiting. Known filament nucleators include the Arp2/3 complex and its large family of nucleation promoting factors (NPFs), formins, Spire, Cobl, VopL/VopF, TARP and Lmod. These molecules control the time and location for polymerization, and additionally influence the structures of the actin networks that they generate. Filament nucleators are generally unrelated, but with the exception of formins they all use the WASP-Homology 2 domain (WH2 or W), a small and versatile actin-binding motif, for interaction with actin. A common architecture, found in Spire, Cobl and VopL/VopF, consists of tandem W domains that bind three to four actin subunits to form a nucleus. Structural considerations suggest that NPFs-Arp2/3 complex can also be viewed as a specialized form of tandem W-based nucleator. Formins are unique in that they use the formin-homology 2 (FH2) domain for interaction with actin and promote not only nucleation, but also processive barbed end elongation. In contrast, the elongation function among W-based nucleators has been "outsourced" to a dedicated family of proteins, Eva/VASP, which are related to WASP-family NPFs.

Biochemical mechanisms for regulating protrusion by nematode major sperm protein

Crawling motion is ubiquitous in eukaryotic cells and contributes to important processes such as immune response and tumor growth. To crawl, a cell must adhere to the substrate, while protruding at the front and retracting at the rear. In most crawling cells protrusion is driven by highly regulated polymerization of the actin cytoskeleton, and much of the biochemical network for this process is known. Nematode sperm utilize a cytoskeleton composed of Major Sperm Protein (MSP), which is considered to form a simpler, yet similar, crawling motility apparatus. Key components involved in the polymerization of MSP have been identified; however, little is known about the chemical kinetics for this system. Here we develop a model for MSP polymerization that takes into account the effects of several of the experimentally identified cytosolic and membrane-bound proteins. To account for some of the data, the model requires force-dependent polymerization, as is predicted by Brownian ratchet mechanisms. Using the tethered polymerization ratchet model with our biochemical kinetic model for MSP polymerization, we find good agreement with experimental data on MSP-driven protrusion. In addition, our model predicts the force-velocity relation that is expected for in vitro protrusion assays.

Platelet-associated IgAs and impaired GPVI responses in platelets lacking WIP

The role of the Wiskott-Aldrich syndrome protein (WASp) in platelet function is unclear because platelets that lack WASp function normally. WASp constitutively associates with WASp-interacting protein (WIP) in resting and activated platelets. The role of WIP in platelet function was investigated using mice that lack WIP or WASp. WIP knockout (KO) platelets lack WASp and thus are double deficient. WIP KO mice have a thrombocytopenia, similar to WASp KO mice, resulting in part from enhanced platelet clearance. Most WIP KO, but not WASp KO, mice evolved platelet-associated immunoglobulins (Ig) of the IgA class, which normalize their platelet survival but diminish their glycoprotein VI (GPVI) responses. Protein tyrosine phosphorylation, including that of phospholipase C-gamma2, and calcium mobilization are impaired in IgA-presenting WIP KO platelets stimulated through GPVI, resulting in defects in alpha-granule secretion, integrin alphaIIbbeta3 activation, and actin assembly. The anti-GPVI antibody JAQ1 induces the irreversible loss of GPVI from circulating platelets in wild-type mice, but not in WIP KO mice that bear high levels of platelet-associated IgAs. Together, the data indicate that platelet-associated IgAs negatively modulate GPVI signaling and function in WIP KO mice.

Wiskott-Aldrich syndrome protein is an effector of Kit signaling

The pleiotropic receptor tyrosine kinase Kit can provide cytoskeletal signals that define cell shape, positioning, and migration, but the underlying mechanisms are less well understood. In this study, we provide evidence that Kit signals through Wiskott-Aldrich syndrome protein (WASP), the central hematopoietic actin nucleation-promoting factor and regulator of the cytoskeleton. Kit ligand (KL) stimulation resulted in transient tyrosine phosphorylation of WASP, as well as interacting proteins WASP-interacting protein and Arp2/3. KL-induced filopodia in bone marrow-derived mast cells (BMMCs) were significantly decreased in number and size in the absence of WASP. KL-dependent regulation of intracellular Ca(2+) levels was aberrant in WASP-deficient BMMCs. When BMMCs were derived from WASP-heterozygous female mice using KL as a growth factor, the cultures eventually developed from a mixture of WASP-positive and -negative populations into a homogenous WASP-positive culture derived from the WASP-positive progenitors. Thus, WASP expression conferred a selective advantage to the development of Kit-dependent hematopoiesis consistent with the selective advantage of WASP-positive hematopoietic cells observed in WAS-heterozygous female humans. Finally, KL-mediated gene expression in wild-type and WASP-deficient BMMCs was compared and revealed that approximately 30% of all Kit-induced changes were WASP dependent. The results indicate that Kit signaling through WASP is necessary for normal Kit-mediated filopodia formation, cell survival, and gene expression, and provide new insight into the mechanism in which WASP exerts a strong selective pressure in hematopoiesis.