Michael Freeley

Regulation of Protein Kinase C function by phosphorylation on conserved and non-conserved sites.

Protein Kinase C (PKC) is a family of serine/threonine kinases whose function is influenced by phosphorylation. In particular, three conserved phosphorylation sites known as the activation-loop, the turn-motif and the hydrophobic-motif play important roles in controlling the catalytic activity, stability and intracellular localisation of the enzyme. Prevailing models of PKC phosphorylation suggest that phosphorylation of these sites occurs shortly following synthesis and that these modifications are required for the processing of newly-transcribed PKC to the mature (but still inactive) form; phosphorylation is therefore a priming event that enables catalytic activation in response to lipid second messengers. However, many studies have also demonstrated inducible phosphorylation of PKC isoforms at these sites following stimulation, highlighting that our understanding of PKC phosphorylation and its impact on enzymatic function is incomplete. Furthermore, inducible phosphorylation at these sites is often interpreted as catalytic activation, which could be misleading for some isoforms. Recent studies that include systems-wide phosphoproteomic profiling of cells has revealed a host of additional (and in many cases non-conserved) phosphorylation sites on PKC family members that influence their function. Many of these may in fact be more suitable than previously described sites as surrogate markers of catalytic activation. Here we discuss the role of phosphorylation in controlling PKC function and outline our current understanding of the mechanisms that regulate these phosphorylation sites.

L-Plastin Regulates Polarization and Migration in Chemokine-Stimulated Human T Lymphocytes

Chemokines such as SDF-1α play a crucial role in orchestrating T lymphocyte polarity and migration via polymerization and reorganization of the F-actin cytoskeleton, but the role of actin-associated proteins in this process is not well characterized. In this study, we have investigated a role for L-plastin, a leukocyte-specific F-actin–bundling protein, in SDF-1α–stimulated human T lymphocyte polarization and migration. We found that L-plastin colocalized with F-actin at the leading edge of SDF-1α–stimulated T lymphocytes and was also phosphorylated at Ser5, a site that when phosphorylated regulates the ability of L-plastin to bundle F-actin. L-plastin phosphorylation was sensitive to pharmacological inhibitors of protein kinase C (PKC), and several PKC isoforms colocalized with L-plastin at the leading edge of SDF-1α–stimulated lymphocytes. However, PKC ζ, an established regulator of cell polarity, was the only isoform that regulated L-plastin phosphorylation. Knockdown of L-plastin expression with small interfering RNAs demonstrated that this protein regulated the localization of F-actin at the leading edge of chemokine-stimulated cells and was also required for polarization, lamellipodia formation, and chemotaxis. Knockdown of L-plastin expression also impaired the Rac1 activation cycle and Akt phosphorylation in response to SDF-1α stimulation. Furthermore, L-plastin also regulated SDF-1α–mediated lymphocyte migration on the integrin ligand ICAM-1 by influencing velocity and persistence, but in a manner that was independent of LFA-1 integrin activation or adhesion. This study, therefore, demonstrates an important role for L-plastin and the signaling pathways that regulate its phosphorylation in response to chemokines and adds L-plastin to a growing list of proteins implicated in T lymphocyte polarity and migration.

Phosphorylation of Rab5a protein by protein kinase Cϵ is crucial for T-cell migration

Background: Rab5a GTPase plays important roles in intracellular transport and cell signaling. Results: T-cell stimulation through the integrin LFA-1 or the chemokine receptor CXCR4 induces PKCϵ-dependent phosphorylation of Rab5a at Thr-7, which is crucial for cytoskeleton remodeling and cell migration. Conclusion: PKCϵ-Rab5a-Rac1 axis regulates T-cell motility. Significance: The study provides novel insights into the role of Rab5a in the adaptive immune response. Rab GTPases control membrane traffic and receptor-mediated endocytosis. Within this context, Rab5a plays an important role in the spatial regulation of intracellular transport and signal transduction processes. Here, we report a previously uncharacterized role for Rab5a in the regulation of T-cell motility. We show that Rab5a physically associates with protein kinase Cϵ (PKCϵ) in migrating T-cells. After stimulation of T-cells through the integrin LFA-1 or the chemokine receptor CXCR4, Rab5a is phosphorylated on an N-terminal Thr-7 site by PKCϵ. Both Rab5a and PKCϵ dynamically interact at the centrosomal region of migrating cells, and PKCϵ-mediated phosphorylation on Thr-7 regulates Rab5a trafficking to the cell leading edge. Furthermore, we demonstrate that Rab5a Thr-7 phosphorylation is functionally necessary for Rac1 activation, actin rearrangement, and T-cell motility. We present a novel mechanism by which a PKCϵ-Rab5a-Rac1 axis regulates cytoskeleton remodeling and T-cell migration, both of which are central for the adaptive immune response.

A High-Content Analysis Toolbox Permits Dissection of Diverse Signaling Pathways for T Lymphocyte Polarization

RNA interfering (RNAi) screening strategies offer the potential to elucidate the signaling pathways that regulate integrin and adhesion receptor-mediated changes in T lymphocyte morphology. Of crucial importance, however, is the definition of key sets of parameters that will provide accurate, quantitative, and nonredundant information to flag relevant hits in such assays. In this study, the authors have used an image-based high-content analysis (HCA) technology platform and a panel of 24 pharmacological inhibitors, at a range of concentrations, to define key sets of parameters that enables sensitive and quantitative effects on integrin (LFA-1)–mediated lymphocyte morphology to be evaluated. In particular, multiparametric analysis of lymphocyte morphology that was based on intracellular staining of both the F-actin and α-tubulin cytoskeleton resulted in improved ability to discriminate morphological behavior compared to F-actin staining alone. Morphological and fluorescence intensity/distribution profiling of pharmacologically treated lymphocytes stimulated with integrin (LFA-1) and adhesion receptors (CD44) also revealed notable differences in their sensitivity to inhibitors. The assay described here may be used in HCA strategies such as RNAi screening assays to elucidate the signaling pathways and molecules that regulate integrin/adhesion receptor-mediated T lymphocyte polarization.

Hepatitis C virus targets the T cell secretory machinery as a mechanism of immune evasion

T cell activation and the resultant production of interleukin (IL‐2) is a central response of the adaptive immune system to pathogens, such as hepatitis C virus (HCV). HCV uses several mechanisms to evade both the innate and adaptive arms of the immune response. Here we demonstrate that liver biopsy specimens from individuals infected with HCV had significantly lower levels of IL‐2 compared with those with other inflammatory liver diseases. Cell culture–grown HCV particles inhibited the production of IL‐2 by normal peripheral blood mononuclear cells, as did serum from HCV‐infected patients. This process was mediated by the interaction of HCV envelope protein E2 with tetraspanin CD81 coreceptor. HCV E2 attenuated IL‐2 production at the level of secretion and not transcription by targeting the translocation of protein kinase C beta (PKCβ), which is essential for IL‐2 secretion, to lipid raft microdomains. The lipid raft disruptor methyl‐β‐cyclodextrin reversed HCV E2‐mediated inhibition of IL‐2 secretion, but not in the presence of a PKCβ‐selective inhibitor. HCV E2 further inhibited the secretion of other cytokines, including interferon‐γ. Conclusion: These data suggest that HCV E2–mediated disruption of the association of PKCβ with the cellular secretory machinery represents a novel mechanism for HCV to evade the human immune response and to establish persistent infection. (HEPATOLOGY 2011;)

LFA-1/ICAM-1 Ligation in Human T Cells Promotes Th1 Polarization through a GSK3β Signaling–Dependent Notch Pathway

In this study, we report that the integrin LFA-1 cross-linking with its ligand ICAM-1 in human PBMCs or CD4+ T cells promotes Th1 polarization by upregulating IFN-γ secretion and T-bet expression. LFA-1 stimulation in PBMCs, CD4+ T cells, or the T cell line HuT78 activates the Notch pathway by nuclear translocation of cleaved Notch1 intracellular domain (NICD) and upregulation of target molecules Hey1 and Hes1. Blocking LFA-1 by a neutralizing Ab or specific inhibition of Notch1 by a γ-secretase inhibitor substantially inhibits LFA-1/ICAM-1–mediated activation of Notch signaling. We further demonstrate that the Notch pathway activation is dependent on LFA-1/ICAM-1–induced inactivation of glycogen synthase kinase 3β (GSK3β), which is mediated via Akt and ERK. Furthermore, in silico analysis in combination with coimmunoprecipitation assays show an interaction between NICD and GSK3β. Thus, there exists a molecular cross-talk between LFA-1 and Notch1 through the Akt/ERK–GSK3β signaling axis that ultimately enhances T cell differentiation toward Th1. Although clinical use of LFA-1 antagonists is limited by toxicity related to immunosuppression, these findings support the concept that Notch inhibitors could be attractive for prevention or treatment of Th1-related immunologic disorders and have implications at the level of local inflammatory responses.

Analysis of dynamic tyrosine phosphoproteome in LFA-1 triggered migrating T-cells

The ordered, directional migration of T‐lymphocytes is a key process during immune surveillance and response. This requires cell adhesion to the high endothelial venules or to the extracellular matrix by a series of surface receptor/ligand interactions involving adhesion molecules of the integrin family including lymphocyte function associated molecule‐1 (LFA‐1) and intercellular adhesion molecules (ICAMs). Reversible protein phosphorylation is emerging as a key player in the regulation of biological functions with tyrosine phosphorylation playing a crucial role in signal transduction. Thus, the study of this type of post‐translational modification at the proteomic level has great biological significance. In this work, phospho‐enriched cell lysates from LFA‐1‐triggered migrating human T‐cells were subjected to immunoaffinity purification of tyrosine phosphorylated proteins, mass spectrometric, and bioinformatic analysis. In addition to the identification of several well‐documented proteins, the analysis suggested involvement of a number of new and novel proteins in LFA‐1 induced T‐cell migration. This dataset expands the list of the signaling components of the LFA‐1 induced phosphotyrosine protein complexes in migrating T‐cells that will be extremely useful in the study of their specific roles within LFA‐1 associated signaling pathways. Identification of proteins previously not reported in the context of LFA‐1 stimulated signal transduction might provide new insights into understanding the LFA‐1 signaling networks and aid in the search for new potential therapeutic targets. J. Cell. Physiol. 226: 1489–1498, 2011.

Differential modulation of Helicobacter pylori lipopolysaccharide-mediated TLR2 signaling by individual Pellino proteins.

Eradication rates for current H. pylori therapies have fallen in recent years, in line with the emergence of antibiotic resistant infections. The development of therapeutic alternatives to antibiotics, such as immunomodulatory therapy and vaccines, requires a more lucid understanding of host–pathogen interactions, including the relationships between the organism and the innate immune response. Pellino proteins are emerging as key regulators of immune signaling, including the Toll‐like receptor pathways known to be regulated by H. pylori. The aim of this study was to characterize the role of Pellino proteins in the innate immune response to H. pylori lipopolysaccharide.

The two hit hypothesis: An improved method for siRNA-mediated gene silencing in stimulated primary human T cells ☆

Small interfering RNAs (siRNAs) have revolutionised cellular and molecular biology by uncovering new roles for genes in various biological processes and by providing new opportunities to silence gene expression for therapeutic purposes. A limiting factor of siRNA-mediated gene silencing, however, is the ability to efficiently deliver these molecules into hard-to-transfect cell types such as primary T cells. Nucleofection® technology, marketed by Lonza (Amaxa®), is an electroporation-based method that is commonly used for the delivery of siRNAs and plasmids into primary T cells. In this study we found that the recommended programs for nucleofection of stimulated primary human T cells with siRNAs inhibited cellular proliferation and were associated with a significant loss of cell viability. Furthermore, viable cells that survived the nucleofection procedure were perturbed in their ability to polarise in response to chemokine stimulation in comparison to mock nucleofections. We therefore evaluated other nucleofection programs and highlight one that resulted in significant silencing at the protein level following nucleofection with siRNAs, while maintaining cell viability and responsiveness to chemokine stimulation. Further optimisation of this method revealed that a second nucleofection with siRNAs after 72 h significantly increased silencing compared to a single nucleofection. This new and improved two-hit nucleofection method for siRNA-mediated gene silencing in stimulated primary human T cells will therefore permit the investigation of genes and signalling pathways in the T cell immune response.

Protein kinase C delta is a substrate of tissue transglutaminase and a novel autoantigen in coeliac disease

Post-translational modification of proteins by deamidation or transamidation by tissue transglutaminase (tTG) has been suggested as a possible mechanism for the development of autoimmunity. Sequence analysis of protein kinase C delta (PKCδ) identified an amino acid motif that suggested the possibility that PKCδ was a glutamine substrate of tTG and MALDI-TOF analysis of synthesised peptides from PKCδ proved that this was the case. Polymerisation experiments using recombinant tTG and biotinylated hexapeptide substrate incorporation assays demonstrated that PKCδ is a substrate for tTG-mediated transamidation. Elevated levels of anti-PKCδ antibodies were detected in sera from patients with coeliac disease (p<0.0001) but not from patients with other autoimmune disorders. These data suggest that a subset of patients with coeliac disease produce autoantibodies against PKCδ and that this response may stem from a tTG-PKCδ substrate interaction.