Negative regulation of TCR signaling by ubiquitination of Zap-70 Lys-217

Cyclophilin A associates with and regulates the activity of ZAP70 in TCR/CD3-stimulated T cells

The ZAP70 protein tyrosine kinase (PTK) couples stimulated T cell antigen receptors (TCRs) to their downstream signal transduction pathways and is sine qua non for T cell activation and differentiation. TCR engagement leads to activation-induced post-translational modifications of ZAP70, predominantly by kinases, which modulate its conformation, leading to activation of its catalytic domain. Here, we demonstrate that ZAP70 in TCR/CD3-activated mouse spleen and thymus cells, as well as human Jurkat T cells, is regulated by the peptidyl-prolyl cis-trans isomerase (PPIase), cyclophilin A (CypA) and that this regulation is abrogated by cyclosporin A (CsA), a CypA inhibitor. We found that TCR crosslinking promoted a rapid and transient, Lck-dependent association of CypA with the interdomain B region, at the ZAP70 regulatory domain. CsA inhibited CypA binding to ZAP70 and prevented the colocalization of CypA and ZAP70 at the cell membrane. In addition, imaging analyses of antigen-specific T cells stimulated by MHC-restricted antigen-fed antigen-presenting cells revealed the recruitment of ZAP70-bound CypA to the immunological synapse. Enzymatically active CypA downregulated the catalytic activity of ZAP70 in vitro, an effect that was reversed by CsA in TCR/CD3-activated normal T cells but not in CypA-deficient T cells, and further confirmed in vivo by FRET-based studies. We suggest that CypA plays a role in determining the activity of ZAP70 in TCR-engaged T cells and impact on T cell activation by intervening with the activity of multiple downstream effector molecules.

The T cell CD6 receptor operates a multitask signalosome with opposite functions in T cell activation

To determine the respective contribution of the LAT transmembrane adaptor and CD5 and CD6 transmembrane receptors to early TCR signal propagation, diversification, and termination, we describe a CRISPR/Cas9-based platform that uses primary mouse T cells and permits establishment of the composition of their LAT, CD5, and CD6 signalosomes in only 4 mo using quantitative mass spectrometry. We confirmed that positive and negative functions can be solely assigned to the LAT and CD5 signalosomes, respectively. In contrast, the TCR-inducible CD6 signalosome comprised both positive (SLP-76, ZAP70, VAV1) and negative (UBASH3A/STS-2) regulators of T cell activation. Moreover, CD6 associated independently of TCR engagement to proteins that support its implication in inflammatory pathologies necessitating T cell transendothelial migration. The multifaceted role of CD6 unveiled here accounts for past difficulties in classifying it as a coinhibitor or costimulator. Congruent with our identification of UBASH3A within the CD6 signalosome and the view that CD6 constitutes a promising target for autoimmune disease treatment, single-nucleotide polymorphisms associated with human autoimmune diseases have been found in the Cd6 and Ubash3a genes.

Coordinating Cytoskeleton and Molecular Traffic in T Cell Migration, Activation, and Effector Functions

Dynamic localization of receptors and signaling molecules at the plasma membrane and within intracellular vesicular compartments is crucial for T lymphocyte sensing environmental cues, triggering membrane receptors, recruiting signaling molecules, and fine-tuning of intracellular signals. The orchestrated action of actin and microtubule cytoskeleton and intracellular vesicle traffic plays a key role in all these events that together ensure important steps in T cell physiology. These include extravasation and migration through lymphoid and peripheral tissues, T cell interactions with antigen-presenting cells, T cell receptor (TCR) triggering by cognate antigen-major histocompatibility complex (MHC) complexes, immunological synapse formation, cell activation, and effector functions. Cytoskeletal and vesicle traffic dynamics and their interplay are coordinated by a variety of regulatory molecules. Among them, polarity regulators and membrane-cytoskeleton linkers are master controllers of this interplay. Here, we review the various ways the T cell plasma membrane, receptors, and their signaling machinery interplay with the actin and microtubule cytoskeleton and with intracellular vesicular compartments. We highlight the importance of this fine-tuned crosstalk in three key stages of T cell biology involving cell polarization: T cell migration in response to chemokines, immunological synapse formation in response to antigen cues, and effector functions. Finally, we discuss two examples of perturbation of this interplay in pathological settings, such as HIV-1 infection and mutation of the polarity regulator and tumor suppressor adenomatous polyposis coli (Apc) that leads to familial polyposis and colorectal cancer.

Integrative proteomics reveals an increase in non-degradative ubiquitylation in activated CD4+ T cells

Despite gathering evidence that ubiquitylation can direct non-degradative outcomes, most investigations of ubiquitylation in T cells have focused on degradation. Here, we integrated proteomic and transcriptomic datasets from primary mouse CD4⁺ T cells to establish a framework for predicting degradative or non-degradative outcomes of ubiquitylation. Di-glycine remnant profiling was used to reveal ubiquitylated proteins, which in combination with whole-cell proteomic and transcriptomic data allowed prediction of protein degradation. Analysis of ubiquitylated proteins identified by di-glycine remnant profiling indicated that activation of CD4⁺ T cells led to an increase in non-degradative ubiquitylation. This correlated with an increase in non-proteasome-targeted K29, K33 and K63 polyubiquitin chains. This study revealed over 1,200 proteins that were ubiquitylated in primary mouse CD4⁺ T cells and highlighted the relevance of non-proteasomally targeted ubiquitin chains in T cell signaling.

Discovery and Characterization of Two Classes of Selective Inhibitors of the Suppressor of the TCR Signaling Family of Proteins

The suppressor of T-cell receptor signaling (Sts) proteins, Sts-1, has recently emerged as a potential immunostimulatory target for drug development. Genetic inactivation of the Sts proteins dramatically increases host survival of systemic infection and leads to improved pathogen clearance. The protein tyrosine phosphatase (PTP) activity of these proteins arises from a C-terminal 2-histidine phosphatase (HP) domain. To identify new inhibitors of the HP activity of Sts-1, we miniaturized a phosphatase assay to a 1536-well format and conducted a 20 580 compound screen. Among the hits were two classes of structurally related compounds, tetracycline variants and sulfonated azo dyes. These hits had low micromolar to nanomolar IC₅₀ values. Orthogonal screening confirmed the validity of these inhibitors and demonstrated that both act competitively on Sts-1 phosphatase activity. When tested on other PTPs, PTP1B and SHP1, it was found that the tetracycline PTP1B, SHP1, the tetracycline variant (doxycycline), and the sulfonated azo dye (Congo red) are selective inhibitors of Sts-1_HP, with selectivity indices ranging from 19 to as high as 200. The planar polyaromatic moieties present in both classes of compounds suggested a common binding mode. The mutation of either tryptophan 494 or tyrosine 596, located near the active site of the protein, reduced the K_i of the inhibitors from 3- to 18-fold, indicating that these residues may help to promote the binding of substrates with aromatic groups. This work provides new insights into substrate selectivity mechanisms and describes two classes of compounds that can serve as probes of function or as a basis for future drug discovery.

A highly conserved redox-active Mx(2)CWx(6)R motif regulates Zap70 stability and activity

ζ-associated protein of 70 kDa (Zap70) is crucial for T-cell receptor (TCR) signaling. Loss of Zap70 in both humans and mice results in severe immunodeficiency. On the other hand, the expression of Zap70 in B-cell malignancies correlates with the severity of the disease. Because of its role in immune-related disorders, Zap70 has become a therapeutic target for the treatment of human diseases. It is well-established that the activity/expression of Zap70 is regulated by post-translational modifications of crucial amino acids including the phosphorylation of tyrosines and the ubiquitination of lysines. Here, we have investigated whether also oxidation of cysteine residues regulates Zap70 functions. We have identified C575 as a major sulfenylation site of Zap70. A C575A substitution results in protein instability, reduced activity, and increased dependency on the Hsp90/Cdc37 chaperone system. Indeed, Cdc37 overexpression reconstituted partially the expression but fully the function of Zap70C575A. C575 lies within a Mx₍₂₎CWx₍₆₎R motif which is highly conserved among almost all human tyrosine kinases. Mutation of any of the conserved amino acids, but not of a non-conserved residue preceding the cysteine, also results in Zap70 instability. Collectively, we have identified a new redox-active motif which is crucial for the regulation of Zap70 stability/activity. We believe that this motif has the potential to become a novel target for the development of therapeutic tools to modulate the expression/activity of kinases.

Targeting p38 MAP kinase signaling in cancer through post-translational modifications

The p38 MAPK signaling pathway is a key signal transduction cascade that cancer cells employ to sense and adapt to a plethora of environmental stimuli, and has attracted much attention as a promising target for cancer therapy. Accumulating evidence suggests a dual role of p38 signaling in various types of cancers, wherein the p38 pathway can both suppress and promote tumor growth, metastasis and chemoresistance. This dual role of p38 signaling, along with its context dependence and versatility, poses a great challenge for developing efficient anticancer treatment. An increasing number of studies showed that p38 signaling is subject to regulation by a variety of post-translational modifications (PTMs). Recently, large-scale proteomics profilings have identified a large number of PTMs on key components of the p38 pathway. However, the majority of these modifications and their biological significance in cancer remain uncharacterized. In this review, we highlight a series of studies that focus on the PTMs in the p38 cascade landscape, and discuss the complexity and implications of these PTMs in p38 MAPK signaling regulation.