Chenqi Xu

Regulation of T Cell Receptor Activation by Dynamic Membrane Binding of the CD3ɛ Cytoplasmic Tyrosine-Based Motif

Many immune system receptors signal through cytoplasmic tyrosine-based motifs (ITAMs), but how receptor ligation results in ITAM phosphorylation remains unknown. Live-cell imaging studies showed a close interaction of the CD3epsilon cytoplasmic domain of the T cell receptor (TCR) with the plasma membrane through fluorescence resonance energy transfer between a C-terminal fluorescent protein and a membrane fluorophore. Electrostatic interactions between basic CD3epsilon residues and acidic phospholipids enriched in the inner leaflet of the plasma membrane were required for binding. The nuclear magnetic resonance structure of the lipid-bound state of this cytoplasmic domain revealed deep insertion of the two key tyrosines into the hydrophobic core of the lipid bilayer. Receptor ligation thus needs to result in unbinding of the CD3epsilon ITAM from the membrane to render these tyrosines accessible to Src kinases. Sequestration of key tyrosines into the lipid bilayer represents a previously unrecognized mechanism for control of receptor activation.

Self-antigen tetramers discriminate between myelin autoantibodies to native or denatured protein

The role of autoantibodies in the pathogenesis of multiple sclerosis (MS) and other demyelinating diseases is controversial, in part because widely used western blotting and ELISA methods either do not permit the detection of conformation-sensitive antibodies or do not distinguish them from conformation-independent antibodies. We developed a sensitive assay based on self-assembling radiolabeled tetramers that allows discrimination of antibodies against folded or denatured myelin oligodendrocyte glycoprotein (MOG) by selective unfolding of the antigen domain. The tetramer radioimmunoassay (RIA) was more sensitive for MOG autoantibody detection than other methodologies, including monomer-based RIA, ELISA or fluorescent-activated cell sorting (FACS). Autoantibodies from individuals with acute disseminated encephalomyelitis (ADEM) selectively bound the folded MOG tetramer, whereas sera from mice with experimental autoimmune encephalomyelitis induced with MOG peptide immunoprecipitated only the unfolded tetramer. MOG-specific autoantibodies were identified in a subset of ADEM but only rarely in adult-onset MS cases, indicating that MOG is a more prominent target antigen in ADEM than MS.

The Structure of the ζζ Transmembrane Dimer Reveals Features Essential for Its Assembly with the T Cell Receptor

The T cell receptor (TCR) alphabeta heterodimer communicates ligand binding to the cell interior via noncovalently associated CD3gammaepsilon, CD3deltaepsilon, and zetazeta dimers. While structures of extracellular components of the TCR-CD3 complex are known, the transmembrane (TM) domains that mediate assembly have eluded structural characterization. Incorporation of the zetazeta signaling module is known to require one basic TCRalpha and two zetazeta aspartic acid TM residues. We report the NMR structure of the zetazeta(TM) dimer, a left-handed coiled coil with substantial polar contacts. Mutagenesis experiments demonstrate that three polar positions are critical for zetazeta dimerization and assembly with TCR. The two aspartic acids create a single structural unit at the zetazeta interface stabilized by extensive hydrogen bonding, and there is evidence for a structural water molecule (or molecules) within close proximity. This structural unit, representing only the second transmembrane dimer interface solved to date, serves as a paradigm for the assembly of all modules involved in TCR signaling.

Potentiating the antitumour response of CD8+ T cells by modulating cholesterol metabolism

CD8+ T cells have a central role in antitumour immunity, but their activity is suppressed in the tumour microenvironment. Reactivating the cytotoxicity of CD8+ T cells is of great clinical interest in cancer immunotherapy. Here we report a new mechanism by which the antitumour response of mouse CD8+ T cells can be potentiated by modulating cholesterol metabolism. Inhibiting cholesterol esterification in T cells by genetic ablation or pharmacological inhibition of ACAT1, a key cholesterol esterification enzyme, led to potentiated effector function and enhanced proliferation of CD8+ but not CD4+ T cells. This is due to the increase in the plasma membrane cholesterol level of CD8+ T cells, which causes enhanced T-cell receptor clustering and signalling as well as more efficient formation of the immunological synapse. ACAT1-deficient CD8+ T cells were better than wild-type CD8+ T cells at controlling melanoma growth and metastasis in mice. We used the ACAT inhibitor avasimibe, which was previously tested in clinical trials for treating atherosclerosis and showed a good human safety profile, to treat melanoma in mice and observed a good antitumour effect. A combined therapy of avasimibe plus an anti-PD-1 antibody showed better efficacy than monotherapies in controlling tumour progression. ACAT1, an established target for atherosclerosis, is therefore also a potential target for cancer immunotherapy.

Ca2+ regulates T-cell receptor activation by modulating the charge property of lipids.

Ionic protein–lipid interactions are critical for the structure and function of membrane receptors, ion channels, integrins and many other proteins. However, the regulatory mechanism of these interactions is largely unknown. Here we show that Ca2+ can bind directly to anionic phospholipids and thus modulate membrane protein function. The activation of T-cell antigen receptor–CD3 complex (TCR), a key membrane receptor for adaptive immunity, is regulated by ionic interactions between positively charged CD3ε/ζ cytoplasmic domains (CD3CD) and negatively charged phospholipids in the plasma membrane. Crucial tyrosines are buried in the membrane and are largely protected from phosphorylation in resting T cells. It is not clear how CD3CD dissociates from the membrane in antigen-stimulated T cells. The antigen engagement of even a single TCR triggers a Ca2+ influx and TCR-proximal Ca2+ concentration is higher than the average cytosolic Ca2+ concentration. Our biochemical, live-cell fluorescence resonance energy transfer and NMR experiments showed that an increase in Ca2+ concentration induced the dissociation of CD3CD from the membrane and the solvent exposure of tyrosine residues. As a consequence, CD3 tyrosine phosphorylation was significantly enhanced by Ca2+ influx. Moreover, when compared with wild-type cells, Ca2+ channel-deficient T cells had substantially lower levels of CD3 phosphorylation after stimulation. The effect of Ca2+ on facilitating CD3 phosphorylation is primarily due to the charge of this ion, as demonstrated by the fact that replacing Ca2+ with the non-physiological ion Sr2+ resulted in the same feedback effect. Finally, 31P NMR spectroscopy showed that Ca2+ bound to the phosphate group in anionic phospholipids at physiological concentrations, thus neutralizing the negative charge of phospholipids. Rather than initiating CD3 phosphorylation, this regulatory pathway of Ca2+ has a positive feedback effect on amplifying and sustaining CD3 phosphorylation and should enhance T-cell sensitivity to foreign antigens. Our study thus provides a new regulatory mechanism of Ca2+ to T-cell activation involving direct lipid manipulation.

Regulation of EGFR nanocluster formation by ionic protein-lipid interaction

The abnormal activation of epidermal growth factor receptor (EGFR) is strongly associated with a variety of human cancers but the underlying molecular mechanism is not fully understood. By using direct stochastic optical reconstruction microscopy (dSTORM), we find that EGFR proteins form nanoclusters in the cell membrane of both normal lung epithelial cells and lung cancer cells, but the number and size of clusters significantly increase in lung cancer cells. The formation of EGFR clusters is mediated by the ionic interaction between the anionic lipid phosphatidylinositol-4,5-bisphosphate (PIP2) in the plasma membrane and the juxtamembrane (JM) region of EGFR. Disruption of EGFR clustering by PIP2 depletion or JM region mutation impairs EGFR activation and downstream signaling. Furthermore, JM region mutation in constitutively active EGFR mutant attenuates its capability of cell transformation. Collectively, our findings highlight the key roles of anionic phospholipids in EGFR signaling and function, and reveal a novel mechanism to explain the aberrant activation of EGFR in cancers.

Ionic protein-lipid interaction at the plasma membrane: what can the charge do?

Phospholipids are the major components of cell membranes, but they have functional roles beyond forming lipid bilayers. In particular, acidic phospholipids form microdomains in the plasma membrane and can ionically interact with proteins via polybasic sequences, which can have functional consequences for the protein. The list of proteins regulated by ionic protein-lipid interaction has been quickly expanding, and now includes membrane proteins, cytoplasmic soluble proteins, and viral proteins. Here we review how acidic phospholipids in the plasma membrane regulate protein structure and function via ionic interactions, and how Ca(2+) regulates ionic protein-lipid interactions via direct and indirect mechanisms.

BmTx3, a scorpion toxin with two putative functional faces separately active on A-type K+ and HERG currents

A novel HERG channel blocker was isolated from the venom of the scorpion Buthus martensi Karsch, sequenced and characterized at the pharmacological level after chemical synthesis. According to the determined amino acid sequence, the cDNA and genomic genes were then cloned. The genomic gene consists of two exons interrupted by an intron of 65 bp at position -6 upstream from the mature toxin. The protein sequence of this toxin was completely identical with that of a known A-type K+ current blocker BmTx3, belonging to scorpion alpha-KTx subfamily 15. Thus BmTx3 is the first reported alpha-KTx peptide also showing HERG-blocking activity, like gamma-KTx peptides. Moreover, different from classical alpha-KTx peptides, such as charybdotoxin, BmTx3 cannot block Shaker -type K+ channels. Phylogenetic tree analysis reveals that this toxin takes an intermediate position between classical alpha-KTx and gamma-KTx toxins. From a structural point of view, we propose that two separate functional faces might exist on the BmTx3 molecule, responsible for the two different K+-current-blocking functions. Face A, composed of Arg18 and Lys19 in the alpha-helix side, might correspond to HERG blocking activity, whereas Face B, containing a putative functional dyad (Lys27 and Tyr36) in the beta-sheet side, might correspond to A-type blocking activity. A specific deletion mutant with the disrupted Face B, BmTx3-Y36P37del, loses the A-type current-blocking activity, but keeps a similar HERG-blocking activity, as seen with the wild-type toxin.

The clathrin adaptor Numb regulates intestinal cholesterol absorption through dynamic interaction with NPC1L1

Hypercholesterolemia, typically due to excessive cholesterol uptake, is a major risk factor for cardiovascular disease, which is responsible for ∼50% of all deaths in developed societies. Although it has been shown that intestinal cholesterol absorption is mediated by vesicular endocytosis of the Niemann-Pick C1–like 1 (NPC1L1) protein, the mechanism of sterol-stimulated NPC1L1 internalization is still mysterious. Here, we identified an endocytic peptide signal, YVNXXF (where X stands for any amino acid), in the cytoplasmic C-terminal tail of NPC1L1. Cholesterol binding on the N-terminal domain of NPC1L1 released the YVNXXF-containing region of NPC1L1 from association with the plasma membrane and enabled Numb binding. We also found that Numb, a clathrin adaptor, specifically recognized this motif and recruited clathrin for internalization. Disrupting the NPC1L1-Numb interaction decreased cholesterol uptake. Ablation of Numb in mouse intestine significantly reduced dietary cholesterol absorption and plasma cholesterol level. Together, these data show that Numb is a pivotal protein for intestinal cholesterol absorption and may provide a therapeutic target for hypercholesterolemia.

A membrane-proximal tetracysteine motif contributes to assembly of CD3deltaepsilon and CD3gammaepsilon dimers with the T cell receptor.

Assembly of the T cell receptor (TCR) with its dimeric signaling modules, CD3δϵ, CD3γϵ, and ζζ, is organized by transmembrane (TM) interactions. Each of the three assembly steps requires formation of a three-helix interface involving one particular basic TCR TM residue and two acidic TM residues of the respective signaling dimer. The extracellular domains of CD3δϵ and CD3γϵ contribute to assembly, but TCR interaction sites on CD3 dimers have not been defined. The structures of the extracellular domains of CD3δϵ and CD3γϵ demonstrated parallel β-strands ending at the first cysteine in the CXXCXEXXX motif present in the stalk segment of each CD3 chain. Mutation of the membrane-proximal cysteines impaired assembly of either CD3 dimer with TCR, and little complex was isolated when all four membrane-proximal cysteines were mutated to alanine. These mutations had, however, no discernable effect on CD3δϵ or CD3γϵ dimerization. CD3δϵ assembled with a TCRα mutant that lacked both immunoglobulin domains, but shortening of the TCRα connecting peptide reduced assembly, consistent with membrane-proximal TCRα-CD3δϵ interactions. Chelation of divalent cations did not affect assembly, indicating that coordination of a cation by the tetracysteine motif was not required. The membrane-proximal cysteines were within close proximity but only formed covalent CD3 dimers when one cysteine was mutated. The four cysteines may thus form two intrachain disulfide bonds integral to the secondary structure of CD3 stalk regions. The three-chain interaction theme first established for the TM domains thus extends into the membrane-proximal domains of TCRα-CD3δϵ and TCRβ-CD3γϵ.