The Wiskott-Aldrich syndrome: disordered actin dynamics in haematopoietic cells

Frontier and hotspot evolution in Wiskott-Aldrich syndrome: A bibliometric analysis from 2001 to 2021

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency disorder. Despite our enormous progress in the strategies used to diagnose, treat, and cure WAS, no bibliometric studies have been performed in this research field. This study explored the trends in WAS research through a bibliometric analysis evaluating relevant literature quantitatively and qualitatively.

METHODS

The literature concerning WAS from 2001 to 2021 was retrieved from the Science Citation Index Expanded (SCI-expanded) of the Web of Science Core Collection database. Acquired data were then visually analyzed using CiteSpace and VOSviewer.

RESULTS

2036 papers were included in the final analysis. The annual publication outputs reached its peak in 2013 but declined in recent years. The dominant position of the United States in WAS research was quite obvious. Harvard University (USA), University College London (UK), and Inserm (France) were the three most prolific institutions. Adrian J. Thrasher exerted significant publication impact and made the most notable contributions in the field of WAS. Blood was the most influential journal with the highest publication outputs, and nearly all the top 10 journals and co-cited journals belonged to Q1. Immune dysregulation, thrombocytopenia, syndrome protein deficiency, stem cell, mutation, and diagnosis were the keywords with the strongest citation burst.

CONCLUSION

From 2001 to 2021, the United States was a global leader in the WAS research. Collaboration between countries and institutions is expected to deepen and strengthen in the future. Research hotspots included pathogenesis, clinical manifestations, diagnosis, and therapy. Our results suggest a greater understanding of the mechanistic underpinnings of immune dysfunction in WAS patients, the application of targeted therapies for individual complications, and the development of curative approaches, which will remain research hotspots in the future.

Loss of Hem1 disrupts macrophage function and impacts migration, phagocytosis, and integrin-mediated adhesion

Hematopoietic-specific protein 1 (Hem1) is an essential subunit of the WAVE regulatory complex (WRC) in immune cells. WRC is crucial for Arp2/3 complex activation and the protrusion of branched actin filament networks. Moreover, Hem1 loss of function in immune cells causes autoimmune diseases in humans. Here, we show that genetic removal of Hem1 in macrophages diminishes frequency and efficacy of phagocytosis as well as phagocytic cup formation in addition to defects in lamellipodial protrusion and migration. Moreover, Hem1-null macrophages displayed strong defects in cell adhesion despite unaltered podosome formation and concomitant extracellular matrix degradation. Specifically, dynamics of both adhesion and de-adhesion as well as concomitant phosphorylation of paxillin and focal adhesion kinase (FAK) were significantly compromised. Accordingly, disruption of WRC function in non-hematopoietic cells coincided with both defects in adhesion turnover and altered FAK and paxillin phosphorylation. Consistently, platelets exhibited reduced adhesion and diminished integrin αIIbβ3 activation upon WRC removal. Interestingly, adhesion phenotypes, but not lamellipodia formation, were partially rescued by small molecule activation of FAK. A full rescue of the phenotype, including lamellipodia formation, required not only the presence of WRCs but also their binding to and activation by Rac. Collectively, our results uncover that WRC impacts on integrin-dependent processes in a FAK-dependent manner, controlling formation and dismantling of adhesions, relevant for properly grabbing onto extracellular surfaces and particles during cell edge expansion, like in migration or phagocytosis.

Oxidized LDL phagocytosis during foam cell formation in atherosclerotic plaques relies on a PLD2-CD36 functional interdependence

The uptake of cholesterol carried by low-density lipoprotein (LDL) is tightly controlled in the body. Macrophages are not well suited to counteract the cellular consequences of excess cholesterol leading to their transformation into "foam cells," an early step in vascular plaque formation. We have uncovered and characterized a novel mechanism involving phospholipase D (PLD) in foam cell formation. Utilizing bone marrow-derived macrophages from genetically PLD deficient mice, we demonstrate that PLD2 (but not PLD1)-null macrophages cannot fully phagocytose aggregated oxidized LDL (Agg-Ox-LDL), which was phenocopied with a PLD2-selective inhibitor. We also report a role for PLD2 in coupling Agg-oxLDL phagocytosis with WASP, Grb2, and Actin. Further, the clearance of LDL particles is mediated by both CD36 and PLD2, via mutual dependence on each other. In the absence of PLD2, CD36 does not engage in Agg-Ox-LDL removal and when CD36 is blocked, PLD2 cannot form protein-protein heterocomplexes with WASP or Actin. These results translated into humans using a GEO database of microarray expression data from atheroma plaques versus normal adjacent carotid tissue and observed higher values for NFkB, PLD2 (but not PLD1), WASP, and Grb2 in the atheroma plaques. Human atherectomy specimens confirmed high presence of PLD2 (mRNA and protein) as well as phospho-WASP in diseased arteries. Thus, PLD2 interacts in macrophages with Actin, Grb2, and WASP during phagocytosis of Agg-Ox-LDL in the presence of CD36 during their transformation into "foam cells." Thus, this study provides new molecular targets to counteract vascular plaque formation and atherogenesis.

Metalloproteinase MT1-MMP islets act as memory devices for podosome reemergence

The authors find that matrix metalloproteinase MT1-MMP is enriched at the plasma membrane of macrophage podosomes, where it persists beyond podosome lifetime and, through binding to the subcortical actin cytoskeleton, forms subcellular signposts that facilitate podosome reformation.

Podosomes are dynamic cell adhesions that are also sites of extracellular matrix degradation, through recruitment of matrix-lytic enzymes, particularly of matrix metalloproteinases. Using total internal reflection fluorescence microscopy, we show that the membrane-bound metalloproteinase MT1-MMP is enriched not only at podosomes but also at distinct “islets” embedded in the plasma membrane of primary human macrophages. MT1-MMP islets become apparent upon podosome dissolution and persist beyond podosome lifetime. Importantly, the majority of MT1-MMP islets are reused as sites of podosome reemergence. siRNA-mediated knockdown and recomplementation analyses show that islet formation is based on the cytoplasmic tail of MT1-MMP and its ability to bind the subcortical actin cytoskeleton. Collectively, our data reveal a previously unrecognized phase in the podosome life cycle and identify a structural function of MT1-MMP that is independent of its proteolytic activity. MT1-MMP islets thus act as cellular memory devices that enable efficient and localized reformation of podosomes, ensuring coordinated matrix degradation and invasion.

Tools of the trade: podosomes as multipurpose organelles of monocytic cells

Podosomes are adhesion and invasion structures that are particularly prominent in cells of the monocytic lineage such as macrophages, dendritic cells, and osteoclasts. They are multifunctional organelles that combine several key abilities required for cell migration and invasion. The podosome repertoire includes well-established functions such as cell-substrate adhesion, and extracellular matrix degradation, recently discovered abilities such as rigidity and topology sensing as well as antigen sampling, and also more speculative functions such as cell protrusion stabilization and transmigration. Collectively, podosomes not only enable dynamic interactions of cells with their surroundings, they also gather information about the pericellular environment, and are actively involved in its reshaping. This review presents an overview of the current knowledge on podosome composition, architecture, and regulation. We focus in particular on the growing list of podosome functions and discuss the specific properties of podosomes in macrophages, dendritic cells, and osteoclasts. Moreover, this article highlights podosome-related intracellular transport processes, the formation of podosomes in 3D environments as well as potentially podosome-associated diseases involving monocytic cells.

The central role of the cytoskeleton in mechanisms and functions of the NK cell immune synapse

Natural killer (NK) cells discriminate between healthy and unhealthy target cells through a balance of activating and inhibitory signals at direct intercellular contacts called immune synapses. Rearrangements in the cellular cytoskeleton have long been known to be critical in assembly of immune synapses. Here, through bringing together the vast literature on this subject, the number of different ways in which the cytoskeleton is important becomes evident. The dynamics of filamentous actin are critical in (i) creating the nanometer-scale organization of NK cell receptors, (ii) establishing cellular polarity, (iii) coordinating immune receptor and integrin-mediated signaling, and (iv) directing secretion of lytic granules and cytokines. The microtubule network also is important in the delivery of lytic granules and vesicles containing cytokines to the immune synapse. Together, these data establish that the cytoskeleton acts as a central regulator of this complex and dynamic process - and an enormous amount of NK cell biology is controlled through the cytoskeleton.

WIP modulates dendritic spine actin cytoskeleton by transcriptional control of lipid metabolic enzymes

We identify Wiskott-Aldrich syndrome protein (WASP)-interacting protein (WIP) as a novel component of neuronal synapses whose absence increases dendritic spine size and filamentous actin levels in an N-WASP/Arp2/3-independent, RhoA/ROCK/profilinIIa-dependent manner. These effects depend on the reduction of membrane sphingomyelin (SM) due to transcriptional upregulation of neutral sphingomyelinase (NSM) through active RhoA; this enhances RhoA binding to the membrane, raft partitioning and activation in steady state but prevents RhoA changes in response to stimulus. Inhibition of NSM or SM addition reverses RhoA, filamentous actin and functional anomalies in synapses lacking WIP. Our findings characterize WIP as a link between membrane lipid composition and actin cytoskeleton at dendritic spines. They also contribute to explain cognitive deficits shared by individuals bearing mutations in the region assigned to the gene encoding for WIP.

Imaging cells at the nanoscale

Recently developed super-resolution techniques in optical microscopy have pushed the length scale at which cellular structure can be observed down to tens of nanometres. A wide array of methods have been described that fall under the umbrella term of super-resolution microscopy and each of these methods has different requirements for acquisition speed, experimental complexity, fluorophore requirements and post-processing of data. For example, experimental complexity can be decreased by using a standard widefield microscope for acquisition, but this requires substantial processing of the data to extract the super-resolution information. These powerful techniques are bringing new insights into the nanoscale structure of sub-cellular assemblies such as podosomes, which are an ideal system to observe with super-resolution microscopy as the structures are relatively thin and they form and dissociate over a period of several minutes. Here we discuss the major classes of super-resolution microscopy techniques, and demonstrate their relative performance by imaging podosomes.

A novel phospholipase D2-Grb2-WASp heterotrimer regulates leukocyte phagocytosis in a two-step mechanism

Phagocytosis is a primary innate response of both macrophages and neutrophils involving the formation of filamentous actin (F-actin)-rich protrusions that are extended around opsonized pathogens to form a phagocytic cup, resulting in their subsequent internalization. The molecular mechanism for this is still not completely understood. We now show for the first time that phospholipase D2 (PLD2) binds to growth factor receptor-bound protein 2 (Grb2) and to the Wiskott-Aldrich syndrome protein (WASp) to form a heterotrimer complex, PLD2-Grb2-WASp, and present the mechanism of interaction. Grb2 binds to the Y169/Y179 residues of PLD2 using its only SH2 domain, and it interacts with the poly-proline region of WASp using its two SH3 domains. The PLD2-Grb2-WASp heterotrimer can be visualized in early phagocytic cups of macrophages ingesting opsonized red blood cells, where it associates with polymerized actin. Cup colocalization and phagocytosis are disrupted with mutants that alter binding at either of the two proteins or by silencing Grb2 with RNA interference (RNAi). WASp association to PLD2-K758R, a lipase-inactive mutant, still occurs, albeit at lower levels, indicating that PLD2 plays a second role in phagocytosis, which is the production of phosphatidic acid (PA) and activation of phosphatidylinositol 5-kinase (PI5K) with subsequent synthesis of phosphatidylinositol 4,5-bisphosphate (PIP(2)). The latter can be blocked with RNAi, which negates phagocytosis. Lastly, a constitutively "open" active form of WASp (WASp-L270P) brings phagocytosis to its maximum level, which can be mimicked with WASp-WT plus PLD2 or plus PA. Since neither a protein-protein disruption nor lack of PLD activity completely negates cup formation or phagocytosis, we posit a two-step mechanism: PLD2 anchors WASp at the phagocytic cup through Grb2 following protein-protein interactions and also activates it, making key lipids available locally. The heterotrimer PLD2-Grb2-WASp then enables actin nucleation at the phagocytic cup and phagocytosis, which are at the center of the innate immune system function.

Degrading devices: invadosomes in proteolytic cell invasion

Podosomes and invadopodia, collectively known as invadosomes, are cell-matrix contacts in a variety of cell types, such as monocytic cells or cancer cells, that have to cross tissue barriers. Both structures share an actin-rich core, which distinguishes them from other matrix contacts, and are regulated by a multitude of signaling pathways including RhoGTPases, kinases, actin-associated proteins, and microtubule-dependent transport. Invadosomes recruit and secrete proteinases and are thus able to lyse extracellular matrix components. They are therefore considered to be potential key structures in proteolytic cell invasion in both physiological and pathological settings. This review provides an overview of the field, with special focus on current developments such as intracellular transport processes, ultrastructural analysis, the possible involvement of invadosomes in disease, and the tentative identification of invadosomes in 3D environments and in vivo.

Hematopoietic cell transplantation for Wiskott-Aldrich syndrome: advances in biology and future directions for treatment

The Wiskott-Aldrich syndrome (WAS) is an X-linked disorder characterized by a triad of diagnostic clinical elements: immunodeficiency, eczema, and hemorrhage caused by thrombocytopenia with small-sized platelets. The formal proof that hematopoietic cell transplantation (HCT) could be used to cure WAS revealed a requirement for both immunosuppression and myelosuppression that still underlies the standard approach to curative therapy today. The current short- and long-term toxicities of HCT are the main stumbling block for the ability to cure every patient with WAS and X-linked thrombocytopenia, and much remains to be done.

Revertant somatic mosaicism in the Wiskott-Aldrich syndrome

Up to 11% of patients affected with Wiskott-Aldrich syndrome (WAS) have presented with somatic mosaicism due to spontaneous in vivo reversion to normal of the original mutation or second-site compensatory mutations that restored production of the WAS gene product. The reasons underlying the high prevalence of this phenomenon in WAS are unclear and may include strong selective advantage of revertant cells over mutated populations, abnormally high general mutation rate and/or increased susceptibility of specific WAS gene sequences to DNA polymerase errors. Additional studies in human samples and in vitro/in vivo models of the disease will likely yield further insights into the mechanisms responsible for the occurrence of revertant mosaicism in WAS and elucidate additional biological characteristics of the WAS gene and protein.

Role for MKL1 in megakaryocytic maturation

Megakaryoblastic leukemia 1 (MKL1), identified as part of the t(1;22) translocation specific to acute megakaryoblastic leukemia, is highly expressed in differentiated muscle cells and promotes muscle differentiation by activating serum response factor (SRF). Here we show that Mkl1 expression is up-regulated during murine megakaryocytic differentiation and that enforced overexpression of MKL1 enhances megakaryocytic differentiation. When the human erythroleukemia (HEL) cell line is induced to differentiate with 12-O-tetradecanoylphorbol 13-acetate, overexpression of MKL1 results in an increased number of megakaryocytes with a concurrent increase in ploidy. MKL1 overexpression also promotes megakaryocytic differentiation of primary human CD34(+) cells cultured in the presence of thrombopoietin. The effect of MKL1 is abrogated when SRF is knocked down, suggesting that MKL1 acts through SRF. Consistent with these findings in human cells, knockout of Mkl1 in mice leads to reduced platelet counts in peripheral blood, and reduced ploidy in bone marrow megakaryocytes. In conclusion, MKL1 promotes physiologic maturation of human and murine megakaryocytes.

Clinical immunology review series: an approach to the patient with recurrent superficial abscesses

Patients may be referred to the immunology clinic for investigation of recurrent superficial abscess formation. In the majority of adult patients this clinical presentation does not equate with an underlying primary immune deficiency. Nevertheless, recurrent mucocutaneous abscesses can be associated with significant morbidity and long-term complications, including scarring and fistula formation, and may be associated with underlying immune-mediated disease. This review sets out an approach to the patient with recurrent superficial abscesses, focusing on the differential diagnoses, investigation and management of both the common causes and those associated with specific immune deficiency.

A crucial role for macrophages in the pathology of K/B x N serum-induced arthritis

Autoantibodies in the form of immune complexes are known to be crucial mediators in initiating inflammation in a variety of autoimmune diseases. This has been well documented in the anti-collagen II antibody-induced arthritis animal model for a long time now. Recently, in the K/B x N mouse model (the F1 of the TCR-transgenic KRN and the diabetic NOD mice), anti-glucose-6-phosphate isomerase (GPI) autoantibodies have been shown to induce arthritis. Experimental work in the K/B x N model demonstrated key roles of autoantigenic immune complexes activating the alternative pathway of complement, the subsequent association with C5aR and Fc gammaRIII-mediated cell activation and production of the inflammatory cytokines IL-1 and TNF-alpha, finally leading to joint destruction. The presence of high amounts of inflammatory cytokines and matrix-degrading proteases at sites of inflammation obviously put the cytokine-producing macrophages as the next target for investigation in this model. Here, we show that mice depleted of macrophages by clodronate liposome treatment are completely resistant to K/B x N serum-induced arthritis. Reconstituting clodronate liposome-treated mice with macrophages from naive animals could reverse this resistance. Also, we found that deficiencies in the Wiskott-Aldrich syndrome protein and CD40, which are both implicated in macrophage activation, chemotaxis and phagocytosis, are not essential in serum-induced arthritis. Mast cell degranulation was seen in arthritogenic serum-treated mice even in the absence of macrophages, possibly suggesting that mast cell degranulation/activation acts hierarchically before macrophages in the inflammatory cascade of anti-GPI antibody-induced arthritis.

The leukocyte podosome

Myeloid leukocytes are the first line of host defence. When they sense perturbations in tissue homeostasis such as infection, inflammation and ischemia, they respond by trafficking. Whilst neutrophils and macrophages migrate to sites of infection, dendritic cells (DC) migrate from tissue-resident sites back into lymph nodes where they activate T and B lymphocytes. The directed migration of these leukocytes through peripheral tissues is thus crucial for their function. This article considers recent advances in our understanding of the adhesive and motile behaviour of macrophages and DC, with particular emphasis on the podosomes that appear to be required for normal migration through extracellular matrices.

Actin polymerization in the equatorial and postacrosomal regions of guinea pig spermatozoa during the acrosome reaction is regulated by G proteins

The acrosome reaction (AR) is an exocytotic process of spermatozoa, and an absolute requirement for fertilization. During AR, actin polymerization is necessary in the equatorial and postacrosomal regions of guinea pig sperm for spermatozoa incorporation deep into the egg cytoplasm, but not for plasma membrane (PM) fusion nor the early steps of egg activation. To identify the mechanisms involved in this sperm actin polymerization, we searched for the protein members, known to be involved in a highly conserved model, that may apply to any cellular process in which de novo actin polymerization occurs from G protein activation. WASP, Arp 2/3, profilins I and II, and Cdc42, RhoA and RhoB GTPases were localized by indirect immunofluorescence (IIF) in guinea pig spermatozoa and their presence corroborated by Western blotting. WASP and profilin II were translocated to the postacrosomal region (Arp2/3 already were there) in long-term capacitated and acrosome-reacted spermatozoa, at the same time as actin polymerization occurred. These events were inhibited by GDP-beta-S and promoted by lysophosphatidic acid (LPA) and GTP-gamma-S, a small GTPase inhibitor and two activators, respectively. By immunoprecipitation, Cdc42-WASp association was identified in capacitated but not in noncapacitated gametes. Polymerized actin in the postacrosomal region is apparently anchored both to the postacrosomal perinuclear theca region and the overlying PM. Results suggest that GTPases are involved in sperm actin polymerization, in the postacrosomal region and the mechanism for polymerization might fit a previously proposed model (Mullins, 2000: Curr Opin Cell Biol 12:91-96).

WASP- mice exhibit defective immune responses to influenza A virus, Streptococcus pneumoniae, and Mycobacterium bovis BCG

OBJECTIVE

To quantify the immune response of WASP- mice to three different pathogens: influenza A virus, Streptococcus pneumoniae, and Mycobacterium bovis.

METHODS

Primary and secondary T-cell responses to influenza A virus were quantified via tetramer assays. Viral clearance from lung was also measured. Lethality of intranasal inoculation with luminescent S. pneumoniae was assessed by dose escalation and direct luminescence imaging. After intravenous inoculation with M. bovis, residual mycobacteria in lung, liver, and spleen were measured by standard culture methods.

RESULTS

The reduced secondary T-cell response to influenza A virus correlates with a relative but not absolute loss of splenic T and B cells similar to that seen in clinical Wiskott-Aldrich Syndrome (WAS), and slower clearance of virus from lung. The reduced magnitude of the secondary T-cell response correlates with a progressive loss of influenza-specific T cells after primary inoculation. WASP- mice show an increased susceptibility to lethal pneumonia after intranasal inoculation with S. pneumoniae, which is among the most frequent causes of clinical complications in WAS patients. WASP- mice clear M. bovis bacille Calmette-Guerin (BCG) more slowly from lung, liver, and spleen. Bone marrow-derived macrophages, however, show normal ex vivo cytokine secretion in response to M. bovis.

CONCLUSIONS

These results demonstrate that WASP- mice are functionally immunodeficient in regard to three different pathogens, and provide relevant end points for the study of treatment modalities in this model. They also suggest a specific physiologic mechanism, failure to accumulate memory T cells, for at least one of the defective immune responses.

Small Rho GTPases Regulate Antigen Presentation in Dendritic Cells

Dendritic cells (DC) are involved in the regulation of innate and adaptive immunity. However, the molecular mechanisms maintaining DC function remain to be elucidated. In this study, we report on the role of small Rho GTPases: Cdc42, Rac1, and RhoA in the regulation of DC adherence, Ag presentation, migration, chemotaxis, and endocytosis. Murine DC were transfected with vaccinia virus-based constructs, encoding dominant-negative or constitutively active (ca) mutant forms of Rho GTPases. We demonstrate that Cdc42 plays a major role in the regulation of DC adhesion, because caCdc42-transfected DC had significant up-regulation of adhesion to extracellular matrix, which was blocked by the Rho GTPase inhibitor toxin B (ToxB). In contrast, caRho-transfected DC only modestly elevated DC adhesion, and caRac had no effect. Additionally, caCdc42 and caRho increased the ability of DC to present OVA peptide to specific T cells. This effect was abrogated by ToxB. Activation of Cdc42 in DC significantly inhibited spontaneous and chemokine-induced DC migration. Furthermore, uptake of dextran 40 by DC was significantly enhanced by Rho GTPase activators cytotoxic necrotizing factor 1 and PMA, and reduced by ToxB. caCdc42 also increased endocytotic activity of DC, whereas dominant-negative Cdc42 blocked it. Thus, Rho GTPases Cdc42, RhoA, and Rac1 regulate DC functions that are critical for DC-mediated immune responses in vivo.

A Dictyostelium homologue of WASP is required for polarized F-actin assembly during chemotaxis

The actin cytoskeleton controls the overall structure of cells and is highly polarized in chemotaxing cells, with F-actin assembled predominantly in the anterior leading edge and to a lesser degree in the cell's posterior. Wiskott-Aldrich syndrome protein (WASP) has emerged as a central player in controlling actin polymerization. We have investigated WASP function and its regulation in chemotaxing Dictyostelium cells and demonstrated the specific and essential role of WASP in organizing polarized F-actin assembly in chemotaxing cells. Cells expressing very low levels of WASP show reduced F-actin levels and significant defects in polarized F-actin assembly, resulting in an inability to establish axial polarity during chemotaxis. GFP-WASP preferentially localizes at the leading edge and uropod of chemotaxing cells and the B domain of WASP is required for the localization of WASP. We demonstrated that the B domain binds to PI(4,5)P2 and PI(3,4,5)P3 with similar affinities. The interaction between the B domain and PI(3,4,5)P3 plays an important role for the localization of WASP to the leading edge in chemotaxing cells. Our results suggest that the spatial and temporal control of WASP localization and activation is essential for the regulation of directional motility.

Wiskott-Aldrich syndrome protein and the cytoskeletal dynamics of dendritic cells

The regulated migration and spatial localization of dendritic cells in response to environmental signals are critical events during the initiation of physiological immune responses and maintenance of tolerance. Cells deficient in the Wiskott-Aldrich syndrome protein (WASP) have been used to demonstrate the importance of the dynamic remodelling of the actin-based cytoskeleton during the selective adhesion and migration of these cells. Unlike most cell types, macrophages, dendritic cells, and osteoclasts utilize a specialized adhesive array termed the podosome in order to migrate. Podosomes are composed of many of the same structural and regulatory proteins as seen in the more commonly found focal adhesion, but are unique in their requirement for WASP. Without WASP, podosomes cannot form and the affected cells are obliged to use focal adhesions for their migratory activities. Once activated by a series of upstream regulatory proteins, WASP acts as a scaffold for the binding of the potent actin nucleating protein complex known as Arp2/3. This article reviews the available evidence that suggests that failures in the regulation of the actin cytoskeleton may contribute significantly to the immunopathology of the Wiskott-Aldrich syndrome.

WASp deficiency in mice results in failure to form osteoclast sealing zones and defects in bone resorption

No defects related to deficiency of the Wiskott-Aldrich Syndrome protein (WASp) have been described in osteoclasts. Here we show that there are significant morphologic and functional abnormalities. WASp-null cells spread over a much larger surface area and are highly polykaryotic. In their migratory phase, normal cells assemble clusters of podosomes behind their leading edges, whereas during the bone resorptive phase multiple podosomes are densely aggregated in well-defined actin rings forming the sealing zone. In comparison, WASp-null osteoclasts in either phase are markedly depleted of podosomes. On bone surfaces, this results in a failure to form actin rings at sealing zones. Complementation of WASp-null osteoclasts with an enhanced green fluorescent protein (eGFP)-WASp fusion protein restores normal cytoarchitecture. These structural disturbances translate into abnormal patterns of bone resorption both in vitro on bone slices and in vivo. Although physiologic steady-state levels of bone resorption are maintained, a major impairment is observed when WASp-null animals are exposed to a resorptive challenge. Our results provide clear evidence that WASp is a critical component of podosomes in osteoclasts and indicate a nonredundant role for WASp in the dynamic organization of these actin structures during bone resorption. (Blood. 2004;103:3552-3561)

Autoimmunity in Wiskott-Aldrich syndrome

As many as 40% of patients with Wiskott-Aldrich syndrome may eventually suffer from an autoimmune disorder, with an increased chance of developing a malignancy. Vasculitides and autoimmune hemolytic anemia are the two most common autoimmune manifestations and often cause considerable morbidity and mortality, because they may require treatment with bone marrow transplantation. Insights into the mechanisms of autoimmunity have provided clues to the pathogenesis of these disorders in Wiskott-Aldrich syndrome. Chronic inflammation, interleukin-2 deficiency, and increased apoptosis may all play a possible role in the loss of peripheral tolerance to self-antigens in this disease. This article reviews the manifestations and consequences of autoimmunity in Wiskott-Aldrich syndrome, its possible mechanisms, and available treatments.

Podosomes: adhesion hot-spots of invasive cells

Podosomes are highly dynamic, actin-rich adhesion structures of monocyte-derived cells, certain transformed fibroblasts and carcinoma cells and have recently also been discovered in an increasing number of other cell types. Because they are found mainly in motile cells and control the activity of matrix metalloproteases, podosomes are thought to contribute to tissue invasion and matrix remodeling. Importantly, podosomes are physiologically relevant organelles because they can be found in ex vivo models of invasive cells. Regulators of podosome turnover include tyrosine kinases, RhoGTPases, actin regulators and the microtubule system. Podosomes might also serve as an attractive model to study how integration of various signaling pathways controls actin dynamics. Here, we summarize and discuss the known structural, regulatory and functional features of podosomes, our aim being to stimulate further research into these unique structures.

Functional correction of T cells derived from patients with the Wiskott-Aldrich syndrome (WAS) by transduction with an oncoretroviral vector encoding the WAS protein

T-cell dysfunction is thought to be central to the immunodeficiency state seen in patients with the Wiskott-Aldrich syndrome (WAS). Aspects of the WAS phenotype have been corrected in other cell types on introduction of the normal WAS protein (WASP), but the potential for correction of the T-cell defects has not been evaluated. Here we demonstrate that an oncoretroviral vector encoding WASP and green fluorescent protein (GFP), and pseudotyped with the RD114 envelope protein, efficiently transduces primary human T cells derived from WAS patients. Transcription initiated at the oncoretroviral long terminal repeat (LTR) results in levels of WASP that, while lower than those seen in normal control T cells, resulted in correction of the deficient proliferative response to T-cell receptor (TCR) stimulation characteristic of WAS. IL2 secretion after TCR stimulation was partially corrected. Control primary T cells transduced with the same vector responded normally to TCR stimulation, and showed no increase in WASP expression. The demonstration that correction of T cell defects can be achieved by gene transfer supports continued efforts to develop gene therapy for WAS.

Leukocyte uropod formation and membrane/cytoskeleton linkage in immune interactions

The acquisition of a cell polarity is a crucial requirement for migration, activation, and apoptosis of leukocytes. The polarization of leukocytes involves the formation of two distinct poles: the leading edge--the attachment cell site to the substrate allowing directional movements of the cell--and on the opposite side, the uropod--mostly involved in cell-to-cell interaction and in a variety of leukocyte activities including activation and apoptosis. However, the uropod takes shape in neutrophils, monocytes, and natural killer cells, and the formation of this cell protrusion seems to exert an important role in immune interactions. In fact, the polarization sites of leukocytes are involved in a complex cross-talk between cells and extracellular matrix components, and a number of receptors and counter-receptors crowd in the contact sites to allow efficient cell-to-cell or cell-substrate interaction. The membrane/cytoskeleton interaction plays a crucial role in tuning these activities and in "predisposing" leukocytes to their function through the acquisition of a polarized phenotype. This review is focused on the mechanisms underlying the formation of the leukocyte uropod, the role of cytoskeleton in defining its structure and function, and the involvement of the uropod in the complex interplay between immune cells.

A Rac/Cdc42-specific exchange factor, GEFT, induces cell proliferation, transformation, and migration

The Rho family of small GTPases, including Rho, Rac, and Cdc42, play essential roles in diverse cellular functions. The ability of Rho family GTPases to participate in signaling events is determined by the ratio of inactive (GDP-bound) and active (GTP-bound) forms in the cell. The activation of Rho family proteins requires the exchange of bound GDP for GTP, a process catalyzed by the Dbl family of guanine nucleotide exchange factors (GEFs). The GEFs have high affinity for the guanine nucleotide-free state of the GTPases and are thought to promote GDP release by stabilizing an intermediate transition state. In this study, we have identified and characterized a new Rac/Cdc42-specific Dbl family guanine nucleotide exchange factor, named GEFT. GEFT is highly expressed in the excitable tissues, including brain, heart, and muscle. Low or very little expression was detected in other nonexcitable tissues. GEFT has specific exchange activity for Rac and Cdc42 in our in vitro GTPase exchange assays and glutathione S-transferase-PAK pull-down assays with GTP-bound Rac1 and Cdc42. Overexpression of GEFT leads to changes in cell morphology and actin cytoskeleton re-organization, including the formation of membrane microspikes, filopodia, and lamilliopodia. Furthermore, expression of GEFT in NIH3T3 cells promotes foci formation, cell proliferation, and cell migration, possibly through the activation of transcriptional factors involved in cell growth and proliferation. Together, our data suggest that GEFT is a Rac/Cdc42-specific GEF protein that regulates cell morphology, cell proliferation, and transformation.

A WASp homolog powers actin polymerization-dependent motility of endosomes in vivo

BACKGROUND

WASp/SCAR proteins activate the Arp2/3 complex to nucleate actin filament assembly and are thought to have important roles in endocytosis. WASp is required for efficient endocytosis of antigen receptors, N-WASp promotes actin polymerization-dependent movement of endomembrane vesicles, and Las17 (a yeast WASp homolog) is required for endocytic internalization. However, it is unknown whether movement of endosomes or other organelles requires activation of the Arp2/3 complex by members of the WASp/SCAR family.

RESULTS

Fluorescence video microscopy of yeast cells expressing a GFP-tagged G protein-coupled receptor (Ste2-GFP) as an endocytic marker revealed that endosomes and the lysosome-like vacuole are highly motile. Endosome/vacuole motility required actin polymerization, as indicated by sensitivity to latrunculin A, whereas microtubules were uninvolved. Endosome/vacuole motility did not require actin cables or myosin V (a MYO2 gene product), which moves secretory vesicles and the Golgi apparatus and mediates vacuole segregation. However, endosome motility required Las17, a WASp homolog. In contrast to other processes involving Las17, endosome/vacuole motility required the WCA domain of Las17, which is necessary and sufficient to activate the Arp2/3 complex.

CONCLUSIONS

Endosome/vacuole motility in vivo requires actin polymerization stimulated by the WASp homolog Las17. WASp/SCAR family members in mammalian cells may have similar functions. Defects in endosome/lysosome motility may contribute to deficits in lymphocyte or macrophage function observed in human patients lacking WASp or developmental defects in N-WASp-deficient mice.

Mechanisms of CD47-induced caspase-independent cell death in normal and leukemic cells: link between phosphatidylserine exposure and cytoskeleton organization

Dying cells, apoptotic or necrotic, are swiftly eliminated by professional phagocytes. We previously reported that CD47 engagement by CD47 mAb or thrombospondin induced caspase-independent cell death of chronic lymphocytic leukemic B cells (B-CLL). Here we show that human immature dendritic cells (iDCs) phagocytosed the CD47 mAb-killed leukemic cells in the absence of caspases 3, 7, 8, and 9 activation in the malignant lymphocytes. Yet the dead cells displayed the cytoplasmic features of apoptosis, including cell shrinkage, phosphatidylserine exposure, and decreased mitochondrial transmembrane potential (DeltaPsim). CD47 mAb-induced cell death also occurred in normal resting and activated lymphocytes, with B-CLL cells demonstrating the highest susceptibility. Importantly, iDCs and CD34(+) progenitors were resistant. Structure-function studies in cell lines transfected with various CD47 chimeras demonstrated that killing exclusively required the extracellular and transmembrane domains of the CD47 molecule. Cytochalasin D, an inhibitor of actin polymerization, and antimycin A, an inhibitor of mitochondrial electron transfer, completely suppressed CD47-induced phosphatidylserine exposure. Interestingly, CD47 ligation failed to induce cell death in mononuclear cells isolated from Wiskott-Aldrich syndrome (WAS) patients, suggesting the involvement of Cdc42/WAS protein (WASP) signaling pathway. We propose that CD47-induced caspase-independent cell death be mediated by cytoskeleton reorganization. This form of cell death may be relevant to maintenance of homeostasis and as such might be explored for the development of future therapeutic approaches in lymphoid malignancies.

Wiskott-Aldrich syndrome protein is required for NK cell cytotoxicity and colocalizes with actin to NK cell-activating immunologic synapses

The Wiskott-Aldrich syndrome (WAS) is a primary immunodeficiency disorder caused by a mutation in WAS protein (WASp) that results in defective actin polymerization. Although the function of many hematopoietic cells requires WASp, the specific expression and function of this molecule in natural killer (NK) cells is unknown. Here, we report that WAS patients have increased percentages of peripheral blood NK cells and that fresh enriched NK cells from two patients with a WASp mutation have defective cytolytic function. In normal NK cells, WASp was expressed and localized to the activating immunologic synapse (IS) with filamentous actin (F-actin). Perforin also localized to the NK cell-activating IS but at a lesser frequency than F-actin and WASp. The accumulation of F-actin and WASp at the activating IS was decreased significantly in NK cells that had been treated with the inhibitor of actin polymerization, cytochalasin D. NK cells from WAS patients lacked expression of WASp and accumulated F-actin at the activating IS infrequently. Thus, WASp has an important function in NK cells. In patients with WASp mutations, the resulting NK cell defects are likely to contribute to their disease.

Restoration of podosomes and chemotaxis in Wiskott-Aldrich syndrome macrophages following induced expression of WASp

We used a direct-viewing (Dunn) chemotaxis chamber to analyse the chemotactic responses of human normal and Wiskott-Aldrich syndrome (WAS) macrophages to the cytokine colony stimulating factor-1 (CSF-1). In five patients with classic WAS, where specialised adhesion complexes called podosomes are absent, chemotaxis of macrophages was abolished. The deficient chemotactic responses of WAS macrophages following cytokine stimulation could be correlated with abnormalities in cell polarisation and actin organisation. In a series of cell microinjection studies we found that normal chemotactic responses were restored in WASp macrophages transfected with a full-length human WAS construct. Expression of exogenous WAS protein (WASp) in these cells also restored normal polarised cell morphology and the ability to form podosomes.

The invasive phenotype in HMT-3522 cells requires increased EGF receptor signaling through both PI 3-kinase and ERK 1,2 pathways

We studied the invasion of HMT-3522 breast epithelial cells in response to epidermal growth factor (EGF), and the associated signaling pathways. HMT-3522 T4-2 cells were shown to invade Matrigel-coated Transwell membranes in response to EGF while HMT-3522 S-1 cells failed to invade when treated with EGF. Studies utilizing specific molecular inhibitors showed the importance of beta1 integrin, phosphatidylinositol 3 kinase (PI 3-kinase), p38, extracellular regulated kinase 1, 2 (Erk 1,2) MAP kinases, and metalloproteinases in invasion and motility. T4-2 cell invasion was shown to be time-dependent and also gene transcription-dependent as shown by inhibition with Actinomycin D. T4-2 cells exhibited an increased activation of MAP kinases Erk 1,2 (2-fold), EGF receptor (3-fold), and PI 3-kinase (3- to 4-fold) when compared to the S-1 cells. In response to EGF, T4-2 cells showed a 5-fold greater secretion of matrix metalloproteinase-9 (MMP-9) as compared to S-1 cells, and this increase was largely dependent on the activity of PI 3-kinase. These findings indicate that expression of the invasive phenotype in these breast epithelial cells requires increased EGF receptor signaling, involving both PI 3-kinase and Erk 1,2 activities, which leads to multiple downstream effects, including enhanced secretion of MMP-9 and transcription of invasion-related genes.

Monocytes from Wiskott-Aldrich patients differentiate in functional mature dendritic cells with a defect in CD83 expression

Wiskott-Aldrich syndrome (WAS) is an X-linked disorder characterized by congenital thrombocytopenia and progressive deterioration of the immune function. Dendritic cells (DC) are key effectors in the induction of specific immunity and are highly specialized in antigen uptake and subsequent migration to draining lymph nodes. DC were generated in vitro from circulating monocytes from ten WAS patients characterized by a different disease score. Immature DC showed similar morphology and membrane phenotype, as compared to normal DC. In chemotaxis assay, immature DC had a reduced migration in response to MIP-1alpha/CCL3, but efficiently endocytosed the macromolecules FITC-dextran and FITC-albumin. Upon terminal differentiation with LPS or CD40 ligand, the acquisition of a mature surface phenotype was variably achieved among WAS patients, with increased expression of CD80, CD86 and DC-LAMP. In contrast, the expression of CD83 was usually low. A defective up-regulation of CD83 was also observed in the lymph node from one WAS patient, whose DC stained positively for DC-LAMP. Mature DC from all the patients tested, but one, significantly migrated in vitro in response to MIP-3beta, a finding confirmed in vivo by the detection of HLA-DR/DC LAMP-positive cells in secondary lymphoid organs. When tested in MLR assays, both immature and mature WAS DC induced allogenic T cell proliferation in a manner comparable to control DC. Collectively these results suggest that, although many functional activities of WAS DC are essentially similar to normal DC, subtle and selective alterations of DC differentiation were also observed, with reduced migratory activity of immature DC and defective CD83 expression upon maturation.

Leukocyte transendothelial migration: orchestrating the underlying molecular machinery

Transendothelial migration of leukocytes involves the spatiotemporal regulation of adhesion molecules, chemokines and cytoskeletal regulators. Recent results show that distinct steps of leukocyte transendothelial migration are regulated by sequential integrin activation and coordinated Rho family GTPase activity. Progress has been made in understanding how the dynamic regulation of these molecules translates into leukocyte transmigration.