Alexander B. Sigalov

Enzymatic repair of oxidative damage to human apolipoprotein A-I

Oxidative damage to apolipoprotein A‐I that occurs in vivo commonly involves methionine oxidation, and is accompanied by alterations in structure, lipid association, and cholesterol efflux function. We have used the enzyme peptide methionine sulfoxide reductase (PMSR) to reverse this damage, and shown by a variety of criteria that enzyme treatment restores the primary, secondary, and tertiary structure and lipid association characteristic of the native unoxidized protein. Lipid‐associated as well as lipid‐free apolipoprotein A‐I reacts with PMSR, suggesting that enzymatic reduction of oxidized apolipoprotein A‐I in high density lipoproteins can result in restoration of biological activity and the ability to promote cholesterol efflux from cells.

The intrinsically disordered cytoplasmic domain of the T cell receptor zeta chain binds to the nef protein of simian immunodeficiency virus without a disorder-to-order transition.

Intrinsically disordered proteins are thought to undergo coupled binding and folding upon interaction with their folded partners. In this study, we investigate whether binding of the intrinsically disordered T cell receptor zeta cytoplasmic tail to the well-folded simian immunodeficiency virus Nef core domain is accompanied by a disorder-to-order transition. We show that zeta forms a 1:1 complex with Nef and remains unfolded in the complex. Thus, our findings oppose the generally accepted view of the behavior of intrinsically disordered proteins and provide new evidence of the existence of specific interactions for unfolded protein molecules.

Oxidation of methionine residues affects the structure and stability of apolipoprotein A-I in reconstituted high density lipoprotein particles.

To determine the effect of oxidative damage to lipid-bound apolipoprotein A-I (apo A-I) on its structure and stability that might be related to previously observed functional disorders of oxidized apo A-I in high density lipoproteins (HDL), we prepared homogeneous reconstituted HDL (rHDL) particles containing unoxidized apo A-I and its commonly occurring oxidized form (Met-112, 148 bis-sulfoxide). The size of the obtained discoidal rHDL particles ranged from 9.0 to 10.0 nm and did not depend upon the content of the oxidized protein. Using circular dichroism methods, no change in the secondary structure of lipid-bound oxidized apo A-I was found. Isothermal and thermal denaturation experiments showed a significant destabilization of the oxidized protein to denaturation by guanidine hydrochloride or heat. This effect was observed with and without co-reconstituted apolipoprotein A-II. Limited tryptic digestion indicated that the central region of oxidatively damaged apo A-I becomes exposed to proteolysis in the rHDL particles. Implications of these data for apolipoprotein function are discussed.

Structure Induction of the T‐Cell Receptor ζ‐Chain upon Lipid Binding Investigated by NMR Spectroscopy

The conformation of the cytoplasmic part of the ζ‐chain of the T‐cell receptor (TCR) in its free form and bound to detergent micelles has been investigated by heteronuclear NMR spectroscopy. The ζ‐chain is considered to be a mediator between the extracellular antigen and the intracellular signal‐transduction cascade leading to T‐cell activation. Earlier studies suggested a T‐cell activation mechanism that involved a TCR‐state‐dependent lipid incorporation propensity of the ζ‐chain accompanied by a helical folding transition. In order to support this proposed mechanism, standard protein NMR assignment and secondary‐structure‐elucidation techniques have been applied to the free TCR ζ‐chain and to the ζ‐chain bound to the detergent LMPG, which forms a micelle, in order to obtain the structural characteristics of this folding transition in a residue‐resolved manner. We could assign the resonances of the free ζ‐chain at 278 K, and this formed the basis for chemical‐shift‐perturbation studies to identify lipid binding sites. Our NMR results show that the free TCR ζ‐chain is indeed intrinsically unstructured. Regions around the ITAM2 and ITAM3 sequences are involved in a highly dynamic binding of the free ζ‐chain to a detergent micelle formed by the acidic lipid LMPG.

Membrane binding mode of intrinsically disordered cytoplasmic domains of T cell receptor signaling subunits depends on lipid composition

Intrinsically disordered cytoplasmic domains of T cell receptor (TCR) signaling subunits including zeta(cyt) and CD3epsilon(cyt) all contain one or more copies of an immunoreceptor tyrosine-based activation motif (ITAM), tyrosine residues of which are phosphorylated upon receptor triggering. Membrane binding-induced helical folding of zeta(cyt) and CD3epsilon(cyt) ITAMs is thought to control TCR activation. However, the question whether or not lipid binding of zeta(cyt) and CD3epsilon(cyt) is necessarily accompanied by a folding transition of ITAMs remains open. In this study, we investigate whether the membrane binding mechanisms of zeta(cyt) and CD3epsilon(cyt) depend on the membrane model used. Circular dichroic and fluorescence data indicate that binding of zeta(cyt) and CD3epsilon(cyt) to detergent micelles and unstable vesicles is accompanied by a disorder-to-order transition, whereas upon binding to stable vesicles these proteins remain unfolded. Using electron microscopy and dynamic light scattering, we show that upon protein binding, unstable vesicles fuse and rupture. In contrast, stable vesicles remain intact under these conditions. This suggests different membrane binding modes for zeta(cyt) and CD3epsilon(cyt) depending on the bilayer stability: (1) coupled binding and folding, and (2) binding without folding. These findings explain the long-standing puzzle in the literature and highlight the importance of the choice of an appropriate membrane model for protein-lipid interactions studies.

Signaling Chain Homooligomerization (SCHOOL) Model

Multichain immune recognitionreceptors (MIRRs) represent a family of surfacereceptors expressed on different cells of the hematopoietic system and function to transduce signals leading to a variety of biologic responses. The most intriguing and distinct structural feature ofMIRR family members is that extracellular recognition domains and intracellular signaling domains are located on separate subunits. The biochemical cascades triggered by MIRRs are understood in significant detail, however, the mechanism by which extracellular ligand binding initiates intracellular signal transduction processes is not clear and no model fully explains how MIRR signaling commences. In this Chapter, I describe a novel mechanistic model of MIRR-mediated signal transduction, the signaling chain homooligomerization (SCHOOL) model. The basic concept of this model assumes that the structural similarity of the MIRRs provides the basis for the similarity in the mechanisms ofMIRR-mediated transmembrane signaling. Within the SCHOOL model, MIRR triggering is considered to be a result of the ligand-induced interplay between (1) intrareceptor transmembrane interactions between MIRR recognition and signaling subunits that stabilize and maintain receptor integrity and (2) interreceptor homointeractions between MIRR signaling subunits that lead to the formation of oligomeric signaling structures, thus triggering the receptors and initiating the signaling cascade. Thus, the SCHOOL model is based on specific protein-protein interactions--biochemical processes that can be influenced and controlled. In this context, this plausible and easily testable model is fundamentally different from those previously suggested for particular MIRRs and has several important advantages. The basic principles oftransmembrane signaling learned from the SCHOOL model may be used in different fields of immunology and cell biology to describe, explain and predict immunological phenomena and processes mediated by structurally related but functionally different membrane receptors. Important applications of the SCHOOL model in clinical immunology, molecular pharmacology and virology are described in the Chapters 20 and 22 of this book.

Viral Pathogenesis, Modulation of Immune Receptor Signaling and Treatment

During the co-evolution of viruses and their hosts, the latter have equipped themselves with an elaborate immune system to defend themselves from the invading viruses. In order to establish a successful infection, replicate and persist in the host, viruses have evolved numerous strategies to counter and evade host antiviral immune responses as well as exploit them for productive viral replication. These strategies include those that target immune receptor transmembrane signaling. Uncovering the exact molecular mechanisms underlying these critical points in viral pathogenesis will not only help us understand strategies used by viruses to escape from the host immune surveillance but also reveal new therapeutic targets for antiviral as well as immunomodulatory therapy. In this chapter, based on our current understanding of transmembrane signal transduction mediated by multichain immune recognition receptors (MIRRs) and the results of sequence analysis, we discuss the MIRR-targeting viral strategies of immune evasion and suggest their possible mechanisms that, in turn, reveal new points of antiviral intervention. We also show how two unrelated enveloped viruses, human immunodeficiency virus and human cytomegalovirus, use a similar mechanism to modulate the host immune response mediated by two functionally different MIRRs—T-cell antigen receptor and natural killer cell receptor, NKp30. This suggests that it is very likely that similar general mechanisms can be or are used by other viral and possibly nonviral pathogens.

Novel mechanistic insights into viral modulation of immune receptor signaling

To successfully infect, replicate, and persist in the host, viruses have evolved numerous strategies to take control of multiple cellular processes, including those that target transmembrane (TM) signal transduction mediated by immune receptors. Despite tremendous advancement in recent years, the exact molecular mechanisms underlying these critical points in viral pathogenesis remain unknown. In this Opinion, based on a novel model of immune signaling, the Signaling Chain HOmoOLigomerization (SCHOOL) model, I suggest specific mechanisms used by different viruses such as human immunodeficiency virus (HIV), cytomegalovirus (CMV), severe acute respiratory syndrome coronavirus (SARS-CoV), herpesvirus saimiri (HVS), human herpesvirus 6 (HHV-6), etc., to modulate the host immune response mediated by members of the family of multichain immune recognition receptors (MIRRs). I also demonstrate how the SCHOOL model, together with the lessons learned from viral pathogenesis, can be used practically for rational drug design and the development of new therapies for immune disorders.

Novel mechanistic concept of platelet inhibition

Background: Activation of circulating platelets by exposed vessel wall collagen is a primary step in the pathogenesis of thrombotic diseases such as heart attack and stroke. Drugs that are capable of blocking platelet activation successfully reduce cardiovascular mortality and morbidity. However, despite intensive research efforts in antithrombotic drug discovery and development, uncontrolled hemorrhage still remains the most common side effect associated with antithrombotic drugs that are currently in use. Objective: The selective inhibition of glycoprotein VI (GPVI), the central platelet collagen receptor, and/or its signaling may inhibit thrombosis without affecting hemostatic plug formation. However, the mechanism of GPVI signaling is not known, hindering the further development of this promising antithrombotic strategy. Methods: This review focuses on an innovative mechanistic concept of platelet inhibition. Results/conclusion: A novel model of GPVI signaling, the signaling chain homooligomerization (SCHOOL) model, has revealed new therapeutic targets for GPVI inhibition, resulting in the development of novel antithrombotic pharmacological approaches and the invention of new platelet inhibitors.

Bi-specific MHC Heterodimers for Characterization of Cross-reactive T Cells

T cell cross-reactivity describes the phenomenon whereby a single T cell can recognize two or more different peptide antigens presented in complex with MHC proteins. Cross-reactive T cells have previously been characterized at the population level by cytokine secretion and MHC tetramer staining assays, but single-cell analysis is difficult or impossible using these methods. In this study, we describe development of a novel peptide-MHC heterodimer specific for cross-reactive T cells. MHC-peptide monomers were independently conjugated to hydrazide or aldehyde-containing cross-linkers using thiol-maleimide coupling at cysteine residues introduced into recombinant MHC heavy chain proteins. Hydrazone formation provided bi-specific MHC heterodimers carrying two different peptides. Using this approach we prepared heterodimers of the murine class I MHC protein H-2Kb carrying peptides from lymphocytic choriomeningitis virus and vaccinia virus, and used these to identify cross-reactive CD8+ T cells recognizing both lymphocytic choriomeningitis virus and vaccinia virus antigens. A similar strategy could be used to develop reagents to analyze cross-reactive T cell responses in humans.