Wiskott-Aldrich Research Articles

Inborn Error of WAS Presenting with SARS-CoV-2-Related Multisystem Inflammatory Syndrome in Children.

Multisystem inflammatory syndrome in children (MIS-C) has been reported in patients with inborn errors of immunity (IEI), providing insights into disease pathogenesis. Here, we present the first case of MIS-C in a child affected by Wiskott-Aldrich syndrome (WAS) gene mutation, elucidating underlying predisposing factors and the involved inflammatory pathways. Genetic analysis revealed a frameshift truncating variant in the WAS gene, resulting in WAS protein expression between mild and severe forms, despite a clinical phenotype resembling X-linked thrombocytopenia (XLT). IL-1β secretion by LPS-stimulated peripheral blood mononuclear cells from patient during MIS-C was lower compared to healthy subjects but increased during follow-up. Conversely, the percentage of ASC (apoptosis-associated speck-like protein containing a CARD) specks in the patient's circulating monocytes during the acute phase was higher than in healthy subjects. The type I interferon (IFN) signature during MIS-C was normal, in contrast to the raised IFN signature measured far from the acute event. This case supports the association of IEI with MIS-C, potentially linked to delayed immune responses to SARS-CoV-2. The XLT phenotype underlies a subclinical immunodysregulation involving the NLRP3 inflammasome and the type-I IFN response.

Disseminated Bacille Calmette-Guérin Vaccine-Induced Disease in a Sample of Iranian Children: A Longitudinal Case-Series Study

Background: Approximately a quarter-million Iranian newborns receive the Bacille Calmette-Guérin (BCG) vaccine annually. However, in immunodeficient infants, this vaccine can lead to disseminated BCG disease (DBD). Objectives: This study aims to determine the clinical presentation, comorbidities, underlying immunodeficiency, and prognosis of DBD in a group of Iranian children. Methods: In a longitudinal case-series study, data were collected from the files of infants and children diagnosed with DBD from 2005 to 2017. Immunodeficiency screening was conducted for each patient. For children with normal immunodeficiency screening results, further testing for Mendelian susceptibility to mycobacterial diseases (MSMD) was performed. Detection of DBD in children was achieved by evaluating gastric lavage and bone marrow aspiration samples for mycobacterium. Results: Most of the 22 patients were immunocompromised, with the following distribution: 31.8% had severe combined immunodeficiency (SCID), 45.5% had MSMD (specifically IL-12Rβ1 deficiency), one patient (4.5%) had Wiskott-Aldrich syndrome (WAS), and the remaining 18.1% had unknown immunodeficiency types. Most patients with MSMD were successfully treated and did not show relapse during the follow-up period, even after discontinuing anti-tuberculosis (TB) medications. Conclusions: Due to the similarity of its manifestations to sepsis, diagnosing systemic infections caused by the BCG vaccine in children requires a high level of clinical suspicion and appropriate diagnostic measures, such as mycobacterial culture, biochemical speciation, or polymerase chain reaction (PCR). These diagnostic steps should be taken promptly in cases of DBD, with concurrent treatment using anti-tuberculosis drugs and, if possible, targeted therapies for underlying immunodeficiency.

Comprehensive newborn screening for severe combined immunodeficiency, X-linked agammaglobulinemia, and spinal muscular atrophy: the Chinese experience.

Newborn screening (NBS) for severe combined immunodeficiency (SCID), X-linked agammaglobulinemia (XLA), and spinal muscular atrophy (SMA) enables early diagnosis and intervention, significantly improving patient outcomes. Advances in real-time polymerase chain reaction (PCR) technology have been instrumental in facilitating their inclusion in NBS programs. We employed multiplex real-time PCR to simultaneously detect T-cell receptor excision circles (TRECs), kappa-deleting recombination excision circles (KRECs), and the absence of the survival motor neuron (SMN) 1 gene in dried blood spots from 103,240 newborns in Zhejiang Province, China, between July 2021 and December 2022. Of all the samples, 122 were requested further evaluation. After flow cytometry evaluation and/or genetic diagnostics, we identified one patient with SCID, two patients with XLA, nine patients with SMA [one of whom also had Wiskott-Aldrich Syndrome (WAS)], and eight patients with other medical conditions. The positive predictive values (PPVs) of NBS for SCID, XLA, and SMA were 2.44%, 2.78%, and 100%, respectively. The estimated prevalence rates in the Chinese population were 1 in 103,240 for SCID, 1 in 51,620 for XLA, and 1 in 11,471 for SMA. This study represents the first large-scale screening in mainland China using a TREC/KREC/SMN1 multiplex assay, providing valuable epidemiological data. Our findings suggest that this multiplex assay is an effective screening method for SCID, XLA, and SMA, potentially supporting the universal implementation of NBS programs across China.

A first-in-class Wiskott-Aldrich syndrome protein activator with antitumor activity in hematologic cancers.

Hematologic cancers are among the most common cancers in adults and children. Despite significant improvements in therapies, many patients still succumb to the disease. Therefore, novel therapies are needed. The Wiskott-Aldrich syndrome protein (WASp) family regulates actin assembly in conjunction with the Arp2/3 complex, a ubiquitous nucleation factor. WASp is expressed exclusively in hematopoietic cells and exists in two allosteric conformations: autoinhibited or activated. Here, we describe the development of EG-011, a first-in-class small molecule activator of the autoinhibited form of WASp. EG-011 possesses in vitro and in vivo antitumor activity as a single agent in lymphoma, leukemia, and multiple myeloma, including models of secondary resistance to PI3K, BTK, and proteasome inhibitors. The in vitro activity was confirmed in a lymphoma xenograft. Actin polymerization and WASp binding were demonstrated using multiple techniques. Transcriptome analysis highlighted homology with drugs inducing actin polymerization.

WAVE1 and WAVE2 facilitate human papillomavirus-driven actin polymerization during cellular entry.

Human PapillomavirusType 16 (HPV16) is an etiological agent of human cancers that requires endocytosis to initiate infection. HPV16 entry into epithelial cells occurs through a non-canonical endocytic pathway that is actin-driven, but it is not well understood how HPV16-cell surface interactions trigger actin reorganization in a way that facilitates entry. This study provides evidence that Wiskott-Aldrich syndrome protein family verprolin-homologous proteins 1 and 2 (WAVE1 and WAVE2) are molecular mediators of the actin polymerization that facilitates HPV endocytosis and intracellular trafficking. We demonstrate through post-transcriptional gene silencing and genome editing that WAVE1 and WAVE2 are critical for efficient HPV16 infection, and that restoration of each in knockout cells rescues HPV16 infection. Cells lacking WAVE1, WAVE2, or both, internalize HPV16 at a significantly reduced rate. Analysis of fluorescently labeled cells exposed to HPV16 and acquired by confocal fluorescence microscopy revealed that HPV16, WAVE1, WAVE2, and actin are all colocalized at the cellular dorsal surface. We also found that HPV16 stimulates WAVE1 and WAVE2-mediated cellular dorsal surface filopodia formation during the viral endocytic process. Taken together, this study provides evidence that the HPV endocytic process needed for infection is controlled by actin reorganization into filopodial protrusions and that this process is mediated by WAVE1 and WAVE2.

Pathophysiology of Congenital High Production of IgE and Its Consequences: A Narrative Review Uncovering a Neglected Setting of Disorders.

Elevated serum IgE levels serve as a critical marker for uncovering hidden immunological disorders, particularly inborn errors of immunity (IEIs), which are often misdiagnosed as common allergic conditions. IgE, while typically associated with allergic diseases, plays a significant role in immune defense, especially against parasitic infections. However, extremely high levels of IgE can indicate more severe conditions, such as Hyper-IgE syndromes (HIES) and disorders with similar features, including Omenn syndrome, Wiskott-Aldrich syndrome, and IPEX syndrome. Novel insights into the genetic mutations responsible for these conditions highlight their impact on immune regulation and the resulting clinical features, including recurrent infections, eczema, and elevated IgE. This narrative review uniquely integrates recent advances in the genetic understanding of IEIs and discusses how these findings impact both diagnosis and treatment. Additionally, emerging therapeutic strategies, such as hematopoietic stem cell transplantation (HSCT) and gene therapies, are explored, underscoring the potential for personalized treatment approaches. Emphasizing the need for precise diagnosis and tailored interventions aims to enhance patient outcomes and improve the quality of care for those with elevated IgE levels and associated immunological disorders.

X-linked Thrombocytopenia with Normal Wiskott-Aldrich Syndrome Protein Expression in Lymphocytes and a Novel Wiskott-Aldrich Syndrome Protein Gene Variant: A Case Report and Brief Review of the Literature

We present a case of X-linked thrombocytopenia (XLT) with a novel WAS gene variant expressing a normal amount of Wiskott-Aldrich syndrome protein (WASp) in lymphocytes. XLT usually reduces WASp expression not only in platelets but also in lymphocytes. However, there were cases, such as the present one, in which WASp was expressed normally in lymphocytes and absent only in platelets. Our finding suggests that it is greater diagnostic sensitivity to perform an expression analysis of WASp in both platelets and lymphocytes when XLT is suspected.

A human DCC variant causing mirror movement disorder reveals that the WAVE regulatory complex mediates axon guidance by netrin-1-DCC.

The axon guidance cue netrin-1 signals through its receptor DCC (deleted in colorectal cancer) to attract commissural axons to the midline. Variants in DCC are frequently associated with congenital mirror movements (CMMs). A CMM-associated variant in the cytoplasmic tail of DCC is located in a conserved motif predicted to bind to a regulator of actin dynamics called the WAVE (Wiskott-Aldrich syndrome protein-family verprolin homologous protein) regulatory complex (WRC). Here, we explored how this variant affects DCC function and may contribute to CMM. We found that a conserved WRC-interacting receptor sequence (WIRS) motif in the cytoplasmic tail of DCC mediated the interaction between DCC and the WRC. This interaction was required for netrin-1-mediated axon guidance in cultured rodent commissural neurons. Furthermore, the WIRS motif of Fra, the Drosophila DCC ortholog, was required for attractive signaling in vivo at the Drosophila midline. The CMM-associated R1343H variant of DCC, which altered the WIRS motif, prevented the DCC-WRC interaction and impaired axon guidance in cultured commissural neurons and in Drosophila. The findings reveal the WRC as a pivotal component of netrin-1-DCC signaling and uncover a molecular mechanism explaining how a human genetic variant in the cytoplasmic tail of DCC may lead to CMM.

Clinical Manifestations of Wiskott-Aldrich Syndrome in an Iranian Patient

Wiskott-Aldrich Syndrome (WAS) is an immunodeficiency disorder resulting from genetic mutations in the WAS protein (WASP) gene in the X chromosome, characterized by thrombocytopenia, eczema, and infections. This case report focused on a 12-year-old Iranian male with WAS with a history of Crohn’s disease, meningitis, and bilateral hernia. His WAS was diagnosed at age six with a hemizygous c.777+1 G>A mutation in the WASP gene. The patient was referred to our clinic with symptoms including fever, abdominal pain, thrombocytopenia, and elevated ESR. Clinical Imaging revealed a significant lung nodule align bronchiectasis, mild ascites, bilateral epididymitis, and lymphadenopathy. Nephrotic syndrome with proteinuria and low levels of albumin have been observed. After six months of receiving intravenous immunoglobulin (IVIG) therapy in addition to antibiotics and antivirals, the patient suffered from arthritis, edema, and fever. Our WAS patient presented the late comorbidity of renal involvement, which highlights the monitoring of this patient, such as those involved in chronic infections. Therefore, a precise treatment approach is needed to manage either the primary immunodeficiency or the late-discovered diseases.

Multifaceted role of the actin-binding protein WIP: Promotor and inhibitor of tumor progression and dissemination.

Cancer cells depend on actin cytoskeleton reorganization to achieve hallmark malignant functions including abnormal activation, proliferation, migration and invasiveness. (Neural)-Wiskott-Aldrich Syndrome protein ((N-)WASP) binds actin and forms a complex with the WASP-interacting protein (WIP), which plays a critical role in regulating the actin cytoskeleton, through (N)-WASP-dependent and independent functions. Mutations in the WIP gene (WIPF1) lead to severe early onset immunodeficiency in humans and severe autoimmunity and shortened lifespan in mice. This review covers the available evidence about the physiological role of WIP in different tissues and its contribution to human disease, focusing on cancer. In solid tumors overexpression of WIP has mostly been associated with tumor initiation, progression and dissemination through matrix degradation by invadopodia, while a suppressive function has been shown for WIP in certain hematological cancers. Interestingly, a minority of studies suggest a protective role for WIP in specific tumor contexts. These data support the need for further research to fully understand the mechanisms underlying WIP's diverse functions in health and disease and raise important questions for future work.

Somatic reversion in Wiskott-Aldrich syndrome: Case reports and mechanistic insights.

This report describes two brothers from India and a Chinese patient with somatic reversion of an inherited deleterious mutation in the WAS gene. Both the Indian siblings had inherited a single nucleotide deletion causing a frameshift mutation (c.1190del, p.Pro397Argfs*48) (variant 1: marked in blue) from the mother. Another variant (variant 2: marked in red), a 12-nucleotide deletion at position 1188-1199 (c.1188_1199del, p.P401_P404del) was also found, which resulted in restoration of the frame and subsequent rescue of the protein sequence. DNA sequencing from buccal mucosal cells revealed only the inherited variant (variant 1), while no reversion mutation was identified in the mucosal cells. Similarly, the Chinese patient was found to have a novel germline 14-base duplication (ACGAAAATGCTTGG) c.120_132 + 1dup (variant 1). This resulted in abolishment of the original splice junction coupled with the creation of a new junction 14 bases 3' and a frameshift mutation with predicted protein truncation p. Thr45Aspfs*. DNA from the patient's PBMC showed co-existence of wild-type and mutated sequences, but only the mutant was present in the buccal cells. Genomic and mRNA analysis of the isolated CD3+ T lymphocytes, CD3- mononuclear cells, and EBV-transformed B lymphocytes indicated that the reverant variant (germline variant was restored to wild-type sequence) were selectively found in CD3+ T lymphocytes.

Structural basis for coupling of the WASH subunit FAM21 with the endosomal SNX27–Retromer complex

Endosomal membrane trafficking is mediated by specific protein coats and formation of actin-rich membrane domains. The Retromer complex coordinates with sorting nexin (SNX) cargo adaptors including SNX27, and the SNX27-Retromer assembly interacts with the Wiskott-Aldrich syndrome protein and SCAR homolog (WASH) complex which nucleates actin filaments establishing the endosomal recycling domain. Crystal structures, modeling, biochemical, and cellular validation reveal how the FAM21 subunit of WASH interacts with both Retromer and SNX27. FAM21 binds the FERM domain of SNX27 using acidic-Asp-Leu-Phe (aDLF) motifs similar to those found in the SNX1 and SNX2 subunits of the ESCPE-1 complex. Overlapping FAM21 repeats and a specific Pro-Leu containing motif bind three distinct sites on Retromer involving both the VPS35 and VPS29 subunits. Mutation of the major VPS35-binding site does not prevent cargo recycling; however, it partially reduces endosomal WASH association indicating that a network of redundant interactions promote endosomal activity of the WASH complex. These studies establish the molecular basis for how SNX27-Retromer is coupled to the WASH complex via overlapping and multiplexed motif-based interactions required for the dynamic assembly of endosomal membrane recycling domains.

PSIP1 promotes gefitinib resistance in lung adenocarcinoma by inducing the expression of WASF3 and its downstream ITGB3/AKT signaling.

Gefitinib (GR), a representative drug of epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI), is a key pillar in the treatment of lung adenocarcinoma (LUAD), but drug resistance is universal. Identifying the potential factors of drug resistance to GR is essential to treat patients with EGFR mutant LUAD. The GR-resistant LUAD cells were established and confirmed by MTT assay. The effects of PC4 and SRSF1 interacting protein 1 (PSIP1) on GR-resistant cell proliferation and apoptosis in vitro and in vivo were detected by colony formation, flow cytometry, tumor-bearing animal model, immunohistochemistry, and TUNEL staining. Western blotting and qPCR were used to determine the expression of relevant markers. The effect of PSIP1 on the promoter region of Wiskott-Aldrich syndrome protein family member 3 (WASF3) was detected by the dual-luciferase assay. The interaction between PSIP1 and RNA polymerase II was evaluated using ChIP-qPCR and Co-IP assays. PSIP1 was highly enriched in GR-resistant LUAD cells. Downregulation of PSIP1 expression significantly inhibited the proliferation of LUAD-resistant cells and promoted apoptosis. WASF3 was shown to have similar effects as PSIP1 in promoting drug resistance in LUAD cells. PSIP1 promoted the transcriptional activity of WASF3, which was achieved by increasing RNA polymerase II recruitment on the WASF3 promoter. Furthermore, PSIP1 positively regulated the expression of the pro-EGFR-TKI resistance factor integrin subunit beta 3 (ITGB3). Our work suggests that PSIP1 promotes resistance to GR in LUAD cells by inducing the expression of WASF3 and its downstream regulator ITGB3.

Interleukin-1 blockade in patients with Wiskott-Aldrich syndrome: a retrospective multinational case series

AbstractUp to 70% of patients with Wiskott-Aldrich syndrome (WAS) develop autoimmune and inflammatory manifestations. Dysregulation of interleukin 1 (IL-1) may be involved in their pathogenesis, yet there is little evidence on treatment with anti–IL-1 agents in these patients. We conducted a multicenter retrospective analysis of 9 patients with WAS treated with anti–IL-1 agents (anakinra or canakinumab). All patients had prominent inflammatory manifestations, including systemic, cutaneous, articular, and intestinal symptoms; 3 patients presented with a severe systemic inflammatory syndrome since the first months of life. Corticosteroid therapy was associated with partial or no response, whereas treatment with anakinra or canakinumab resulted in prompt, often dramatic, responses in all patients, allowing bridging to gene therapy (4 patients) or hematopoietic stem cell transplantation (HSCT; 5 patients). Treatment was overall well tolerated. Low donor myeloid chimerism developed in 4 patients after HSCT and was associated with the appearance or the recurrence of inflammatory manifestations. A second HSCT was performed in 2 patients, achieving full-donor chimerism and resolution of inflammatory manifestation, whereas the other 2 patients were treated with prolonged therapy with anti–IL-1 agents. Our experience demonstrates that some inflammatory manifestations of WAS are dependent on IL-1 and respond well to its pharmacologic blockade.

The sufficiency of genetic diagnosis in managing patients with inborn errors of immunity during prenatal care and childbearing.

Individuals with inborn errors of immunity face challenges in fertility, pregnancy, and genetic disorder transmission. Prenatal genetic counseling is crucial, especially in tribal societies with consanguineous unions. Ten families with confirmed inborn errors of immunity were studied, revealing diverse pregnancy decisions: An architect with autosomal dominant STAT-1 gain of function underwent prenatal diagnosis despite initial plans for preimplantation genetic diagnosis. In a consanguineous family, two children died from leukocyte adhesion deficiency type 1 because the father refused prenatal diagnosis. First cousins opted against terminating the second pregnancy, resulting in two children affected by Bruton disease. Another consanguineous couple, with two children afflicted by ataxia-telangiectasia, chose oocyte donation for their third child, ensuring a healthy birth. Recurrent pregnancy loss was observed in a mother subsequently diagnosed with ZAP70 deficiency. A mother with Wiskott-Aldrich syndrome child opted for in vitro fertilization, leading to a healthy birth post-prenatal diagnosis. A misdiagnosis of anaplastic anemia occurred in a family with multiple instances of Wiskott-Aldrich syndrome. A leukocyte adhesion deficiency type 1 case led to parental dissolution due to the father's refusal to acknowledge the condition. In a non-consanguineous couple, the father's diagnosis of TACI deficiency influenced the mother's decision to discontinue pregnancy post-prenatal diagnosis. Genetic diagnosis alone cannot optimize prenatal care for immune dysregulation disorders. Various factors, including patient education, societal norms, ethics, and economics, impact pregnancy decisions. Clinical immunologists must integrate these elements into guidance strategies to enhance patient outcomes.

Role of WAVE3 as an of actin binding protein in the pathology of triple negative breast cancer.

Breast cancer, a prevalent global health concern, has sparked extensive research efforts, particularly focusing on triple negative breast cancer (TNBC), a subtype lacking estrogen receptor (ER), progesterone receptor, and epidermal growth factor receptor. TNBC's aggressive nature and resistance to hormone-based therapies heightens the risk of tumor progression and recurrence. Actin-binding proteins, specifically WAVE3 from the Wiskott-Aldrich syndrome protein (WASP) family, have emerged as major drivers in understanding TNBC biology. This review delves into the intricate molecular makeup of TNBC, shedding light on actin's fundamental role in cellular processes. Actin, a structural element in the cytoskeleton, regulates various cellular pathways essential for homeostasis. Its dynamic nature enables functions such as cell migration, motility, intracellular transport, cell division, and signal transduction. Actin-binding proteins, including WAVE3, play pivotal roles in these processes. WAVE3, a member of the WASP family, remains the focus of this review due to its potential involvement in TNBC progression. While actin-binding proteins are studied for their roles in healthy cellular cycles, their significance in TNBC remains underexplored. This review aims to discuss WAVE3's impact on TNBC, exploring its molecular makeup, functions, and significance in tumor progression. The intricate structure of WAVE3, featuring elements like the verprolin-cofilin-acidic domain and regulatory elements, plays a crucial role in regulating actin dynamics. Dysregulation of WAVE3 in TNBC has been linked to enhanced cell migration, invasion, extracellular matrix remodeling, epithelial-mesenchymal transition, tumor proliferation, and therapeutic resistance. Understanding the role of actin-binding proteins in cancer biology has potential clinical implications, making them potential prognostic biomarkers and promising therapeutic targets. The review emphasizes the need for further research into actin-binding proteins' clinical applications, diagnostic value, and therapeutic interventions. In conclusion, this comprehensive review explores the complex interplay between actin and actin-binding proteins, with special emphasis on WAVE3, in the context of TNBC. By unraveling the molecular intricacies, structural characteristics, and functional significance, the review paves the way for future research directions, clinical applications, and potential therapeutic strategies in the challenging landscape of TNBC.

Brucella NpeA is a secreted Type IV effector containing an N-WASP-binding short linear motif that promotes niche formation.

The modulation of actin polymerization is a common theme among microbial pathogens. Even though microorganisms show a wide repertoire of strategies to subvert the activity of actin, most of them converge in the ones that activate nucleating factors, such as the Arp2/3 complex. Brucella spp. are intracellular pathogens capable of establishing chronic infections in their hosts. The ability to subvert the host cell response is dependent on the capacity of the bacterium to attach, invade, avoid degradation in the phagocytic compartment, replicate in an endoplasmic reticulum-derived compartment and egress. Even though a significant number of mechanisms deployed by Brucella in these different phases have been identified and characterized, none of them have been described to target actin as a cellular component. In this manuscript, we describe the identification of a novel virulence factor (NpeA) that promotes niche formation. NpeA harbors a short linear motif (SLiM) present within an amphipathic alpha helix that has been described to bind the GTPase-binding domain (GBD) of N-WASP and stabilizes the autoinhibited state. Our results show that NpeA is secreted in a Type IV secretion system-dependent manner and that deletion of the gene diminishes the intracellular replication capacity of the bacterium. In vitro and ex vivo experiments demonstrate that NpeA binds N-WASP and that the short linear motif is required for the biological activity of the protein.IMPORTANCEThe modulation of actin-binding effectors that regulate the activity of this fundamental cellular protein is a common theme among bacterial pathogens. The neural Wiskott-Aldrich syndrome protein (N-WASP) is a protein that several pathogens target to hijack actin dynamics. The highly adapted intracellular bacterium Brucella has evolved a wide repertoire of virulence factors that modulate many activities of the host cell to establish successful intracellular replication niches, but, to date, no effector proteins have been implicated in the modulation of actin dynamics. We present here the identification of a virulence factor that harbors a short linear motif (SLiM) present within an amphipathic alpha helix that has been described to bind the GTPase-binding domain (GBD) of N-WASP stabilizing its autoinhibited state. We demonstrate that this protein is a Type IV secretion effector that targets N-WASP-promoting intracellular survival and niche formation.

Polyglutamine binding protein 1 regulates neurite outgrowth through recruiting N-WASP

Neurite outgrowth is a critical step in neural development, leading to the generation of neurite branches that allow individual neurons to make contacts with multiple neurons within the target region. Polyglutamine-binding protein 1 (PQBP1) is a highly conserved protein with a key role in neural development. Our recent mass spectrometric analysis showed that PQBP1 associates with neural Wiskott-Aldrich syndrome protein (N-WASP), an important actin polymerization-promoting factor involved in neurite outgrowth. Here, we report that the WW domain of PQBP1 directly interacts with the proline-rich domain of N-WASP. The disruption of this interaction leads to impaired neurite outgrowth and growth cone size. Furthermore, we demonstrate that PQBP1/N-WASP interaction is critical for the recruitment of N-WASP to the growth cone, but does not affect N-WASP protein levels or N-WASP-induced actin polymerization. Our results indicated that PQBP1 regulates neurite outgrowth by recruiting N-WASP to the growth cone, thus representing an alternative molecular mechanism via which PQBP1-mediates neurite outgrowth.

Update on the Use of Thrombopoietin-Receptor Agonists in Pediatrics.

This review summarizes the rationale and current data on the use of thrombopoietin receptor agonists (TPO-RAs) for treating severe thrombocytopenia in infants, children, and adolescents. It focuses on substances that have been approved by the U.S. Food and Drug Administration (FDA) and European Medicines Agency (EMA) for pediatric patients. Romiplostim and eltrombopag are already established as second-line treatment for persistent or chronic immune thrombocytopenia (ITP). As in adults, TPO-RAs are currently also evaluated in severe aplastic anemia (SAA), chemotherapy-induced thrombocytopenia (CIT), myelodysplastic syndromes (MDS), and poor engraftment after hematopoietic stem cell transplantation in pediatric and adolescent patients. Moreover, studies on the implication of TPO-RA in treating rare inherited thrombocytopenias, such as Wiskott-Aldrich syndrome (WAS), congenital amegakaryocytic thrombocytopenia (CAMT), or MYH9-associated thrombocytopenia, deserve future attention. Current developments include testing of avatrombopag and lusutrombopag that are approved for the treatment of thrombocytopenia associated with chronic liver disease (CLD) in adult patients. In pediatric and adolescent medicine, we expect in the near future a broader use of TPO-RAs as first-line treatment in primary ITP, thereby considering immunomodulatory effects that increase the rate of sustained remission off-treatment, and a selective use in rare inherited thrombocytopenias based on current clinical trials.

Mechanisms of actin filament severing and elongation by formins.

Humans express 15 formins that play crucial roles in actin-based processes, including cytokinesis, cell motility and mechanotransduction1,2. However, the lack of structures bound to the actin filament (F-actin) has been a major impediment to understanding formin function. Whereas formins are known for their ability to nucleate and elongate F-actin3-7, some formins can additionally depolymerize, sever or bundle F-actin. Two mammalian formins, inverted formin 2 (INF2) and diaphanous 1 (DIA1, encoded by DIAPH1), exemplify this diversity. INF2 shows potent severing activity but elongates weakly8-11 whereas DIA1 has potent elongation activity but does not sever4,8. Using cryo-electron microscopy (cryo-EM)we show five structural states of INF2 and two of DIA1 bound to the middle and barbed end of F-actin. INF2 and DIA1 bind differently to these sites, consistent with their distinct activities. The formin-homology 2 and Wiskott-Aldrich syndrome protein-homology 2 (FH2 andWH2, respectively) domains of INF2 are positioned to sever F-actin, whereas DIA1 appears unsuited for severing. These structures also show how profilin-actin is delivered to the fast-growing barbed end, and how this is followed by a transition of the incoming monomer into the F-actin conformation and the release of profilin. Combined, the seven structures presented here provide step-by-step visualization of the mechanisms of F-actin severing and elongation by formins.