Rapid platelet turnover in WASP(-) mice correlates with increased ex vivo phagocytosis of opsonized WASP(-) platelets

High ploidy large cytoplasmic megakaryocytes are hematopoietic stem cells regulators and essential for platelet production

Megakaryocytes (MK) generate platelets. Recently, we and others, have reported MK also regulate hematopoietic stem cells (HSC). Here we show high ploidy large cytoplasmic megakaryocytes (LCM) are critical negative regulators of HSC and critical for platelet formation. Using a mouse knockout model (Pf4-Srsf3^Δ/Δ) with normal MK numbers, but essentially devoid of LCM, we demonstrate a pronounced increase in BM HSC concurrent with endogenous mobilization and extramedullary hematopoiesis. Severe thrombocytopenia is observed in animals with diminished LCM, although there is no change in MK ploidy distribution, uncoupling endoreduplication and platelet production. When HSC isolated from a microenvironment essentially devoid of LCM reconstitute hematopoiesis in lethally irradiated mice, the absence of LCM increases HSC in BM, blood and spleen, and the recapitulation of thrombocytopenia. In contrast, following a competitive transplant using minimal numbers of WT HSC together with HSC from a microenvironment with diminished LCM, sufficient WT HSC-generated LCM regulates a normal HSC pool and prevents thrombocytopenia. Importantly, LCM are conserved in humans.

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.

Yunnan Baiyao Adjuvant Treatment for Patients with Hemoptysis: A Systematic Review and Meta-Analysis

Background

Yunnan Baiyao (YNBY) is a traditional Chinese medicine used to treat bleeding. We evaluated the efficacy of YNBY plus conventional pharmaceutical treatment (CPT) versus CPT alone in patients with hemoptysis.

Methods

A total of eight electronic databases were searched. The outcomes in the included studies were effective rate, hemoptysis volume, duration of hemoptysis and hospitalization, number of cases requiring endotracheal intubation, and adverse events (AEs). The studies were used to calculate risk ratios (RRs) or mean differences (MDs) with corresponding 95% confidence intervals. Risk of bias for included trials was assessed using the Cochrane risk of bias tool.

Results

Thirteen RCTs were analyzed consisting of a total of 1379 patients. Treatment with YNBY + CPT had a greater effective rate than CPT alone (RR: 1.18; 95% CI: 1.13 to 1.23; P < 0.001; I² = 0%), a lower hemoptysis volume (MD: −107.37; 95% CI: −121.69 to −93.06; P < 0.001; I² = 0%), a shorter duration of hemoptysis (MD: −2.70; 95% CI: −2.96 to 2.43; P < 0.001; I² = 0%) and hospitalization (MD: −2.38; 95% CI: −2.93 to −1.83; P < 0.001; I² = 9%), and a reduction in the incidence of AEs (RR: 0.34; 95% CI: 0.23 to 0.51; P < 0.001; I² = 0%). YNBY + CPT treatment provided no significant difference in reducing the number of cases requiring endotracheal intubation compared to CPT alone (RR: 0.49; 95% CI: 0.15 to 1.60; P=0.24; I² = 0%).

Conclusion

YNBY plus CPT showed better efficacy than CPT for patients with hemoptysis. Our study provides medical evidence for the efficacy and safety of YNBY for hemoptysis.

The Role of WAVE2 Signaling in Cancer

The Wiskott–Aldrich syndrome protein (WASP) and WASP family verprolin-homologous protein (WAVE)—WAVE1, WAVE2 and WAVE3 regulate rapid reorganization of cortical actin filaments and have been shown to form a key link between small GTPases and the actin cytoskeleton. Upon receiving upstream signals from Rho-family GTPases, the WASP and WAVE family proteins play a significant role in polymerization of actin cytoskeleton through activation of actin-related protein 2/3 complex (Arp2/3). The Arp2/3 complex, once activated, forms actin-based membrane protrusions essential for cell migration and cancer cell invasion. Thus, by activation of Arp2/3 complex, the WAVE and WASP family proteins, as part of the WAVE regulatory complex (WRC), have been shown to play a critical role in cancer cell invasion and metastasis, drawing significant research interest over recent years. Several studies have highlighted the potential for targeting the genes encoding either part of or a complete protein from the WASP/WAVE family as therapeutic strategies for preventing the invasion and metastasis of cancer cells. WAVE2 is well documented to be associated with the pathogenesis of several human cancers, including lung, liver, pancreatic, prostate, colorectal and breast cancer, as well as other hematologic malignancies. This review focuses mainly on the role of WAVE2 in the development, invasion and metastasis of different types of cancer. This review also summarizes the molecular mechanisms that regulate the activity of WAVE2, as well as those oncogenic pathways that are regulated by WAVE2 to promote the cancer phenotype. Finally, we discuss potential therapeutic strategies that target WAVE2 or the WAVE regulatory complex, aimed at preventing or inhibiting cancer invasion and metastasis.

Photobiomodulation therapy for thrombocytopenia by upregulating thrombopoietin expression via the ROS-dependent Src/ERK/STAT3 signaling pathway

BACKGROUND

Chemotherapy-induced thrombocytopenia (CIT) can increase the risk of bleeding, which may delay or prevent the administration of anticancer treatment schedules. Photobiomodulation therapy (PBMT), a non-invasive physical treatment, has been proposed to improve thrombocytopenia; however, its underlying regulatory mechanism is not fully understood.

OBJECTIVE

To further investigate the mechanism of thrombopoietin (TPO) in megakaryocytopoiesis and thrombopoiesis.

METHODS

Multiple approaches such as western blotting, cell transfection, flow cytometry, and animal studies were utilized to explore the effect and mechanism of PBMT on thrombopoiesis.

RESULTS

PBMT prevented a severe drop in platelet count by increasing platelet production, and then ameliorated CIT. Mechanistically, PBMT significantly upregulated hepatic TPO expression in a thrombocytopenic mouse model, which promoted megakaryocytopoiesis and thrombopoiesis. The levels of TPO mRNA and protein increased by PBMT via the Src/ERK/STAT3 signaling pathway in hepatic cells. Furthermore, the generation of the reactive oxygen species was responsible for PBMT-induced activation of Src and its downstream target effects.

CONCLUSIONS

Our research suggests that PBMT is a promising therapeutic strategy for the treatment of CIT.

Efficacy of romiplostim in treatment of thrombocytopenia in children with Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a life-threatening primary immunodeficiency associated with bleeding of variable severity due to thrombocytopenia. Correction of the thrombocytopenia is of paramount importance for most WAS patients. We report a retrospective analysis of the safety and efficacy of romiplostim treatment in reducing thrombocytopenia and bleeding tendency in 67 children (median age 1·3 years) with genetically confirmed WAS, followed in eight months (range, 1-12 months). Complete or partial primary responses regarding platelet counts were observed in 22 (33%) and 18 (27%) subjects, respectively. Yet, even in the non-responder group, the risk of haemorrhagic events decreased significantly, to 21%, after the first month of treatment. The responses tended to be durable and stable over time, with no significant fluctuations in platelets counts. The results of this retrospective study of a large cohort of WAS patients demonstrates that romiplostim can be used to increase platelet counts and reduce the risks of life-threatening bleeding in WAS patients awaiting haematopoietic stem cell transplantation or forgoing the procedure for various reasons.

Wiskott-Aldrich Syndrome in four male siblings from a consanguineous family from Lebanon

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is a rare X-linked primary immunodeficiency disorder (PID) characterized by microthrombocytopenia, bloody diarrhea, eczema, recurrent infections, and a high incidence of autoimmunity and malignancy.

OBJECTIVE

To investigate the mechanism of thrombocytopenia and infections in four boys of consanguineous parents from Lebanon.

METHODS

Patient gDNA was studied using Next Generation Sequencing and Sanger Sequencing. Protein expression was determined by immunoblotting, and mRNA expression by semi-quantitative RT-PCR. F-actin polymerization and cellular proliferation were assayed by flow cytometry.

RESULTS

We identified a threonine to a methionine change at position 45 (T45M) of the WAS protein (WASp) that abolished protein expression and disturbed F-actin polymerization and T cell proliferation, but not B cell proliferation. In addition, the levels of the WAS-interacting protein (WIP) were significantly decreased in the patients.

CONCLUSION

The mutation identified severely destabilizes WASp and affects the downstream signaling events important for T cell function, but not B cell function. It was previously known that the stability of WASp depends on WIP. In this manuscript, we report that the stability of WIP also depends on WASp. Finally, it is important to suspect X-linked PIDs even in consanguineous families.

CLINICAL IMPLICATIONS

The patients are above the optimal age for transplant in WAS, and it is difficult to identify one or more donors for four patients, therefore, they represent ideal candidates for gene therapy or interleukin-2 therapy.

New insights into cytoskeletal remodeling during platelet production

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

Mechanisms of increased mitochondria-dependent necrosis in Wiskott-Aldrich syndrome platelets

Wiskott-Aldrich syndrome (WAS) is associated with thrombocytopenia of unclear origin. We investigated real-time cytosolic calcium dynamics, mitochondrial membrane potential and phoszphatidylserine (PS) exposure in single fibrinogen-bound platelets using confocal microscopy. The WAS platelets had higher resting calcium levels, more frequent spikes, and their mitochondria more frequently lost membrane potential followed by PS exposure (in 22.9% of platelets vs. 3.9% in controls; P<0.001) after the collapse of the last mitochondria. This phenomenon was inhibited by the mitochondrial permeability transition pore inhibitor cyclosporine A, as well by xestospongin C and lack of extracellular calcium. Thapsigargin by itself caused accelerated cell death in the WAS platelets. The number of mitochondria was predictive of PS exposure: 33% of platelets from WAS patients with fewer than five mitochondria exposed PS, while only 12% did among those that had five or more mitochondria. Interestingly, healthy donor platelets with fewer mitochondria also more readily became procoagulant upon PAR1/PAR4 stimulation. Collapse of single mitochondria led to greater cytosolic calcium increase in WAS platelets if they had one to three mitochondria compared with platelets containing higher numbers. A computer systems biology model of platelet calcium homeostasis showed that smaller platelets with fewer mitochondria could have impaired calcium homeostasis because of higher surface-to-volume ratio and greater metabolic load, respectively. There was a correlation (C=0.81, P<0.02) between the mean platelet size and platelet count in the WAS patients. We conclude that WAS platelets readily expose PS via a mitochondria-dependent necrotic mechanism caused by their smaller size, which could contribute to the development of thrombocytopenia.

D-dencichine Regulates Thrombopoiesis by Promoting Megakaryocyte Adhesion, Migration and Proplatelet Formation

Life-threatening chemotherapy-induced thrombocytopenia can increase the risk of bleeding due to a dramatic low platelet count, which may limit or delay treatment schedules in cancer patients. The pressing need for the rapid alleviation of the symptoms of thrombocytopenia has prompted us to search for novel highly effective and safe thrombopoietic agents. Pharmacological investigations have indicated that dencichine can prevent and treat blood loss and increase the number of platelets. On the basis of the neurotoxicity of dencichine, D-dencichine is artificially synthesized in the laboratory. Our initial results showed that D-dencichine had potential to elevate peripheral platelet levels in mice with carboplatin-induced thrombocytopenia. However, the mechanisms of D-dencichine on thrombopoiesis have been poorly understood. In this study, we found that sequential administration of D-dencichine had a distinct ability to elevate numbers of reticulated platelets, and did not alter their clearance. Moreover, we demonstrated that D-dencichine was able to modulate the return of hematopoietic factors to normal levels, including thrombopoietin and IL-6. However, subsequent analysis revealed that D-dencichine treatment had no direct effects on megakaryocytes proliferation, differentiation, and polyploidization. Further in vitro studies, we demonstrated for the first time that D-dencichine significantly stimulated megakaryocyte adhesion, migration, and proplatelet formation in a dose-dependent manner through extracellular regulated protein kinases1/2 (ERK1/2) and v-akt murine thymoma viral oncogene homolog (AKT) signaling pathways. This study sufficiently characterized the role of the effects of D-dencichine treatment on the regulation of thrombopoiesis and provided a promising avenue for CIT treating.

Autonomous role of Wiskott-Aldrich syndrome platelet deficiency in inducing autoimmunity and inflammation

BACKGROUND

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency characterized by eczema, infections, and susceptibility to autoimmunity and malignancies. Thrombocytopenia is a constant finding, but its pathogenesis remains elusive.

OBJECTIVE

To dissect the basis of the WAS platelet defect, we used a novel conditional mouse model (CoWas) lacking Wiskott-Aldrich syndrome protein (WASp) only in the megakaryocytic lineage in the presence of a normal immunologic environment, and in parallel we analyzed samples obtained from patients with WAS.

METHODS

Phenotypic and functional characterization of megakaryocytes and platelets in mutant CoWas mice and patients with WAS with and without autoantibodies was performed. Platelet antigen expression was examined through a protein expression profile and cluster proteomic interaction network. Platelet immunogenicity was tested by using ELISAs and B-cell and platelet cocultures.

RESULTS

CoWas mice showed increased megakaryocyte numbers and normal thrombopoiesis in vitro, but WASp-deficient platelets had short lifespan and high expression of activation markers. Proteomic analysis identified signatures compatible with defects in cytoskeletal reorganization and metabolism yet surprisingly increased antigen-processing capabilities. In addition, WASp-deficient platelets expressed high levels of surface and soluble CD40 ligand and were capable of inducing B-cell activation in vitro. WASp-deficient platelets were highly immunostimulatory in mice and triggered the generation of antibodies specific for WASp-deficient platelets, even in the context of a normal immune system. Patients with WAS also showed platelet hyperactivation and increased plasma soluble CD40 ligand levels correlating with the presence of autoantibodies.

CONCLUSION

Overall, these findings suggest that intrinsic defects in WASp-deficient platelets decrease their lifespan and dysregulate immune responses, corroborating the role of platelets as modulators of inflammation and immunity.

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

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

The contribution of mouse models to the understanding of constitutional thrombocytopenia

Constitutional thrombocytopenias result from platelet production abnormalities of hereditary origin. Long misdiagnosed and poorly studied, knowledge about these rare diseases has increased considerably over the last twenty years due to improved technology for the identification of mutations, as well as an improvement in obtaining megakaryocyte culture from patient hematopoietic stem cells. Simultaneously, the manipulation of mouse genes (transgenesis, total or conditional inactivation, introduction of point mutations, random chemical mutagenesis) have helped to generate disease models that have contributed greatly to deciphering patient clinical and laboratory features. Most of the thrombocytopenias for which the mutated genes have been identified now have a murine model counterpart. This review focuses on the contribution that these mouse models have brought to the understanding of hereditary thrombocytopenias with respect to what was known in humans. Animal models have either i) provided novel information on the molecular and cellular pathways that were missing from the patient studies; ii) improved our understanding of the mechanisms of thrombocytopoiesis; iii) been instrumental in structure-function studies of the mutated gene products; and iv) been an invaluable tool as preclinical models to test new drugs or develop gene therapies. At present, the genetic determinants of thrombocytopenia remain unknown in almost half of all cases. Currently available high-speed sequencing techniques will identify new candidate genes, which will in turn allow the generation of murine models to confirm and further study the abnormal phenotype. In a complementary manner, programs of random mutagenesis in mice should also identify new candidate genes involved in thrombocytopenia.

Megakaryocytic Smad4 Regulates Platelet Function through Syk and ROCK2 Expression

Smad4, a key transcription factor in the transforming growth factor-β signaling pathway, is involved in a variety of cell physiologic and pathologic processes. Here, we characterized megakaryocyte/platelet-specific Smad4 deficiency in mice to elucidate its effect on platelet function. We found that megakaryocyte/platelet-specific loss of Smad4 caused mild thrombocytopenia and significantly extended first occlusion time and tail bleeding time in mice. Smad4-deficient platelets showed reduced agonist-induced platelet aggregation. Further studies showed that a severe defect was seen in integrin α_IIbβ₃-mediated bidirectional (inside-out and outside-in) signaling in Smad4-deficient platelets, as evidenced by reduced fibrinogen binding and α-granule secretion, suppressed platelet spreading and clot retraction. Microarray analysis showed that the expression levels of multiple genes were altered in Smad4-deficient platelets. Among these genes, spleen tyrosine kinase (Syk) and Rho-associated coiled-coil containing protein kinase 2 (ROCK2) were downregulated several times as confirmed by quantitative reverse-transcription polymerase chain reaction and immunoblotting. Further research showed that Smad4 directly regulates ROCK2 transcription but indirectly regulates Syk. Megakaryocyte/platelet-specific Smad4 deficiency caused decreased expression levels of Syk and ROCK2 in platelets. These results suggest potential links among Smad4 deficiency, attenuated Syk, and ROCK2 expression and defective platelet activation.

Inherited thrombocytopenias-recent advances in clinical and molecular aspects

Since the beginning of the century, our knowledge of inherited thrombocytopenias greatly advanced, and we presently know 30 forms with well-defined genetic defects. This great advancement changed our view of these disorders, as we realized that most patients have only mild thrombocytopenia with inconspicuous bleeding or no bleeding tendency at all. However, better knowledge of inherited thrombocytopenias also revealed that some of the most prevalent forms expose to the risk of acquiring during infancy or adulthood additional disorders that endanger the life of patients much more than hemorrhages. Thus, inherited thrombocytopenias are complex disorders with quite different clinical features and prognosis. Identification of novel genes whose mutations result in low platelet count greatly advanced also our knowledge of the megakaryocyte biology and proved beyond any doubt that the defective proteins play an essential role in platelet biogenesis or survival in humans. Based on the study of inherited thrombocytopenias, we better understood the sequence of molecular events regulating megakaryocyte differentiation, maturation, and platelet release. Since nearly 50% of patients have as yet unidentified genetic or molecular mechanisms underlying their inherited thrombocytopenia, further studies are expected to reveal new clinical entities and new molecular mechanisms of platelet production.

Numerical analysis of in vivo platelet consumption data from ITP patients

Background

Numerical methods have recently allowed quantitative interpretation of in vivo murine platelet consumption data in terms of values for the random destruction rate constant (RD), intrinsic lifespan (LS), and the standard deviation of ln LS (SD), as well as the platelet production rate (PR) and age distribution (AD). But application of these methods to data obtained in thrombocytopenic patients is problematic for two reasons. First, such data has in all cases been obtained with radiolabeled platelets, and uptake of the radio-isotope by long lived cells complicates the analysis. Second, inferred values of the platelet production rate (PR) and random destruction rate (RD) are difficult to interpret, since increased RD can occur either as a cause or a consequence of thrombocytopenia.

Methods

We used a numerical method to analyze in vivo platelet consumption data from a series of 41 patients with immune thrombocytopenic purpura (ITP). An additional parameter, the fraction of labeled long-lived cells (LL), was evaluated concurrently with RD, LS, and SD. To provide a basis for interpreting these values, we used an iterative interpolation process to predict their response to different pathophysiologic mechanisms. The process also generates predicted effects on the widely used immature platelet fraction (IPF).

Results

Optimal parameter value sets were identified in 76 % (31 of 41) of the data sets. 27 of 31 ITP patients showed no substantial homeostatic increase in platelet production, with the remaining 4 showing both augmented platelet consumption and a compensatory increase in PR. Up to 1/3 of the patients showed the degree of increased RD expected to result from reduced thrombopoiesis only. “Jacknife” resampling yielded CV values of <0.5 in over 75 % of the evaluable data sets. Predicted platelet age distributions indicate that interpretation of the IPF and absolute IPF (aIPF) is a complex function of platelet count. We found, counter-intuitively, that reduced PR can increase the IPF, and increased RD can reduce the aIPF.

Conclusions

Our findings support the feasibility of using numerical analysis to quantitatively interpret in vivo platelet consumption data, to identify likely etiologies of thrombocytopenias, and to assess the utility of IPF measurements in that context.

Electronic supplementary material

The online version of this article (doi:10.1186/s12878-015-0034-4) contains supplementary material, which is available to authorized users.

Pak2 restrains endomitosis during megakaryopoiesis and alters cytoskeleton organization

Megakaryocyte maturation and polyploidization are critical for platelet production; abnormalities in these processes are associated with myeloproliferative disorders, including thrombocytopenia. Megakaryocyte maturation signals through cascades that involve p21-activated kinase (Pak) function; however, the specific role for Pak kinases in megakaryocyte biology remains elusive. Here, we identify Pak2 as an essential effector of megakaryocyte maturation, polyploidization, and proplatelet formation. Genetic deletion of Pak2 in murine bone marrow is associated with macrothrombocytopenia, altered megakaryocyte ultrastructure, increased bone marrow megakaryocyte precursors, and an elevation of mature CD41(+) megakaryocytes, as well as an increased number of polyploid cells. In Pak2(-/-) mice, platelet clearance rate was increased, as was production of newly synthesized, reticulated platelets. In vitro, Pak2(-/-) megakaryocytes demonstrate increased polyploidization associated with alterations in β1-tubulin expression and organization, decreased proplatelet extensions, and reduced phosphorylation of the endomitosis regulators LIM domain kinase 1, cofilin, and Aurora A/B/C. Together, these data establish a novel role for Pak2 as an important regulator of megakaryopoiesis, polyploidization, and cytoskeletal dynamics in developing megakaryocytes.

Clinical spectrum, pathophysiology and treatment of the Wiskott-Aldrich syndrome

PURPOSE OF REVIEW

The Wiskott-Aldrich syndrome (WAS), caused by mutations in the WAS gene, is a complex and diverse disorder with X-linked inheritance. This review focuses on recent developments in the understanding of its basic pathophysiology, diverse clinical phenotypes and optimal patient management including novel therapies.

RECENT FINDINGS

The protein encoded by the WAS gene is a multifunctional signaling element expressed in immune and hematopoietic cells that plays a critical role in cytoskeletal reorganization, immune synapse formation and intracellular signaling. The type of specific mutation, its location within the gene and its effect on protein expression play a major role in determining an individual patient's clinical phenotype. Recent clinical observations and molecular studies have created a sophisticated picture of the disease spectrum. The improved outcome of stem cell transplantation from related and unrelated matched donors and promising early results from the first clinical gene therapy trial have added new therapeutic options for these patients.

SUMMARY

Classic WAS, X-linked thrombocytopenia and X-linked neutropenia are caused by WAS gene mutations, each having a distinct pattern of clinical symptoms and disease severity. New developments in the understanding of these syndromes and novel therapeutic options will have a major impact on the treatment of individuals with WAS mutations.

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.

Increased uptake by splenic red pulp macrophages contributes to rapid platelet turnover in WASP(-) mice

Thrombocytopenia caused by rapid platelet consumption contributes to the severe thrombocytopenia of Wiskott-Aldrich syndrome (WAS) and to the milder thrombocytopenia seen in murine WAS. We show that rapid clearance of ¹¹¹In-labeled murine WASP(-) platelets correlates with enhanced splenic uptake. Using platelets labeled with a pH-sensitive fluorescent marker (pHrodo), we quantify normal platelet uptake by red pulp macrophages (RPMs), and demonstrate its enhancement after in vivo opsonization of platelets. The spleens of WASP(-) mice contain an increased number of RPM, and rapid clearance of WASP(-) platelets in WASP(-) mice in turn generates an increased number of pHrodo(+) splenic RPMs. To separately assess the platelet intrinsic and recipient-dependent functions involved in the clearance and splenic phagocyte uptake of WASP(-) platelets, we performed "crossed" pHrodo(+) platelet injection studies (wild type [WT] to WASP(-), WASP(-) to WT). We show that an extrinsic effect of recipient WASP deficiency on the clearance of WASP(-) platelets correlates with increased platelet uptake by RPMs. An intrinsic effect of platelet WASP deficiency on platelet clearance does not, however, correlate with increased total uptake by WT or WASP(-) RPMs. In contrast to other published findings, we find no evidence of a baseline or antibody-induced increase in phosphatidyl serine exposure on WASP(-) platelets. Our findings suggest that an increased number of RPMs in WASP(-) mice contributes significantly to the increased platelet consumption rate in WASP(-) mice. This might explain the consistent efficacy of splenectomy in murine and clinical WAS.

Pathogenesis and management of inherited thrombocytopenias: rationale for the use of thrombopoietin-receptor agonists

Knowledge in the field of inherited thrombocytopenias (ITs) has considerably improved over the recent years. In the last 5 years, nine new genes whose mutations are responsible for thrombocytopenia have been identified, and this also led to the recognition of several novel nosographic entities, such as thrombocytopenias deriving from mutations in CYCS, TUBB1, FLNA, ITGA2B/ITGB3, ANKRD26 and ACTN1. The identification of novel molecular alterations causing thrombocytopenia together with improvement of methodologies to study megakaryopoiesis led to considerable advances in understanding pathophysiology of ITs, thus providing the background for proposing new treatments. Thrombopoietin-receptor agonists (TPO-RAs) represent an appealing therapeutic hypothesis for ITs and have been tested in a limited number of patients. In this review, we provide an updated description of pathogenetic mechanisms of thrombocytopenia in the different forms of ITs and recapitulate the current management of these disorders. Moreover, we report the available clinical and preclinical data about the role of TPO-RAs in ITs and discuss the rationale for the use of these molecules in view of pathogenesis of the different forms of thrombocytopenia of genetic origin.

Biochemical and functional significance of F-BAR domain proteins interaction with WASP/N-WASP

The Bin-Amphiphysin-Rvs (BAR) domain family of proteins includes groups which promote positive (classical BAR, N-BAR, and F-BAR) and negative (I-BAR) membrane deformation. Of these groups, the F-BAR subfamily is the most diverse in its biochemical properties. F-BAR domain proteins dimerize to form a tight scaffold about the membrane. The F-BAR domain provides a banana-shaped, alpha-helical structure that senses membrane curvature. Different types of F-BAR domain proteins contain tyrosine kinase or GTPase activities; some interact with phosphatases and RhoGTPases. Most possess an SH3 domain that facilitates the recruitment and activation of WASP/N-WASP. Thus, F-BAR domain proteins affect remodeling of both membrane and the actin cytoskeleton. The purpose of this review is to highlight the role of F-BAR proteins in coupling WASP/N-WASP to cytoskeletal remodeling. A role for F-BAR/WASP interaction in human diseases affecting nervous, blood, and neoplastic tissues is discussed.

A numerical analysis model for the interpretation of in vivo platelet consumption data

Unlike anemias, most thrombocytopenias cannot be separated into those due to impaired production and those due to accelerated consumption. While rapid clearance of labeled platelets from the bloodstream can be followed in thrombocytopenic individuals, no model exists for quantitatively inferring from autologous or allogeneic platelet consumption data what changes in random consumption, lifespan dependent consumption, and platelet production rate may have caused the thrombocytopenia. Here we describe a numerical analysis model which resolves these issues. The model applies three parameter values (a random consumption rate constant, a lognormally-distributed platelet lifespan, and the standard deviation of the latter) to a matrix comprising a series of platelet cohorts which are sequentially produced and fractionally consumed in a series of time intervals. The cohort platelet counts achieved after equilibration of production and consumption both enumerate the population age distribution and sum to the population platelet count. Continued platelet consumption after production is halted then serves to model in vivo platelet consumption data, with consumption rate in the first such interval defining the equilibrium platelet production rate. We use a least squares fitting procedure to find parameter values which best fit observed platelet consumption data obtained in WT and thrombocytopenic WASP(-) mice. Equilibrium platelet age distributions are then ‘grafted’ into the matrix to allow modeling of the consumption of WT platelets in WASP(-) recipients, and vice versa. The optimal parameter values obtained indicate that random WT platelet consumption accounts for a larger fraction of platelet turnover than was previously suspected. Platelet WASP deficiency accelerates random consumption, and a trans effect of recipient WASP deficiency contributes to this. Application of the model to clinical data will allow distinctions to be made between thrombocytopenias due primarily to impaired platelet production and those due to acceleration of random or lifespan-dependent platelet consumption.

Use of zinc-finger nucleases to knock out the WAS gene in K562 cells: a human cellular model for Wiskott-Aldrich syndrome

Mutations in the WAS gene cause Wiskott-Aldrich syndrome (WAS), which is characterized by eczema, immunodeficiency and microthrombocytopenia. Although the role of WASP in lymphocytes and myeloid cells is well characterized, its role on megakaryocyte (MK) development is poorly understood. In order to develop a human cellular model that mimics the megakaryocytic-derived defects observed in WAS patients we used K562 cells, a well-known model for study of megakaryocytic development. We knocked out the WAS gene in K562 cells using a zinc-finger nuclease (ZFN) pair targeting the WAS intron 1 and a homologous donor DNA that disrupted WASP expression. Knockout of WASP on K562 cells (K562WASKO cells) resulted in several megakaryocytic-related defects such as morphological alterations, lower expression of CD41ɑ, lower increments in F-actin polymerization upon stimulation, reduced CD43 expression and increased phosphatidylserine exposure. All these defects have been previously described either in WAS-knockout mice or in WAS patients, validating K562WASKO as a cell model for WAS. However, K562WASPKO cells showed also increased basal F-actin and adhesion, increased expression of CD61 and reduced expression of TGFβ and Factor VIII, defects that have never been described before for WAS-deficient cells. Interestingly, these phenotypic alterations correlate with different roles for WASP in megakaryocytic differentiation. All phenotypic alterations observed in K562WASKO cells were alleviated upon expression of WAS following lentiviral transduction, confirming the role of WASP in these phenotypes. In summary, in this work we have validated a human cellular model, K562WASPKO, that mimics the megakaryocytic-related defects found in WAS-knockout mice and have found evidences for a role of WASP as regulator of megakaryocytic differentiation. We propose the use of K562WASPKO cells as a tool to study the molecular mechanisms involved in the megakaryocytic-related defects observed in WAS patients and as a cellular model to study new therapeutic strategies.

Platelets from WAS patients show an increased susceptibility to ex vivo phagocytosis

The thrombocytopenia of Wiskott-Aldrich syndrome (WAS) is thought to be due to both reduced platelet production and accelerated platelet consumption. We have previously demonstrated that platelets from WASP-deficient mice are consumed more rapidly in vivo than are WT platelets, and that opsonization accelerates their uptake by bone marrow- derived macrophages more than it does that of WT platelets. Here we asked whether platelets from WAS patients show similar features. We show that ex vivo phagocytosis by activated THP-1 cells of DIO-labeled platelets from a series of WAS or XLT patients is increased in comparison to that of normal control platelets. Using a numerical analysis method, we distinguish this effect from a concurrent effect on the amount of detectable fluorescent signal transferred to the macrophage per phagocytosed platelet. We show that the latter quantity is reduced by platelet WASP deficiency, as might be expected if the fluorescence transferred from these smaller platelets is more rapidly quenched. We are unable to detect a differential effect of opsonization with anti-CD61 antibody on the uptake of WASP(-) vs. WT platelets. However, the high probability of phagocytosis per adsorbed WASP(-) platelet could limit the sensitivity of the assay in this case. We also see no effect of sera from WAS patients on the uptake of normal control platelets, suggesting that in vivo opsonization is not the cause of increased uptake of WASP(-) platelets. Finally, we show little, if any, increase in the reticulated platelet fraction in WAS patients, suggesting that impaired production of reticulated platelets contributes to the thrombocytopenia. Our findings suggest that rapid in vivo platelet consumption contributes significantly to the thrombocytopenia of WAS. They also demonstrate the feasibility of routinely performing functional assays of phagocytosis of small numbers of platelets obtained at remote locations, a method which should be applicable to the study of other types of thrombocytopenia such as ITP.

Calcium- and integrin-binding protein 1 regulates megakaryocyte ploidy, adhesion, and migration

Megakaryocytes are large, polyploid cells that produce platelets. We have previously reported that calcium- and integrin-binding protein 1 (CIB1) regulates endomitosis in Dami cells. To further characterize the role of CIB1 in megakaryopoiesis, we used a Cib1(-/-) mouse model. Cib1(-/-) mice have more platelets and BM megakaryocytes than wild-type (WT) controls (P < .05). Furthermore, subsequent analysis of megakaryocyte-CFU production revealed an increase with Cib1 deletion compared with WT (P < .05). In addition, BM from Cib1(-/-) mice, cultured with thrombopoietin (TPO) for 24 hours, produced more highly polyploid megakaryocytes than WT BM (P < .05). Subsequent analysis of TPO signaling revealed enhanced Akt and ERK1/2 phosphorylation, whereas FAK(Y925) phosphorylation was reduced in Cib1(-/-) megakaryocytes treated with TPO. Conversely, platelet recovery in Cib1(-/-) mice after platelet depletion was attenuated compared with WT (P < .05). This could be the result of impaired adhesion and migration, as adhesion to fibrinogen and fibronectin and migration toward an SDF-1α gradient were reduced in Cib1(-/-) megakaryocytes compared with WT (P < .05). In addition, Cib1(-/-) megakaryocytes formed fewer proplatelets compared with WT (P < .05), when plated on fibrinogen. These data suggest that CIB1 plays a dual role in megakaryopoiesis, initially by negatively regulating TPO signaling and later by augmenting proplatelet production.

A numerical analysis model for interpretation of flow cytometric studies of ex vivo phagocytosis

The study of ex vivo phagocytosis via flow cytometry requires that one distinguish experimentally between uptake and adsorption of fluorescently labeled targets by phagocytes. Removal of the latter quantity from the analysis is the most common means of analyzing such data. Because the probability of phagocytosis is a function of the probability of adsorption, and because partially quenched fluorescence after uptake often overlaps with that of negative controls, this approach is suboptimal at best. Here, we describe a numerical analysis model which overcomes these limitations. We posit that the random adsorption of targets to macrophages, and subsequent phagocytosis, is a function of three parameters: the ratio of targets to macrophages (m), the mean fluorescence intensity imparted to the phagocyte by the internalized target (alpha), and the probability of phagocytosis per adsorbed target (p). The potential values of these parameters define a parameter space and their values at any point in parameter space can be used to predict the fraction of adsorption(+) and [adsorption(-), phagocytosis(+)] cells that might be observed experimentally. By systematically evaluating the points in parameter space for the latter two values and comparing them to experimental data, the model arrives at sets of parameter values that optimally predict such data. Using activated THP-1 cells as macrophages and platelets as targets, we validate the model by demonstrating that it can distinguish between the effects of experimental changes in m, alpha, and p. Finally, we use the model to demonstrate that platelets from a congenitally thrombocytopenic WAS patient show an increased probability of ex vivo phagocytosis. This finding correlates with other evidence that rapid in vivo platelet consumption contributes significantly to the thrombocytopenia of WAS. Our numerical analysis method represents a useful and innovative approach to multivariate analysis.

Mouse models of diseases of megakaryocyte and platelet homeostasis

Platelets are the small anuclear blood cells that are the product of megakaryocytopoiesis, the process of hematopoietic stem cell commitment to megakaryocyte production and the differentiation and maturation of these cells for platelet release. Deregulation or disruption of megakaryocytopoiesis can result in platelet deficiencies, the thrombocytopenias, with attendant risk of hemorrhage or thrombocytosis, a pathological excess of platelet numbers. Mouse models, particularly those engineered to carry genetic alterations modeling mutations associated with human disease, have provided important insights into megakaryocytopoiesis and deregulation of this process in disease. This review focuses on mouse models of diseases of altered megakaryocyte and platelet number, illustrating the profound contribution of these models in validating suspected roles of disease-associated genetic alterations, promoting discovery of new links between genetic mutations and specific diseases, and providing unique tools for better understanding of disease pathophysiology and progression, as well as resources to define drug action or develop new therapeutic strategies.

Novel clearance mechanisms of platelets

PURPOSE OF REVIEW

Blood platelets are involved in primary and secondary hemostasis and thus maintain the integrity of the vasculature. They circulate with an average lifespan of 5-9 days in humans. Thus, the body must generate and clear platelets daily to maintain normal physiological blood platelet counts. Known platelet clearance mechanisms include antibody-mediated clearance by spleen macrophages, as in immune thrombocytopenia, and platelet consumption due to massive blood loss.

RECENT FINDINGS

New concepts in the clearance mechanisms of platelets have recently emerged. New evidence shows that platelets desialyted due to chilling or sepsis are cleared in the liver by macrophages, that is Kupffer cells, as well as hepatocytes, through lectin-mediated recognition of platelet glycans. On the contrary, platelet-associated antibodies normalize the clearance of platelets in a mouse model for Wiskott-Aldrich syndrome.

SUMMARY

The goal of this review is to summarize the latest findings in platelet clearance mechanisms with a focus on lectin-mediated recognition of platelet glycans. Transfusion medicine and treatments of hematopoietic disorders associated with severe thrombocytopenia may benefit from a better understanding of these mechanisms.

Novel GLP-1 fusion chimera as potent long acting GLP-1 receptor agonist

GLP-1 has a variety of anti-diabetic effects. However, native GLP-1 is not suitable for therapy of diabetes due to its short half-life (t1/2<2 min). To circumvent this, we developed a long-lasting GLP-1 receptor agonist by the fusion of GLP-1 with human IgG2 Fc (GLP-1/hIgG2). ELISA-based receptor binding assay demonstrated that GLP-1/hIgG2 had high binding affinity to the GLP-1R in INS-1 cells (Kd = 13.90±1.52 nM). Upon binding, GLP-1/hIgG2 was rapidly internalized by INS-1 cells in a dynamin-dependent manner. Insulin RIA showed that GLP-1/IgG2 dose-dependently stimulated insulin secretion from INS-1 cells. Pharmacokinetic studies in CD1 mice showed that with intraperitoneal injection (i.p.), the GLP-1/hIgG2 peaked at 30 minutes in circulation and maintained a plateau for >168 h. Intraperitoneal glucose tolerance test (IPGTT) in mice showed that GLP-1/hIgG2 significantly decreased glucose excursion. Furthermore, IPGTT performed on mice one week after a single drug-injection also displayed significantly reduced glucose excursion, indicating that GLP-1/hIgG2 fusion protein has long-lasting effects on the modulation of glucose homeostasis. GLP-1/hIgG2 was found to be effective in reducing the incidence of diabetes in multiple-low-dose streptozotocin-induced type 1 diabetes in mice. Together, the long-lasting bioactive GLP-1/hIgG2 retains native GLP-1 activities and thus may serve as a potent GLP-1 receptor agonist.

WASP plays a novel role in regulating platelet responses dependent on alphaIIbbeta3 integrin outside-in signalling

The most consistent feature of Wiskott Aldrich syndrome (WAS) is profound thrombocytopenia with small platelets. The responsible gene encodes WAS protein (WASP), which functions in leucocytes as an actin filament nucleating agent -yet- actin filament nucleation proceeds normally in patient platelets regarding shape change, filopodia and lamellipodia generation. Because WASP localizes in the platelet membrane skeleton and is mobilized by alphaIIbbeta3 integrin outside-in signalling, we questioned whether its function might be linked to integrin. Agonist-induced alphaIIbbeta3 activation (PAC-1 binding) was normal for patient platelets, indicating normal integrin inside-out signalling. Inside-out signalling (fibrinogen, JON/A binding) was also normal for wasp-deficient murine platelets. However, adherence/spreading on immobilized fibrinogen was decreased for patient platelets and wasp-deficient murine platelets, indicating decreased integrin outside-in responses. Another integrin outside-in dependent response, fibrin clot retraction, involving contraction of the post-aggregation actin cytoskeleton, was also decreased for patient platelets and wasp-deficient murine platelets. Rebleeding from tail cuts was more frequent for wasp-deficient mice, suggesting decreased stabilisation of the primary platelet plug. In contrast, phosphatidylserine exposure, a pro-coagulant response, was enhanced for WASP-deficient patient and murine platelets. The collective results reveal a novel function for WASP in regulating pro-aggregatory and pro-coagulant responses downstream of integrin outside-in signalling.

The thrombocytopenia of WAS: a familial form of ITP?

In the first report of the concurrent immunodeficiency, thrombocytopenia, and eczema that we now call the Wiskott-Aldrich Syndrome (WAS), Alfred Wiskott asked whether it could be a familial form of Werlhof's disease (now called ITP). This review summarizes what is known about platelet production, consumption, and function in clinical and murine WAS. Both platelet production and consumption are affected by WASP deficiency. Likely molecular mechanisms have been identified for the former process, but remain problematic for the latter. Recent data in a murine model suggest that WASP deficiency could increase both the incidence of antiplatelet antibodies and susceptibility to their enhancement of platelet consumption. Wiskott's original speculation about the relationship between WAS and ITP may need to be reconsidered.

Antiplatelet antibodies in WASP(-) mice correlate with evidence of increased in vivo platelet consumption

OBJECTIVE

To study the role of antiplatelet antibodies in the thrombocytopenia of murine Wiskott-Aldrich syndrome (WAS).

MATERIALS AND METHODS

A flow cytometric method was developed for detection of serum antiplatelet antibodies via their binding to intact target platelets lacking surface antibodies. Platelets were labeled with 5-chloromethylfluorescein diacetate (CMFDA) in order to track their clearance from the circulation. WASP(-)muMT(-/-) mice were generated by standard breeding methods.

RESULTS

Serum antiplatelet antibodies were detected in approximately 40% of WASP(-) males. The mean level of reticulated platelets is significantly increased in these antibody(+) males. While WASP(-) males show an approximately 50% reduction in platelet counts, 5% to 10% show a more severe thrombocytopenia associated with increased reticulated platelets, suggesting the presence of clearance-inducing antiplatelet antibodies. In support of that inference, 90% of the latter mice show detectable serum antiplatelet antibodies. The antibodies are primarily immunoglobulin G, and are also detected in >30% of CD47(-/-) males. WASP(-)muMT(-/-) males, which demonstrate no serum- or platelet-associated antibodies, show a degree of thrombocytopenia similar to that of WASP(-) males. Their platelet clearance rates remain accelerated--more so in WASP(-)muMT(-/-) than WASP(+)muMT(-/-) recipients.

CONCLUSIONS

These findings suggest that platelet WASP deficiency results in an increase in platelet clearance rates by two mechanisms: an antibody-independent mechanism that largely requires WASP deficiency in trans, and an antibody-dependent mechanism that does not. Both an increased incidence of antiplatelet antibodies and an increased susceptibility to their effects contribute to antibody-dependent clearance of WASP(-) platelets.

Deletion of activating Fcgamma receptors does not confer protection in murine cryoglobulinemia-associated membranoproliferative glomerulonephritis

Many types of glomerulonephritis are initiated by the deposition of immune complexes, which induce tissue injury via either engagement of Fc receptors on effector cells or via complement activation. Four murine Fcgamma receptors (FcgammaRs) have been identified at present. Ligand binding to FcgammaRI, III, and IV induces cell activation via the immunoreceptor tyrosine-based activation motif on the common gamma chain (FcRgamma). In this study, FcRgamma chain knockout (FcRgamma(-/-)) mice were crossed with thymic stromal lymphopoietin transgenic (TSLPtg) mice, which develop cryoglobulinemic membranoproliferative glomerulonephritis (MPGN). Female mice were studied at 30 and 50 days of age, when MPGN is in early and fully developed stages, respectively. Both TSLPtg and TSLPtg/FcRgamma(-/-) mice developed MPGN with massive glomerular immune deposits, mesangial cell proliferation, extensive mesangial matrix accumulation, and macrophage influx. TSLPtg/FcRgamma(-/-) mice had more glomerular immune complex deposits and higher levels of circulating cryoglobulins, IgG2a, IgG2b, and IgM, compared with TSLPtg mice. TSLPtg and TSLPtg/FcRgamma(-/-) mice developed similar levels of proteinuria. These results demonstrated that deletion of activating FcgammaRs does not confer protection in this model of immune complex-mediated MPGN. The findings contradict accepted paradigms on the role of activating FcgammaRs in promoting features of glomerulonephritis as seen in other model systems. We speculate engagement of FcgammaRs on cells such as monocytes/macrophages may be important for the clearance of deposited immune complexes and extracellular matrix proteins.