Yvonne Samstag

Real-time imaging of the intracellular glutathione redox potential

Dynamic analysis of redox-based processes in living cells is now restricted by the lack of appropriate redox biosensors. Conventional redox-sensitive GFPs (roGFPs) are limited by undefined specificity and slow response to changes in redox potential. In this study we demonstrate that the fusion of human glutaredoxin-1 (Grx1) to roGFP2 facilitates specific real-time equilibration between the sensor protein and the glutathione redox couple. The Grx1-roGFP2 fusion protein allowed dynamic live imaging of the glutathione redox potential (EGSH) in different cellular compartments with high sensitivity and temporal resolution. The biosensor detected nanomolar changes in oxidized glutathione (GSSG) against a backdrop of millimolar reduced glutathione (GSH) on a scale of seconds to minutes. It facilitated the observation of redox changes associated with growth factor availability, cell density, mitochondrial depolarization, respiratory burst activity and immune receptor stimulation.

Targeting the Function of Mature Dendritic Cells by Human Cytomegalovirus: A Multilayered Viral Defense Strategy

Human cytomegalovirus (HCMV) can suppress and evade the immune system. We have identified as a mechanism the ability of HCMV to infect dendritic cells (DC), which initiate the antiviral immune response. HCMV-infected DC show enhanced expression of costimulatory molecules. In contrast, MHC molecules are partially downregulated, leading to a reduced antigen-presenting capacity. Moreover, the apoptosis-inducing ligands CD95L (FasL) and tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) are upregulated, thereby enabling HCMV-infected DC to delete activated T lymphocytes. This additional layer of viral defense is complemented by nondeletional mechanisms, which suppress surviving T cells. Thus, infection of DC allows the virus to blunt the antiviral T cell response by a multilayered defense strategy and could play a pivotal role in HCMV-triggered immunosuppression.

Proximity-based protein thiol oxidation by H2O2-scavenging peroxidases

H2O2 acts as a signaling molecule by oxidizing critical thiol groups on redox-regulated target proteins. To explain the efficiency and selectivity of H2O2-based signaling, it has been proposed that oxidation of target proteins may be facilitated by H2O2-scavenging peroxidases. Recently, a peroxidase-based protein oxidation relay has been identified in yeast, namely the oxidation of the transcription factor Yap1 by the peroxidase Orp1. It has remained unclear whether the protein oxidase function of Orp1 is a singular adaptation or whether it may represent a more general principle. Here we show that Orp1 is in fact not restricted to oxidizing Yap1 but can also form a highly efficient redox relay with the oxidant target protein roGFP (redox-sensitive green fluorescent protein) in mammalian cells. Orp1 mediates near quantitative oxidation of roGFP2 by H2O2, and the Orp1-roGFP2 redox relay effectively converts physiological H2O2 signals into measurable fluorescent signals in living cells. Furthermore, the oxidant relay phenomenon is not restricted to Orp1 as the mammalian peroxidase Gpx4 also mediates oxidation of proximal roGFP2 in living cells. Together, these findings support the concept that certain peroxidases harbor an intrinsic and powerful capacity to act as H2O2-dependent protein thiol oxidases when they are recruited into proximity of oxidizable target proteins.

Actin cytoskeletal dynamics in T lymphocyte activation and migration

Dynamic rearrangements of the actin cytoskeleton are crucial for the function of numerous cellular elements including T lymphocytes. They are required for migration of T lymphocytes through the body to scan for the presence of antigens, as well as for the formation and stabilization of the immunological synapse at the interface between antigen‐presenting cells and T lymphocytes. Supramolecular activation clusters within the immunological synapse play an important role for the initiation of T cell responses and for the execution of T cell effector functions. In addition to the T cell receptor/CD3 induced actin nucleation via Wasp/Arp2/3‐activation, signals through accessory receptors of the T cell (i.e., costimulation) regulate actin cytoskeletal dynamics. In this regard, the actin‐binding proteins cofilin and L‐plastin represent prominent candidates linking accessory receptor stimulation to the rearrangement of the actin cytoskeleton. Cofilin enhances actin polymerization via its actin‐severing activity, and as a long‐lasting effect, cofilin generates novel actin monomers through F‐actin depolymerization. L‐plastin stabilizes acin filament structures by means of its actin‐bundling activity.

The serine phosphatases PP1 and PP2A associate with and activate the actin‐binding protein cofilin in human T lymphocytes

Cofilin, an actin‐depolymerizing protein, is essential for the functional dynamics of the actin cytoskeleton and for cell viability. In unstimulated human peripheral blood T lymphocytes cofilin is phosphorylated and localized in the cytoplasm. Following co‐stimulation through accessory receptors (e.g. CD2 or CD28) – however, not following TCR/CD3 stimulation alone – cofilin undergoes dephosphorylation. The subcellular localization as well as the actin‐binding activity of cofilin are regulated by the phosphorylation state of serine‐3. Thus, only the dephosphorylated form of cofilin associates with the actin cytoskeleton and possesses the capability to translocate into the nucleus. Recently, LIM‐kinase 1 was shown to inactivate cofilin through phosphorylation. Here, we have identified the functional counterparts of LIM‐kinase 1: the serine/threonine phosphatases of type 1 and type 2A not only associate with cofilin but also dephosphorylate this 19‐kDa protein and thereby mediate cofilin activation. In malignant T lymphoma cells, activation of these phosphatases occurs spontaneously, independent of external stimuli. In untransformed human peripheral blood T lymphocytes, these phosphatases function through a cyclosporin A/FK506‐resistant co‐stimulatory signaling pathway which is common for the accessory receptors CD2 and CD28. This co‐stimulatory signaling pathway is also not affected by a series of other clinically established immunosuppressive drugs (i.e. rapamycin, dexamethasone, leflunomide or mycophenolic acid).

High affinity interaction of integrin α4β1 (VLA-4) and vascular cell adhesion molecule 1 (VCAM-1) enhances migration of human melanoma cells across activated endothelial cell layers

The capacity of tumor cells to form metastatic foci correlates with their ability to interact with and migrate through endothelial cell layers. This process involves multiple adhesive interactions between tumor cells and the endothelium. Only little is known about the molecular nature of these interactions during extravasation of tumor cells. In human melanoma cells, the integrin αvβ3 is involved in transendothelial migration and its expression correlates with metastasis. However, many human melanoma cells do not express β3 integrins. Therefore, it remained unclear how these cells undergo transendothelial migration. In this study we show that human melanoma cells with different metastatic potency, which do not express β2 or β3 integrins, express the VCAM‐1 receptor α4β1. VCAM‐1 is up‐regulated on activated endothelial cells and is known to promote transendothelial migration of leukocytes. Interestingly, despite comparable cell surface levels of α4β1, only the highly metastatic melanoma cell lines MV3 and BLM, but not the low metastatic cell lines IF6 and 530, bind VCAM‐1 with high affinity without further stimulation, and are therefore able to adhere to and migrate on isolated VCAM‐1. Moreover, we demonstrate that function‐blocking antibodies against the integrin α4β1, as well as siRNA‐mediated knock‐down of the α4 subunit in these highly metastatic human melanoma cells reduce their transendothelial migration. These data imply that only high affinity interactions between the integrin α4β1 on melanoma cells and VCAM‐1 on activated endothelial cells may enhance the metastatic capacity of human β2/β3‐negative melanoma cells. J. Cell. Physiol. 212: 368–374, 2007.

Cofilin peptide homologs interfere with immunological synapse formation and T cell activation

The formation of supramolecular activation clusters within the immunological synapse, crucial for sustained signaling and T lymphocyte activation, requires costimulation-dependent reorganization of the actin cytoskeleton. Here we have identified the actin-remodeling protein cofilin as a key player in this process. Cell-permeable peptides that block costimulation-induced cofilin/F-actin interactions in untransformed human T lymphocytes impair receptor capping and immunological synapse formation at the interface between T cells and antigen-presenting cells. As a consequence, T cell activation, as measured by cytokine production and proliferation, is inhibited.

Oxidation of Cofilin Mediates T Cell Hyporesponsiveness under Oxidative Stress Conditions

Oxidative stress leads to impaired T cell activation. A central integrator of T cell activation is the actin-remodelling protein cofilin. Cofilin is activated through dephosphorylation at Ser3. Activated cofilin enables actin dynamics through severing and depolymerization of F-actin. Binding of cofilin to actin is required for formation of the immune synapse and T cell activation. Here, we showed that oxidatively stressed human T cells were impaired in chemotaxis- and costimulation-induced F-actin modulation. Although cofilin was dephosphorylated, steady-state F-actin levels increased under oxidative stress conditions. Mass spectrometry revealed that cofilin itself was a target for oxidation. Cofilin oxidation induced formation of an intramolecular disulfide bridge and loss of its Ser3 phosphorylation. Importantly, dephosphorylated oxidized cofilin, although still able to bind to F-actin, did not mediate F-actin depolymerization. Impairing actin dynamics through oxidation of cofilin provides a molecular explanation for the T cell hyporesponsiveness caused by oxidative stress.

Dendritic cells control T cell tonic signaling required for responsiveness to foreign antigen

Dendritic cells (DCs) are key components of the adaptive immune system contributing to initiation and regulation of T cell responses. T cells continuously scan DCs in lymphoid organs for the presence of foreign antigen. However, little is known about the functional consequences of these frequent T cell–DC interactions without cognate antigen. Here we demonstrate that these contacts in the absence of foreign antigen serve an important function, namely, induction of a basal activation level in T cells required for responsiveness to subsequent encounters with foreign antigens. This basal activation is provided by self-recognition of MHC molecules on DCs. Following DC depletion in mice, T cells became impaired in TCR signaling and immune synapse formation, and consequently were hyporesponsive to antigen. This process was reversible, as T cells quickly recovered when the number of DCs returned to a normal level. The extent of T cell reactivity correlated with the degree of DC depletion in lymphoid organs, suggesting that a full DC compartment guarantees optimal T cell responsiveness. These findings indicate that DCs are specialized cells that not only present foreign antigen, but also promote a “tonic” state in T cells for antigen responsiveness.

Cofilin: a missing link between T cell co-stimulation and rearrangement of the actin cytoskeleton

The actin cytoskeletal network plays a regulatory role in receptor‐mediated signal‐transducing events. Recently, we have shown that the small actin‐depolymerizing protein cofilin represents a component of a co‐stimulatory signaling pathway in human T cells. Cofilin is dephosphorylated on phosphoserine residues following co‐stimulation via accessory receptors such as CD2, CD4, CD8 or CD28, but not in response to TCR engagement alone. Here we demonstrate that accessory receptor triggering induces the transient association of cofilin with the actin cytoskeleton. Only the dephosphorylated form of cofilin binds to cytoskeletal actin in vivo. The phosphatidylinositol 3‐kinase inhibitors wortmannin and LY294002 block dephosphorylation of cofilin and its association with the actin cytoskeleton. These results suggest that cofilin provides an as yet missing link between functionally crucial T cell surface receptors and rearrangements of the actin cytoskeleton.