T-cell receptor ligation causes Wiskott-Aldrich syndrome protein degradation and F-actin assembly downregulation

Understanding immunoactinopathies: A decade of research on WAS gene defects

Immunoactinopathies caused by mutations in actin-related proteins are a growing group of inborn errors of immunity (IEI). Immunoactinopathies are caused by a dysregulated actin cytoskeleton and affect hematopoietic cells especially because of their unique capacity to survey the body for invading pathogens and altered self, such as cancer cells. These cell motility and cell-to-cell interaction properties depend on the dynamic nature of the actin cytoskeleton. Wiskott-Aldrich syndrome (WAS) is the archetypical immunoactinopathy and the first described. WAS is caused by loss-of-function and gain-of-function mutations in the actin regulator WASp, uniquely expressed in hematopoietic cells. Mutations in WAS cause a profound disturbance of actin cytoskeleton regulation of hematopoietic cells. Studies during the last 10 years have shed light on the specific effects on different hematopoietic cells, revealing that they are not affected equally by mutations in the WAS gene. Moreover, the mechanistic understanding of how WASp controls nuclear and cytoplasmatic activities may help to find therapeutic alternatives according to the site of the mutation and clinical phenotypes. In this review, we summarize recent findings that have added to the complexity and increased our understanding of WAS-related diseases and immunoactinopathies.

Identification of a novel WAS mutation and the non-splicing effect of a second-site mutation in a Chinese pedigree with Wiskott-Aldrich syndrome

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is a rare X-linked immunodeficiency disorder caused by abnormal expression of the WAS protein (WASp) due to mutations in the WAS gene, and is generally characterized by microthrombocytopenia, eczema, recurrent infections, and high susceptibility to autoimmune complications and hematological malignancies.

RESULTS

Herein, we identified a novel WAS mutation (c.158 T > C) using next-generation sequencing in a Chinese pedigree with WAS. The expression of WASp in the patients and their families was detected by flow cytometry and western blot analysis. To explore the exon-splicing effect of intron mutations and the correlation between the genotype and clinical phenotype, four groups of wild-type (WT), exon mutant, intron mutant, and combined mutant recombinant plasmids were transfected into COS-7 cells in vitro. The proband showed dramatically decreased WASp expression, while the female carriers showed a slightly lower level of WASp. The expression of products in the mutant and WT recombinant plasmids was detected by real-time fluorescence quantitative polymerase chain reaction (PCR), which showed a significant reduction in the combined mutant group than in the WT, exon mutant, and intron mutant groups. The length of the expression products in the four groups showed no differences, each containing 360 base pairs. Sequence analysis confirmed that the c.158 T > C mutation appeared in the exon mutant and combined mutant groups, whereas the intron variant c.273 + 14C > T caused no other sequence changes.

CONCLUSION

This study confirmed that the intron mutation did not affect the splicing of exons and excluded the influence of the double mutations at the transcription level on the severe clinical manifestations in the cousin. This in vitro study provided new insights into the pathogenesis of intronic mutations in WAS.

Impairment of cytokine production following immunological synapse formation in patients with Wiskott-Aldrich syndrome and leukocyte adhesion deficiency type 1

T cells following immunological synapse (IS) formation with antigen-presenting cells produce multiple cytokines through T cell receptor, integrin, and costimulatory signaling. Here, we investigated the cytokine profiles following IS formation in response to staphylococcal superantigen exposure in three adolescent patients with classical Wiskott-Aldrich syndrome (WAS) and in one patient with leukocyte adhesion deficiency (LAD) type 1. All WAS patients showed lower Th1 and Th2-skewed cytokine production; similar results were observed in the flow cytometric analysis of IFNγ- and IL-4-producing T cells. The patient with LAD type 1 with somatic mosaicism in 2% of CD8+ T cells showed lower Th1 and Th2 cytokine production than healthy controls. The patients with WAS were susceptible to infections and atopic manifestations, and the patients with LAD type 1 showed cold abscess on their skin, our findings using patient samples provide clinical insights into the mechanisms underlying immunodeficiency related to the symptoms of each disease.

Primary Immunodeficiencies Associated With Early-Onset Inflammatory Bowel Disease in Southeast and East Asia

Background

Causes of early-onset inflammatory bowel disease (IBD) vary, and primary immunodeficiency diseases (PIDs) are associated with early-onset IBD as monogenic disorders.

Aim

This review investigates the prevalence, clinical manifestation, genetic profile, and treatment of patients with early-onset IBD in Southeast and East Asia.

Methods

A systemic review of articles reporting PID patients associated with early-onset IBD in Southeast and East Asia was conducted.

Results

The prevalence of PID associated with IBD was higher than that reported in western nations, and the frequency of patients with bloody stools as an early symptom was relatively higher in monogenic diseases. A total 13 (12.0%) of 108 patients with early-onset IBD were diagnosed as PID by exome sequencing and targeted gene panel analysis in Japan, including four patients with XIAP, three with IL10RA, and two or one patient with other gene mutations. In addition, ten patients were reported as having IL-10 receptor alpha (IL-10RA) deficiency in China and Hong Kong. Allogeneic hematopoietic stem cell transplantation was performed in patients with X-linked inhibitor of apoptosis deficiency, IL-10RA deficiency, or other PID as a curative treatment, and the preferable outcome of reduced-intensity conditioning and complete resolution of IBD symptoms and dysbiosis were achieved.

Conclusion

Comprehensive molecular diagnosis has been widely applied to screen for patients with PID-associated IBD in Southeast and East Asia. These results contributed to the awareness of monogenic PID in early-onset IBD patients and their differences in clinical manifestations and genetic profiles compared to the patients in western counties.

Lymphocytes Utilize Somatic Mutations, Epigenetic Silencing, and the Proteasome to Escape Truncated WASP Expression

Wiskott-Aldrich Syndrome Protein (WASP) deficiency causes Wiskott-Aldrich Syndrome (WAS), a sex-linked disorder characterized by combined immunodeficiency, microthrombocytopenia, and eczema. Like WASP-deficient humans, WASP-deficient mice produce normal numbers of functionally defective T cells. Here, we report a WAS patient with a novel germline frameshifting WAS mutation encoding a truncated form of WASP lacking the C-terminal cofilin homology (C) and the acidic region (A) domains (WASPΔCA). Although stably overexpressed in embryonic kidney cell lines, WASPΔCA was undetectable in circulating patient leukocytes. Deep sequencing, transcript profiling, and protein degradation analyses demonstrated patient lymphocytes employ an array of genetic, epigenetic, and proteasomal strategies to avoid expressing WASPΔCA.

CHIP-dependent regulation of the actin cytoskeleton is linked to neuronal cell membrane integrity

CHIP is an E3-ubiquitin ligase that contributes to healthy aging and has been characterized as neuroprotective. To elucidate dominant CHIP-dependent changes in protein steady-state levels in a patient-derived human neuronal model, CHIP function was ablated using gene-editing and an unbiased proteomic analysis conducted to compare knock-out and wild-type isogenic induced pluripotent stem cell (iPSC)-derived cortical neurons. Rather than a broad effect on protein homeostasis, loss of CHIP function impacted on a focused cohort of proteins from actin cytoskeleton signaling and membrane integrity networks. In support of the proteomics, CHIP knockout cells had enhanced sensitivity to induced membrane damage. We conclude that the major readout of CHIP function in cortical neurons derived from iPSC of a patient with elevate α-synuclein, Parkinson's disease and dementia, is the modulation of substrates involved in maintaining cellular "health". Thus, regulation of the actin cytoskeletal and membrane integrity likely contributes to the neuroprotective function(s) of CHIP.

The Bardet-Biedl syndrome complex component BBS1 controls T cell polarity during immune synapse assembly

Components of the intraflagellar transport (IFT) system that regulates the assembly of the primary cilium are co-opted by the non-ciliated T cell to orchestrate polarized endosome recycling and to sustain signaling during immune synapse formation. Here, we investigated the potential role of Bardet-Biedl syndrome 1 protein (BBS1), an essential core component of the BBS complex that cooperates with the IFT system in ciliary protein trafficking, in the assembly of the T cell synapse. We demonstrated that BBS1 allows for centrosome polarization towards the immune synapse. This function is achieved through the clearance of centrosomal F-actin and its positive regulator WASH1 (also known as WASHC1), a process that we demonstrated to be dependent on the proteasome. We show that BBS1 regulates this process by coupling the 19S proteasome regulatory subunit to the microtubule motor dynein for its transport to the centrosome. Our data identify the ciliopathy-related protein BBS1 as a new player in T cell synapse assembly that functions upstream of the IFT system to set the stage for polarized vesicular trafficking and sustained signaling. This article has an associated First Person interview with the first author of the paper.

Wiskott-Aldrich Syndrome Protein: Roles in Signal Transduction in T Cells

Signal transduction regulates the proper function of T cells in an immune response. Upon binding to its specific ligand associated with major histocompatibility complex (MHC) molecules on an antigen presenting cell, the T cell receptor (TCR) initiates intracellular signaling that leads to extensive actin polymerization. Wiskott-Aldrich syndrome protein (WASp) is one of the actin nucleation factors that is recruited to TCR microclusters, where it is activated and regulates actin network formation. Here we highlight the research that has focused on WASp-deficient T cells from both human and mice in TCR-mediated signal transduction. We discuss the role of WASp in proximal TCR signaling as well as in the Ras/Rac-MAPK (mitogen-activated protein kinase), PKC (protein kinase C) and Ca2+-mediated signaling pathways.

Cytoskeletal tension actively sustains the migratory T-cell synaptic contact

When migratory T cells encounter antigen-presenting cells (APCs), they arrest and form radially symmetric, stable intercellular junctions termed immunological synapses which facilitate exchange of crucial biochemical information and are critical for T-cell immunity. While the cellular processes underlying synapse formation have been well characterized, those that maintain the symmetry, and thereby the stability of the synapse, remain unknown. Here we identify an antigen-triggered mechanism that actively promotes T-cell synapse symmetry by generating cytoskeletal tension in the plane of the synapse through focal nucleation of actin via Wiskott-Aldrich syndrome protein (WASP), and contraction of the resultant actin filaments by myosin II. Following T-cell activation, WASP is degraded, leading to cytoskeletal unraveling and tension decay, which result in synapse breaking. Thus, our study identifies and characterizes a mechanical program within otherwise highly motile T cells that sustains the symmetry and stability of the T cell-APC synaptic contact.

SLAP Is a Negative Regulator of FcεRI Receptor-Mediated Signaling and Allergic Response

Binding of antigen to IgE-high affinity FcεRI complexes on mast cells and basophils results in the release of preformed mediators such as histamine and de novo synthesis of cytokines causing allergic reactions. Src-like adapter protein (SLAP) functions co-operatively with c-Cbl to negatively regulate signaling downstream of the T cell receptor, B cell receptor, and receptor tyrosine kinases (RTK). Here, we investigated the role of SLAP in FcεRI-mediated mast cell signaling, using bone marrow derived mast cells (BMMCs) from SLAP knock out (SLAP KO) mice. Mature SLAP-KO BMMCs displayed significantly enhanced antigen induced degranulation and synthesis of IL-6, TNFα, and MCP-1 compared to wild type (WT) BMMCs. In addition, SLAP KO mice displayed an enhanced passive cutaneous anaphylaxis response. In agreement with a negative regulatory role, SLAP KO BMMCs showed enhanced FcεRI-mediated signaling to downstream effector kinases, Syk, Erk, and Akt. Recombinant GST-SLAP protein binds to the FcεRIβ chain and to the Cbl-b in mast cell lysates, suggesting a role in FcεRI down regulation. In addition, the ubiquitination of FcεRIγ chain and antigen mediated down regulation of FcεRI is impaired in SLAP KO BMMCs compared to the wild type. In line with these findings, stimulation of peripheral blood human basophils with FcεRIα antibody, or a clinically relevant allergen, resulted in increased SLAP expression. Together, these results indicate that SLAP is a dynamic regulator of IgE-FcεRI signaling, limiting allergic responses.

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.

Proteasome Dependent Actin Remodeling Facilitates Antigen Extraction at the Immune Synapse of B Cells

Engagement of the B cell receptor (BCR) with surface-tethered antigens leads to the formation of an immune synapse (IS), where cell signaling and antigen uptake are tightly coordinated. Centrosome re-orientation to the immune synapse has emerged as a critical regulatory step to guide the local recruitment and secretion of lysosomes, which can facilitate the extraction of immobilized antigens. This process is coupled to actin remodeling at the centrosome and at the immune synapse, which is crucial to promote cell polarity. How B cells balance both pools of actin cytoskeleton to achieve a polarized phenotype during the formation of an immune synapse is not fully understood. Here, we reveal that B cells rely on proteasome activity to achieve this task. The proteasome is a multi-catalytic protease that degrades cytosolic and nuclear proteins and its dysfunction is associated with diseases, such as cancer and autoimmunity. Our results show that resting B cells contain an active proteasome pool at the centrosome, which is required for efficient actin clearance at this level. As a result of proteasome inhibition, activated B cells do not deplete actin at the centrosome and are unable to separate the centrosome from the nucleus and thus display impaired polarity. Consequently, lysosome recruitment to the immune synapse, antigen extraction and presentation are severely compromised in B cells with diminished proteasome activity. Additionally, we found that proteasome inhibition leads to impaired actin remodeling at the immune synapse, where B cells display defective spreading responses and distribution of key signaling molecules at the synaptic membrane. Overall, our results reveal a new role for the proteasome in regulating the immune synapse of B cells, where the intracellular compartmentalization of proteasome activity controls cytoskeleton remodeling between the centrosome and synapse, with functional repercussions in antigen extraction and presentation.

Wiskott-Aldrich syndrome protein (WASP) is a tumor suppressor in T cell lymphoma

In T lymphocytes, the Wiskott-Aldrich Syndrome protein (WASP) and WASP-interacting-protein (WIP) regulate T cell antigen receptor (TCR) signaling, but their role in lymphoma is largely unknown. Here we show that the expression of WASP and WIP is frequently low or absent in anaplastic large cell lymphoma (ALCL) compared to other T cell lymphomas. In anaplastic lymphoma kinase-positive (ALK+) ALCL, WASP and WIP expression is regulated by ALK oncogenic activity via its downstream mediators STAT3 and C/EBP-β. ALK+ lymphomas were accelerated in WASP- and WIP-deficient mice. In the absence of WASP, active GTP-bound CDC42 was increased and the genetic deletion of one CDC42 allele was sufficient to impair lymphoma growth. WASP-deficient lymphoma showed increased mitogen-activated protein kinase (MAPK) pathway activation that could be exploited as a therapeutic vulnerability. Our findings demonstrate that WASP and WIP are tumor suppressors in T cell lymphoma and suggest that MAP-kinase kinase (MEK) inhibitors combined with ALK inhibitors could achieve a more potent therapeutic effect in ALK+ ALCL.

Immune Regulation by Ubiquitin Tagging as Checkpoint Code

The immune system is equipped with effective machinery to mobilize its activation to defend invading microorganisms, and at the same time, to refrain from attacking its own tissues to maintain immune tolerance. The balance of activation and tolerance is tightly controlled by diverse mechanisms, since breakdown of tolerance could result in disastrous consequences such as the development of autoimmune diseases. One of the mechanisms is by the means of protein ubiquitination, which involves the process of tagging a small peptide ubiquitin to protein substrates. E3 ubiquitin ligases are responsible for catalyzing the final step of ubiquitin-substrate conjugation by specifically recognizing substrates to determine their fates of degradation or functional modification. The ubiquitination process is reversible, which is carried out by deubiquitinating enzymes to release the ubiquitin molecule from the conjugated substrates. Protein ubiquitination and deubiquitination serve as checkpoint codes in many key steps of lymphocyte regulation including the development, activation, differentiation, and tolerance induction. In this chapter, we will discuss a few E3 ligases and deubiquitinating enzymes that are important in controlling immune responses, with emphasis on their roles in T cells.

Calcium influx through CRAC channels controls actin organization and dynamics at the immune synapse

T cell receptor (TCR) engagement opens Ca²⁺ release-activated Ca²⁺ (CRAC) channels and triggers formation of an immune synapse between T cells and antigen-presenting cells. At the synapse, actin reorganizes into a concentric lamellipod and lamella with retrograde actin flow that helps regulate the intensity and duration of TCR signaling. We find that Ca²⁺ influx is required to drive actin organization and dynamics at the synapse. Calcium acts by promoting actin depolymerization and localizing actin polymerization and the actin nucleation promotion factor WAVE2 to the periphery of the lamellipod while suppressing polymerization elsewhere. Ca²⁺-dependent retrograde actin flow corrals ER tubule extensions and STIM1/Orai1 complexes to the synapse center, creating a self-organizing process for CRAC channel localization. Our results demonstrate a new role for Ca²⁺ as a critical regulator of actin organization and dynamics at the synapse, and reveal potential feedback loops through which Ca²⁺ influx may modulate TCR signaling.

DOI:http://dx.doi.org/10.7554/eLife.14850.001

An effective immune response requires the immune system to rapidly recognize and respond to foreign invaders. Immune cells known as T cells recognize infection through a protein on their surface known as the T cell receptor. The T cell receptor binds to foreign proteins displayed on the surface of other cells. This interaction initiates a chain of events, including the opening of calcium channels embedded in the T cell membrane known as CRAC channels, which allows calcium ions to flow into the cell. These events ultimately lead to the activation of the T cell, enabling it to mount an immune response against the foreign invader.

As part of the activation process, the T cell spreads over the surface of the cell that is displaying foreign proteins to form an extensive interface known as an immune synapse. The movement of the T cell's internal skeleton (the cytoskeleton) is crucial for the formation and function of the synapse. Actin filaments, a key component of the cytoskeleton, flow from the edge of the synapse toward the center; these rearrangements of the actin cytoskeleton help to transport clusters of T cell receptors to the center of the synapse and enable the T cell receptors to transmit signals that lead to the T cell being activated. It is not entirely clear how the binding of T cell receptors to foreign proteins drives the actin rearrangements, but there is indirect evidence suggesting that calcium ions may be involved.

Hartzell et al. have now investigated the interactions between calcium and the actin cytoskeleton at the immune synapse in human T cells. T cells were placed on glass so that they formed immune synapse-like connections with the surface, and actin movements at the synapse were visualized using a specialized type of fluorescence microscopy. When calcium ions were prevented from entering the T cell, the movement of actin stopped almost entirely. Thus, the flow of calcium ions into the T cell through CRAC channels is essential for driving the actin movements that underlie immune synapse development and T cell activation.

In further experiments, Hartzell et al. tracked the movements of CRAC channels and actin at the synapse and found that actin filaments create a constricting “corral” that concentrates CRAC channels in the center of the synapse. Thus, by driving cytoskeleton movement, calcium ions also help to organize calcium channels at the immune synapse. Future work will focus on identifying the actin remodeling proteins that enable calcium ions to control this process.

DOI:http://dx.doi.org/10.7554/eLife.14850.002

F-actin remodeling defects as revealed in primary immunodeficiency disorders

Primary immunodeficiencies (PIDs) are a heterogeneous group of immune-related diseases. PIDs develop due to defects in gene-products that have consequences to immune cell function. A number of PID-proteins is involved in the remodeling of filamentous actin (f-actin) to support the generation of a contact zone between the antigen-specific T cell and antigen presenting cell (APC): the immunological synapse (IS). IS formation is the first step towards T-cell activation and essential for clonal expansion and acquisition of effector function. We here evaluated PIDs in which aberrant f-actin-driven IS formation may contribute to the PID disease phenotypes as seen in patients. We review examples of such contributions to PID phenotypes from literature, and highlight cases in which PID-proteins were evaluated for a role in f-actin polymerization and IS formation. We conclude with the proposition that patient groups might benefit from stratifying them in distinct functional groups in regard to their f-actin remodeling phenotypes in lymphocytes.

WASP-WIP complex in the molecular pathogenesis of Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is an X-linked primary immunodeficiency disease characterized by recurrent infection, thrombocytopenia, and eczema. The gene responsible for X-linked WAS encodes the Wiskott-Aldrich syndrome protein (WASP), which is expressed in hematopoietic cells and which regulates T-cell activation and cytoskeletal reorganization in T-cell receptor (TCR) signaling. Here, I review my recent research on WASP and the WASP-interacting protein (WIP) complex in T cells. I and my colleagues first established a diagnostic screening method using flow cytometry and genetic analysis, and elucidated the molecular pathogenesis in WAS patients with unique clinical manifestations. We investigated the mechanisms by which WASP is recruited to lipid rafts following TCR stimulation and to immunological synapses between antigen-presenting cells and T cells. Subsequently, we elucidated the molecular mechanisms by which WASP is degraded by calpain and ubiquitinated by Cbl-family proteins, which terminate WASP activation. More importantly, we found that WIP plays a critical role in WASP stability in T cells. These results provide new insights into the molecular pathogenesis of X-linked WAS and have facilitated the identification of WIP deficiency as an autosomal recessive form of WAS.

Cinnamomum cassia Suppresses Caspase-9 through Stimulation of AKT1 in MCF-7 Cells but Not in MDA-MB-231 Cells

Background

Cinnamomum cassia bark is a popular culinary spice used for flavoring and in traditional medicine. C. cassia extract (CE) induces apoptosis in many cell lines. In the present study, particular differences in the mechanism of the anti-proliferative property of C. cassia on two breast cancer cell lines, MCF-7 and MDA-MB-231, were elucidated.

Methodology/Principal Findings

The hexane extract of C. cassia demonstrated high anti-proliferative activity against MCF-7 and MDA-MB-231 cells (IC₅₀, 34±3.52 and 32.42 ±0.37 μg/ml, respectively). Oxidative stress due to disruption of antioxidant enzyme (SOD, GPx and CAT) activity is suggested as the probable cause for apoptosis initiation. Though the main apoptosis pathway in both cell lines was found to be through caspase-8 activation, caspase-9 was also activated in MDA-MB-231 cells but suppressed in MCF-7 cells. Gene expression studies revealed that AKT1, the caspase-9 suppressor, was up-regulated in MCF-7 cells while down-regulated in MDA-MB-231 cells. Although, AKT1 protein expression in both cell lines was down-regulated, a steady increase in MCF-7 cells was observed after a sharp decrease of suppression of AKT1. Trans-cinnamaldehyde and coumarin were isolated and identified and found to be mainly responsible for the observed anti-proliferative activity of CE (Cinnamomum cassia).

Conclusion

Activation of caspase-8 is reported for the first time to be involved as the main apoptosis pathway in breast cancer cell lines upon treatment with C. cassia. The double effects of C. cassia on AKT1 gene expression in MCF-7 cells is reported for the first time in this study.

Ubiquitylation as a Rheostat for TCR Signaling: From Targeted Approaches Toward Global Profiling

T cell receptor (TCR) signaling must be precisely tuned to limit collateral damage and prevent reactivity to self, while still allowing robust protective immune responses that control pathogen invasion. One process that can be used to promote, modify, or terminate TCR signaling is ubiquitylation. During ubiquitylation, ubiquitin is covalently attached to target proteins through a multistep process, in which E3 ubiquitin ligases promote the formation of ubiquitin chains on selected substrates. Ubiquitylation can facilitate protein–protein interactions, direct a protein to a specific subcellular location, or initiate protein destruction. Like phosphorylation, ubiquitylation is a reversible process – deubiquitylating enzymes counteract ligase function by removing ubiquitin chains. This reversibility also allows for ubiquitin chain “editing.” Based on an emerging wealth of information from genetic loss-of-function studies showing that deregulation of ubiquitylation pathways leads to immune dysfunction, it has become increasingly apparent that the dynamic process of ubiquitylation is critical for normal immune cell function. In this review, we will describe how ubiquitylation acts as a key modulator and integrator of signaling downstream of TCR engagement. Specifically, we highlight the known roles of the substrate-specific E3 ligases and deubiquitylating enzymes in TCR signaling and T cell activation. While it is clear that ubiquitin enzymes tune T cell signaling and T cell function, elucidating the molecular mechanisms by which these proteins modulate T cells has met with significant challenges. Identifying substrates of these enzymes has been a particular challenge, and thus substrates of many E3 ligases and deubiquitylating enzymes remain largely unknown. To that end, we discuss the promise, and some practical considerations, of using proteomics-based techniques for unbiased identification of putative substrates of ubiquitin cascade proteins within primary T cells. These methods provide an exciting opportunity for further defining how TCR signals are regulated and for identifying new targets for therapeutic modulation.

Analyses of Genetic and Clinical Parameters for Screening Patients With Inherited Thrombocytopenia with Small or Normal-Sized Platelets

BACKGROUND

Childhood thrombocytopenias include immune thrombocytopenic purpura (ITP) and inherited thrombocytopenia; the former is caused by autoantibodies to platelets, whereas the latter can be distinguished by platelet size and underlying genetic mutations. Due to limited methods for the definite diagnosis of ITP, genetic and clinical parameters are required for diagnosing inherited thrombocytopenias with small or normal-sized platelets.

PROCEDURE

In total, 32 Japanese patients with thrombocytopenia with small or normal-sized platelets from 29 families were enrolled. All the patients were under 20 years of age, with family histories of early-onset thrombocytopenia and/or poor response to conventional therapies for ITP. Genotypes and clinical parameters were retrospectively evaluated according to the disease type.

RESULTS

Twelve cases of inherited thrombocytopenia were observed. We identified chromosomal deletions within the WASP gene in two patients with Wiskott-Aldrich syndrome; a missense mutation in a patient with X-linked thrombocytopenia; and mutations in the RUNX1 gene of five patients with familial platelet disorder with propensity to acute myelogenous leukemia, and in the ANKRD26 gene of four patients with autosomal dominant thrombocytopenia-2. All 12 carried germline mutations, three of which were de novo. Furthermore, we observed significantly elevated serum thrombopoietin (TPO) levels and dysplasia of megakaryocytes in patients carrying the RUNX1 and ANKRD26 mutations.

CONCLUSIONS

Genetic analyses and detection of TPO levels and dysmegakaryopoiesis were clinically useful for screening patients with inherited thrombocytopenias, irrespective of the family history. We hypothesize that the WASP, RUNX1, and ANKRD26 genes are important for normal TPO signaling and the network underlying thrombopoiesis.

Maintaining intestinal health: the genetics and immunology of very early onset inflammatory bowel disease

Inflammatory bowel disease (IBD) is a multifactoral disease caused by dysregulated immune responses to commensal or pathogenic microbes in the intestine, resulting in chronic intestinal inflammation. An emerging population of patients with IBD occurring before the age of 5 represent a unique form of disease, termed Very Early Onset (VEO)-IBD, which is phenotypically- and genetically-distinct from older-onset IBD. VEO-IBD is associated with increased disease severity, aggressive progression and poor responsiveness to most conventional therapies. Further investigation into the causes and pathogenesis of VEO-IBD will help improve treatment strategies, and may lead to a better understanding of the mechanisms that are essential to maintain intestinal health or provoke the development of targeted therapeutic strategies to limit intestinal disease. Here we discuss the phenotypic nature of VEO-IBD, the recent identification of novel gene variants associated with disease, and functional immunologic studies interrogating the contribution of specific genetic variants to the development of chronic intestinal inflammation.

Signal Integration during T Lymphocyte Activation and Function: Lessons from the Wiskott-Aldrich Syndrome

Over the last decades, research dedicated to the molecular and cellular mechanisms underlying primary immunodeficiencies (PID) has helped to understand the etiology of many of these diseases and to develop novel therapeutic approaches. Beyond these aspects, PID are also studied because they offer invaluable natural genetic tools to dissect the human immune system. In this review, we highlight the research that has focused over the last 20 years on T lymphocytes from Wiskott–Aldrich syndrome (WAS) patients. WAS T lymphocytes are defective for the WAS protein (WASP), a regulator of actin cytoskeleton remodeling. Therefore, study of WAS T lymphocytes has helped to grasp that many steps of T lymphocyte activation and function depend on the crosstalk between membrane receptors and the actin cytoskeleton. These steps include motility, immunological synapse assembly, and signaling, as well as the implementation of helper, regulatory, or cytotoxic effector functions. The recent concept that WASP also works as a regulator of transcription within the nucleus is an illustration of the complexity of signal integration in T lymphocytes. Finally, this review will discuss how further study of WAS may contribute to solve novel challenges of T lymphocyte biology.

Tyrosine phosphorylation of WIP releases bound WASP and impairs podosome assembly in macrophages

Podosomes are integrin-containing adhesion structures commonly found in migrating leukocytes of the monocytic lineage. The actin cytoskeletal organisation of podosomes is based on a WASP- and Arp2/3-mediated mechanism. WASP also associates with a second protein, WIP (also known as WIPF1), and they co-localise in podosome cores. Here, we report for the first time that WIP can be phosphorylated on tyrosine residues and that tyrosine phosphorylation of WIP is a trigger for release of WASP from the WIP-WASP complex. Using a knockdown approach together with expression of WIP phosphomimics, we show that in the absence of WIP-WASP binding, cellular WASP is rapidly degraded, leading to disruption of podosomes and a failure of cells to degrade an underlying matrix. In the absence of tyrosine phosphorylation, the WIP-WASP complex remains intact and podosome lifetimes are extended. A screen of candidate kinases and inhibitor-based assays identified Bruton's tyrosine kinase (Btk) as a regulator of WIP tyrosine phosphorylation. We conclude that tyrosine phosphorylation of WIP is a crucial regulator of WASP stability and function as an actin-nucleation-promoting factor.

Actin cytoskeletal defects in immunodeficiency

The importance of the cytoskeleton in mounting a successful immune response is evident from the wide range of defects that occur in actin-related primary immunodeficiencies (PIDs). Studies of these PIDs have revealed a pivotal role for the actin cytoskeleton in almost all stages of immune system function, from hematopoiesis and immune cell development, through to recruitment, migration, intercellular and intracellular signaling, and activation of both innate and adaptive immune responses. The major focus of this review is the immune defects that result from mutations in the Wiskott-Aldrich syndrome gene (WAS), which have a broad impact on many different processes and give rise to clinically heterogeneous immunodeficiencies. We also discuss other related genetic defects and the possibility of identifying new genetic causes of cytoskeletal immunodeficiency.

Coordinate control of cytoskeletal remodeling and calcium mobilization during T-cell activation

Ca(2+) mobilization and cytoskeletal reorganization are key hallmarks of T-cell activation, and their interdependence has long been recognized. Recent advances in the field have elucidated the molecular pathways that underlie these events and have revealed several points of intersection. Ca(2+) signaling can be divided into two phases: initial events leading to release of Ca(2+) from endoplasmic reticulum stores, and a second phase involving STIM 1 (stromal interaction molecule 1) clustering and CRAC (calcium release activated calcium) channel activation. Cytoskeletal dynamics promote both phases. During the first phase, the actin cytoskeleton promotes mechanotransduction and serves as a dynamic scaffold for microcluster assembly. Proteins that drive actin polymerization such as WASp (Wiskott-Aldrich syndrome protein) and HS1 (hematopoietic lineage cell-specific protein 1) promote signaling through PLCγ1 (phospholipase Cγ1) and release of Ca(2+) from endoplasmic reticulum stores. During the second phase, the WAVE (WASP-family verprolin homologous protein) complex and the microtubule cytoskeleton promote STIM 1 clustering at sites of plasma membrane apposition, opening Orai channels. In addition, gross cell shape changes and organelle movements buffer local Ca(2+) levels, leading to sustained Ca(2+) mobilization. Conversely, elevated intracellular Ca(2+) activates cytoskeletal remodeling. This can occur indirectly, via calpain activity, and directly, via Ca(2+) -dependent cytoskeletal regulatory proteins such as myosin II and L-plastin. While it is true that the cytoskeleton regulates Ca(2+) responses and vice versa, interdependence between Ca(2+) and the cytoskeleton also encompasses signaling events that occur in parallel, downstream of shared intermediates. Inositol cleavage by PLCγ1 simultaneously triggers both endoplasmic reticulum store release and diacylglycerol-dependent microtubule organizing center reorientation, while depleting the pool of phosphatidylinositol-4,5-bisphosphate, an activator of multiple actin-regulatory proteins. The close interdependence of Ca(2+) signaling and cytoskeletal dynamics in T cells provides positive feedback mechanisms for T-cell activation and allows for finely tuned responses to extracellular cues.

Advances in basic and clinical immunology in 2013

A significant number of contributions to our understanding of primary immunodeficiencies (PIDs) in pathogenesis, diagnosis, and treatment were published in the Journal in 2013. For example, deficiency of mast cell degranulation caused by signal transducer and activator of transcription 3 deficiency was demonstrated to contribute to the difference in the frequency of severe allergic reactions in patients with autosomal dominant hyper-IgE syndrome compared with that seen in atopic subjects with similar high IgE serum levels. High levels of nonglycosylated IgA were found in patients with Wiskott-Aldrich syndrome, and these abnormal antibodies might contribute to the nephropathy seen in these patients. New described genes causing immunodeficiency included caspase recruitment domain 11 (CARD11), mucosa-associated lymphoid tissue 1 (MALT1) for combined immunodeficiencies, and tetratricopeptide repeat domain 7A (TTC7A) for mutations associated with multiple atresia with combined immunodeficiency. Other observations expand the spectrum of clinical presentation of specific gene defects (eg, adult-onset idiopathic T-cell lymphopenia and early-onset autoimmunity might be due to hypomorphic mutations of the recombination-activating genes). Newborn screening in California established the incidence of severe combined immunodeficiency at 1 in 66,250 live births. The use of hematopoietic stem cell transplantation for PIDs was reviewed, with recommendations to give priority to research oriented to establish the best regimens to improve the safety and efficacy of bone marrow transplantation. These represent only a fraction of significant research done in patients with PIDs that has accelerated the quality of care of these patients. Genetic analysis of patients has demonstrated multiple phenotypic expressions of immune deficiency in patients with nearly identical genotypes, suggesting that additional genetic factors, possibly gene dosage, or environmental factors are responsible for this diversity.