WASp family verprolin-homologous protein-2 (WAVE2) and Wiskott-Aldrich syndrome protein (WASp) engage in distinct downstream signaling interactions at the T cell antigen receptor site.

T cell antigen receptor (TCR) engagement has been shown to activate pathways leading to actin cytoskeletal polymerization and reorganization, which are essential for lymphocyte activation and function. Several actin regulatory proteins were implicated in regulating the actin machinery, such as members of the Wiskott-Aldrich syndrome protein (WASp) family. These include WASp and the WASp family verprolin-homologous protein-2 (WAVE2). Although WASp and WAVE2 share several structural features, the precise regulatory mechanisms and potential redundancy between them have not been fully characterized. Specifically, unlike WASp, the dynamic molecular interactions that regulate WAVE2 recruitment to the cell membrane and specifically to the TCR signaling complex are largely unknown. Here, we identify the molecular mechanism that controls the recruitment of WAVE2 in comparison with WASp. Using fluorescence resonance energy transfer (FRET) and novel triple-color FRET (3FRET) technology, we demonstrate how WAVE2 signaling complexes are dynamically regulated during lymphocyte activation in vivo. We show that, similar to WASp, WAVE2 recruitment to the TCR site depends on protein-tyrosine kinase, ZAP-70, and the adaptors LAT, SLP-76, and Nck. However, in contrast to WASp, WAVE2 leaves this signaling complex and migrates peripherally together with vinculin to the membrane leading edge. Our experiments demonstrate that WASp and WAVE2 differ in their dynamics and their associated proteins. Thus, this study reveals the differential mechanisms regulating the function of these cytoskeletal proteins.

Ubiquitylation-dependent negative regulation of WASp is essential for actin cytoskeleton dynamics.

The Wiskott-Aldrich syndrome protein (WASp) is a key regulator of actin dynamics during cell motility and adhesion, and mutations in its gene are responsible for Wiskott-Aldrich syndrome (WAS). Here, we demonstrate that WASp is ubiquitylated following T-cell antigen receptor (TCR) activation. WASp phosphorylation at tyrosine 291 results in recruitment of the E3 ligase Cbl-b, which, together with c-Cbl, carries out WASp ubiquitylation. Lysine residues 76 and 81, located at the WASp WH1 domain, which contains the vast majority of WASp gene mutations, serve as the ubiquitylation sites. Disruption of WASp ubiquitylation causes WASp accumulation and alters actin dynamics and the formation of actin-dependent structures. Our data suggest that regulated degradation of activated WASp might be an efficient strategy by which the duration and localization of actin rearrangement and the intensity of T-cell activation are controlled.

Functional cooperation between the proteins Nck and ADAP is fundamental for actin reorganization.

T cell antigen receptor (TCR) activation triggers profound changes in the actin cytoskeleton. In addition to controlling cellular shape and polarity, this process regulates vital T cell responses, such as T cell adhesion, motility, and proliferation. These depend on the recruitment of the signaling proteins Nck and Wiskott-Aldrich syndrome protein (WASp) to the site of TCR activation and on the functional properties of the adapter proteins linker for activation of T cells (LAT) and SH2-domain-containing leukocyte protein of 76 kDa (SLP76). We now demonstrate that Nck is necessary but insufficient for the recruitment of WASp. We show that two pathways lead to SLP76-dependent actin rearrangement. One requires the SLP76 acidic domain, crucial to association with the Nck SH2 domain, and another requires the SLP76 SH2 domain, essential for interaction with the adhesion- and degranulation-promoting adapter protein ADAP. Functional cooperation between Nck and ADAP mediates SLP76-WASp interactions and actin rearrangement. We also reveal the molecular mechanism linking ADAP to actin reorganization.

Cooperative interactions at the SLP-76 complex are critical for actin polymerization.

T-cell antigen receptor (TCR) engagement induces formation of multi-protein signalling complexes essential for regulating T-cell functions. Generation of a complex of SLP-76, Nck and VAV1 is crucial for regulation of the actin machinery. We define the composition, stoichiometry and specificity of interactions in the SLP-76, Nck and VAV1 complex. Our data reveal that this complex can contain one SLP-76 molecule, two Nck and two VAV1 molecules. A direct interaction between Nck and VAV1 is mediated by binding between the C-terminal SH3 domain of Nck and the VAV1 N-terminal SH3 domain. Disruption of the VAV1:Nck interaction deleteriously affected actin polymerization. These novel findings shed new light on the mechanism of actin polymerization after T-cell activation.

MT1-MMP and RECK are involved in human CD34+ progenitor cell retention, egress, and mobilization.

The mechanisms governing hematopoietic progenitor cell mobilization are not fully understood. We report higher membrane type 1-MMP (MT1-MMP) and lower expression of the MT1-MMP inhibitor, reversion-inducing cysteine-rich protein with Kazal motifs (RECK), on isolated circulating human CD34+ progenitor cells compared with immature BM cells. The expression of MT1-MMP correlated with clinical mobilization of CD34+ cells in healthy donors and patients with lymphoid malignancies. Treatment with G-CSF further increased MT1-MMP and decreased RECK expression in human and murine hematopoietic cells in a PI3K/Akt-dependent manner, resulting in elevated MT1-MMP activity. Blocking MT1-MMP function by Abs or siRNAs impaired chemotaxis and homing of G-CSF-mobilized human CD34+ progenitors. The mobilization of immature and maturing human progenitors in chimeric NOD/SCID mice by G-CSF was inhibited by anti-MT1-MMP treatment, while RECK neutralization promoted motility and egress of BM CD34+ cells. BM c-kit+ cells from MT1-MMP-deficient mice also exhibited inferior chemotaxis, reduced homing and engraftment capacities, and impaired G-CSF-induced mobilization in murine chimeras. Membranal CD44 cleavage by MT1-MMP was enhanced following G-CSF treatment, reducing CD34+ cell adhesion. Accordingly, CD44-deficient mice had a higher frequency of circulating progenitors. Our results reveal that the motility, adhesion, homing, and mobilization of human hematopoietic progenitor cells are regulated in a cell-autonomous manner by dynamic and opposite changes in MT1-MMP and RECK expression.

Low levels of alpha7-nicotinic acetylcholine receptor mRNA on peripheral blood lymphocytes in schizophrenia and its association with illness severity.

BACKGROUND: There is evidence that the nicotinic alpha7-acetylcholine receptor (alpha7-AChR) is involved in the pathophysiology of schizophrenia. Several neurotransmitter receptors, including alpha7-AChR, have been demonstrated on peripheral blood lymphocytes (PBL) and it has been suggested that these peripheral receptors may reflect corresponding brain receptors. OBJECTIVE: In this study we compare alpha7 mRNA expression in PBL between schizophrenia patients and control individuals in order to determine whether any correlation exists between alpha7 mRNA expression in PBL and severity of schizophrenia. In addition, the isoforms of alpha7-AChR expressed are identified. METHOD: Peripheral venous blood samples were collected from individuals with schizophrenia (n = 44) and from healthy subjects (n = 16). Symptomatology and illness severity were assessed using standard clinical psychiatric evaluation scales. RNA was prepared from isolated lymphocytes and alpha7 mRNA was measured by RT-PCR. RESULTS: We observed a significantly lower level of alpha7 mRNA on PBLs of schizophrenia patients in comparison with healthy controls (p < 0.00). A tendency to a negative correlation was noted between the CGI score, reflecting illness severity, and the alpha7-subunit gene expression. CONCLUSION: Observations confirm that the alpha7 mRNA in PBL represents the duplicated alpha7-AChR gene rather than the classic alpha7-AChR gene. Our study observations further substantiate the involvement of alpha7-AChR in the pathophysiology of schizophrenia and, while preliminary, indicate that the alpha7-AChR may be expressed and be readily measured in the peripheral blood circulation.

The alpha7 nicotinic acetylcholine receptor in schizophrenia: decreased mRNA levels in peripheral blood lymphocytes.

Recent studies have suggested that the alpha7 nicotinic acetylcholine receptor (alpha7 AChR) may play a role in the pathogenesis of schizophrenia. In search for peripheral biological markers for schizophrenia we have investigated alpha7 mRNA levels in peripheral blood lymphocytes (PBLs) of schizophrenic patients and healthy controls. Peripheral blood samples were collected from medicated and non-medicated (drug naive) schizophrenic patients as well as from healthy (non-mentally ill) smokers and non-smokers. RNA was prepared from isolated lymphocytes. Polymerase chain reaction products specific for human alpha7 AChR were quantified by densitometry using Scion image-analysis (shared NIH software). We observed a significant decrease of alpha7 mRNA levels on PBLs of schizophrenic patients compared with controls. The decrease in alpha7 mRNA levels was not a result of medication management, because non-medicated schizophrenic patients displayed the same level of reduction in alpha7 mRNA as did patients receiving medication. In addition, we exclude the possibility that the observed decrease in alpha7 mRNA levels resulted from nicotine consumption in smoking, because healthy smokers exhibited the same levels of alpha7 mRNA as non-smokers. We propose that alpha7 AChR may be involved in the pathophysiology of the disease and may serve as a reliable peripheral biological marker in schizophrenia.