The adapter protein ADAP is required for selected dendritic cell functions

ADAP Y571 Phosphorylation Is Required to Prime STAT3 for Activation in TLR4-Stimulated Macrophages

Adhesion and degranulation-promoting adapter protein (ADAP), originally identified as an essential adaptor molecule in TCR signaling and T cell adhesion, has emerged as a critical regulator in innate immune cells such as macrophages; however, its role in macrophage polarization and inflammatory responses remains unknown. In this study, we show that ADAP plays an essential role in TLR4-mediated mouse macrophage polarization via modulation of STAT3 activity. Macrophages from ADAP-deficient mice exhibit enhanced M1 polarization, expression of proinflammatory cytokines and capacity in inducing Th1 responses, but decreased levels of anti-inflammatory cytokines in response to TLR4 activation by LPS. Furthermore, overexpression of ADAP enhances, whereas loss of ADAP reduces, the LPS-mediated phosphorylation and activity of STAT3, suggesting ADAP acts as a coactivator of STAT3 activity and function. Furthermore, the coactivator function of ADAP mostly depends on the tyrosine phosphorylation at Y571 in the motif YDSL induced by LPS. Mutation of Y571 to F severely impairs the stimulating effect of ADAP on STAT3 activity and the ability of ADAP to inhibit M1-like polarization in TLR4-activated mouse macrophages. Moreover, ADAP interacts with STAT3, and loss of ADAP renders mouse macrophages less sensitive to IL-6 stimulation for STAT3 phosphorylation. Collectively, our findings revealed an additional layer of regulation of TLR4-mediated mouse macrophage plasticity whereby ADAP phosphorylation on Y571 is required to prime STAT3 for activation in TLR4-stimulated mouse macrophages.

Immune Cell-Type Specific Ablation of Adapter Protein ADAP Differentially Modulates EAE

The cytosolic adhesion and degranulation-promoting adapter protein ADAP is expressed in various hematopoietic cells including T cells, NK cells, myeloid cells, and platelets but absent in mature B cells. The role of ADAP in T cell activation, proliferation and integrin activation is well-accepted. We previously demonstrated that conventional ADAP knockout mice show a significantly attenuated course of experimental autoimmune encephalomyelitis (EAE). To dissect the impact of different ADAP expressing cell populations on the reduced EAE severity, here, we generated lineage-specific conditional knockout mice. ADAP was deleted in T cells, myeloid cells, NK cells and platelets, respectively. Specific loss of ADAP was confirmed on the protein level. Detailed immunophenotyping was performed to assess the consequence of deletion of ADAP with regard to the maturation and distribution of immune cells in primary and secondary lymphoid organs. The analysis showed equivalent results as for conventional ADAP knockout mice: impaired thymocyte development in ADAP^fl/fl Lck-Cre mice, normal NK cell and myeloid cell distribution in ADAP^fl/fl NKp46-Cre mice and ADAP^fl/fl LysM-Cre mice, respectively as well as thrombocytopenia in ADAP^fl/fl PF4-Cre mice. Active EAE was induced in these animals by immunization with the myelin oligodendrocyte glycoprotein₃₅₋₅₅ peptide. The clinical course of EAE was significantly milder in mice with loss of ADAP in T cells, myeloid cells and NK cells compared to ADAP-sufficient control littermates. Surprisingly, specific deletion of ADAP in platelets resulted in a more exacerbated disease. These data show that T cell-independent as well as T cell-dependent mechanisms are responsible for the complex phenotype observed in conventional ADAP knockout mice.

Integrin Activation Through the Hematopoietic Adapter Molecule ADAP Regulates Dendritic Development of Hippocampal Neurons

Integrin-mediated cell adhesion and signaling is of critical importance for neuronal differentiation. Recent evidence suggests that an “inside-out” activation of β1-integrin, similar to that observed in hematopoietic cells, contributes to the growth and branching of dendrites. In this study, we investigated the role of the hematopoietic adaptor protein adhesion and degranulation promoting adapter protein (ADAP) in these processes. We demonstrate the expression of ADAP in the developing and adult nervous hippocampus, and in outgrowing dendrites of primary hippocampal neurons. We further show that ADAP occurs in a complex with another adaptor protein signal-transducing kinase-associated phosphoprotein-homolog (SKAP-HOM), with the Rap1 effector protein RAPL and the Hippo kinase macrophage-stimulating 1 (MST1), resembling an ADAP/SKAP module that has been previously described in T-cells and is critically involved in “inside-out” activation of integrins. Knock down of ADAP resulted in reduced expression of activated β1-integrin on dendrites. It furthermore reduced the differentiation of developing neurons, as indicated by reduced dendrite growth and decreased expression of the dendritic marker microtubule-associated protein 2 (MAP2). Our data suggest that an ADAP-dependent integrin-activation similar to that described in hematopoietic cells contributes to the differentiation of neuronal cells.

Deleterious mutation in the FYB gene is associated with congenital autosomal recessive small-platelet thrombocytopenia

BACKGROUND

The FYB gene encodes adhesion and degranulation-promoting adaptor protein (ADAP), a hematopoietic-specific protein involved in platelet activation, cell motility and proliferation, and integrin-mediated cell adhesion. No ADAP-related diseases have been described in humans, but ADAP-deficient mice have mild thrombocytopenia and increased rebleeding from tail wounds.

PATIENTS AND METHODS

We studied a previously reported family of five children from two consanguineous sibships of Arab Christian descent affected with a novel autosomal recessive bleeding disorder with small-platelet thrombocytopenia. Homozygosity mapping and exome sequencing were used to identify the genetic lesion causing the disease phenotype on chromosome 5. Bone-marrow morphology and platelet function were analyzed. Platelets were characterized by scanning electron microscopy.

RESULTS

We identified a homozygous deleterious nonsense mutation, c.393G>A, in FYB. A reduced percentage of mature megakaryocytes was found in the bone marrow. Patients' platelets showed increased basal expression of P-selectin and PAC-1, and reduced increments of activation markers after stimulation with ADP, as detected by flow cytometry; they also showed reduced pseudopodium formation and the presence of trapped platelets between the fibrin fibers after thrombin addition, as observed on scanning electron microscopy.

CONCLUSIONS

This is the first report of a disease caused by an FYB defect in humans, manifested by remarkable small-platelet thrombocytopenia and a significant bleeding tendency. The described phenotype shows ADAP to be important for normal platelet production, morphologic changes, and function. It is suggested that mutation analysis of this gene be included in the diagnosis of inherited thrombocytopenia.

Recessive thrombocytopenia likely due to a homozygous pathogenic variant in the FYB gene: case report

Background

Inherited thrombocytopenias (IT) are a heterogeneous group of rare diseases characterized by a reduced number of blood platelets. The frequency of IT is probably underestimated because of diagnostic difficulties and because not all the existing forms have as yet been identified, with some patients remaining without a definitive diagnosis. Exome Sequencing has made possible the identification of almost all variants in the coding regions of protein-coding genes, thereby providing the opportunity to identify the disease causing gene in a number of patients with indefinite diagnoses, specifically in consanguineous families.

Case presentation

Familial thrombocytopenia with small size platelets was present in several members of a highly consanguineous family from Northern Iraq. Genotyping of all affected, their unaffected siblings and parents, followed by exome sequencing revealed a strong candidate loss of function variant in a homozygous state: a frameshift mutation in the FYB gene. The protein encoded by this gene is known to be a cytosolic adaptor molecule expressed by T, natural killer (NK), myeloid cells and platelets, and is involved in platelet activation and controls the expression of interleukin-2. Knock-out mice were reported to show isolated thrombocytopenia.

Conclusion

Inherited thrombocytopenias differ in their presentation, associated features, and molecular etiologies. An accurate diagnosis is needed to provide appropriate management as well as counseling for the individuals and their family members. Exome sequencing may become a first diagnostic tool to identify the molecular basis of undiagnosed familial IT. In this report, the clinical evaluation combined with the power and efficiency of genomic analysis defined the FYB gene as the possible underlying cause of autosomal recessive thrombocytopenia with small platelet size. This is the first report linking pathogenic variants in FYB and thrombocytopenia in humans.

T cell-independent modulation of experimental autoimmune encephalomyelitis in ADAP-deficient mice

The adhesion- and degranulation-promoting adaptor protein (ADAP), expressed in T cells, myeloid cells, and platelets, is known to regulate receptor-mediated inside-out signaling leading to integrin activation and adhesion. In this study, we demonstrate that, upon induction of active experimental autoimmune encephalomyelitis (EAE) by immunization with the myelin oligodendrocyte glycoprotein35-55 peptide, ADAP-deficient mice developed a significantly milder clinical course of EAE and showed markedly less inflammatory infiltrates in the CNS than wild-type mice. Moreover, ADAP-deficient recipients failed to induce EAE after adoptive transfer of myelin oligodendrocyte glycoprotein-specific TCR-transgenic T cells (2D2 T cells). In addition, ex vivo fully activated 2D2 T cells induced significantly less severe EAE in ADAP-deficient recipients. The ameliorated disease in the absence of ADAP was not due to expansion or deletion of a particular T cell subset but rather because of a strong reduction of all inflammatory leukocyte populations invading the CNS. Monitoring the adoptively transferred 2D2 T cells over time demonstrated that they accumulated within the lymph nodes of ADAP-deficient hosts. Importantly, transfer of complete wild-type bone marrow or even bone marrow of 2D2 TCR-transgenic mice was unable to reconstitute EAE in the ADAP-deficient animals, indicating that the milder EAE was dependent on (a) radio-resistant nonhematopoietic cell population(s). Two-photon microscopy of lymph node explants revealed that adoptively transferred lymphocytes accumulated at lymphatic vessels in the lymph nodes of ADAP-deficient mice. Thus, our data identify a T cell-independent mechanism of EAE modulation in ADAP-deficient mice.