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Alexander Sigalov, Ph.D.Academic Role: Research Assistant Professor
Faculty Appointment(s) In:
Other Affiliation(s):
1. Molecular mechanisms of transmembrane signal transduction in the immune system and ways to control and Multichain Immune Recognition Receptors (MIRRs) such as the T cell receptor (TCR), B cell receptor, Fc receptors, natural killer cell receptors, platelet glycoprotein VI (GPVI) and many others represent a family of surface receptors expressed on different cells of the hematopoietic system and function to transduce signals leading to a variety of biologic responses. These receptors share common structural features, including extracellular ligand-binding domains and intracellular signaling domains carried on separate subunits. Although the MIRR-mediated ligand-recognition and the downstream signaling cascades are believed to be among the best-studied in biology in recent years, at present the ligand-induced spatiotemporal organization of MIRR transmembrane signaling and the molecular mechanisms underlying the initiation of this process remain to be elucidated. My central hypothesis is that the similar architecture of the MIRRs dictates similar mechanisms of MIRR-mediated signaling, thus building the basis for existing and future therapeutic strategies targeting MIRRs. Notably, this assumes that clinical knowledge, experience and therapeutic strategies can be transferred between seemingly disparate immune disorders such as atopic dermatitis, allergy, thrombosis, HIV/AIDS and many others. My major interest in this field is to understand in detail the molecular mechanisms underlying MIRR-mediated transmembrane signaling and learn how to control these processes, thus modulating the immune response. In order to achieve this goal, we are using the basic structure-function principles for MIRRs and new powerful molecular tools including rationally designed transmembrane peptides, as suggested by a novel mechanistic model of MIRR transmembrane signal transduction, the Signaling Chain HOmoOLigomerization (SCHOOL) model. Within the model, MIRR triggering is considered to be a result of ligand-induced interplay between (1) intrareceptor transmembrane interactions that stabilize and maintain receptor integrity, and (2) ligand-triggered interreceptor homointeractions between the cytoplasmic domains of MIRR signaling subunits. These specific protein-protein interactions are suggested and proved to be novel attractive therapeutic targets for the existing and future treatment of a variety of immune diseases. The model also reveals the molecular mechanisms of immune signaling modulation following viral infection (Human Immunodeficiency Virus, Human Cytomegalovirus, etc.) that can be used in rational antiviral drug design. In continuing studies, we are using a variety of computational, biophysical, biochemical and cell methods to further study MIRR-mediated transmembrane signaling and develop novel pharmacological approaches to treat diverse immune-mediated diseases. To date, implication of the SCHOOL model to specific molecular processes underlying GPVI-mediated platelet activation has already resulted in the invention of a novel class of platelet inhibitors. Another implication of the SCHOOL model has been suggested for the effect of HIV/SIV Nef in modulating TCR-mediated T cell activation and is currently under investigation.
Figure 1. A. Structural and functional organization of MIRRs. Transmembrane interactions between MIRR ligand-binding and signaling components play a key role in receptor assembly and integrity on resting cells. B. The signaling chain homooligomerization (SCHOOL) model, proposing that the homooligomerization of signaling subunits plays a central role in triggering MIRR-mediated signal transduction.
2. Intrinsically disordered proteins: structure, function and role in immune cell signaling There is a large number of protein domains and even entire proteins, the so-called intrinsically disordered (natively unfolded, intrinsically unstructured) proteins (IDPs), that lack ordered structure under physiological conditions. Intriguingly, many proteins known to be involved in cell signaling have a highly flexible, random coil-like conformation in their native and functional state. Despite the proximity to the cell membrane and crucial role in transmembrane signal transduction, little structural or biochemical information is available for the cytoplasmic domains of MIRR signaling subunits. Recently, the cytoplasmic regions of signaling subunits from many different and functionally unrelated MIRRs, representing a novel class of IDPs, have been discovered to all have a common unique structural feature – the ability to form specific homooligomers under physiological conditions. This finding opposes the generally accepted view on the behavior of IDPs, and this study represents the first work that displays the existence of specific interactions for natively unfolded protein species. An important physiological role of homooligomerization of MIRR signaling subunits in MIRR-mediated signaling has been suggested and used to develop the SCHOOL model of immune cell activation. My major interest in this field is to understand in detail the nature and molecular basis of these unique interactions and learn how to control these specific protein-protein interactions. In order to achieve this goal, we are studying dynamics and residual structure of IDPs as well as specific interactions between unfolded protein molecules using the relevant recombinant proteins and a variety of molecular biology, biochemical and biophysical techniques, including site-directed mutagenesis and multi-dimensional NMR.
3. Oxidative stress, high density lipoproteins, atherogenesis and autoimmune disorders The risk for atherosclerosis and related cardiovascular disease (CVD) is strongly and inversely related to high density lipoprotein (HDL) levels, but the mechanisms of the protective effect of HDL are not fully understood. Now there is a growing body of evidence that these lipoproteins may lose their anti-atherogenic activity during oxidative stress. This may be especially important for all the people who have lipid levels considered normal but who develop heart disease. However, despite extensive studies of oxidized HDL (ox-HDL), little is known of the structures that cause these effects. My major hypothesis is that a higher content of oxidized, functionally impaired and pro-atherogenic HDL particles may contribute to atherogenesis and cardiovascular events in the people who have normal and even high plasma HDL levels. Hypothesizing a Janus-like action of HDL and their transformation into pro-atherogenic species during oxidative stress, I proposed and proved experimentally that the conversion of apolipoprotein (apo) A-I, the main protein constituent of HDL, methionine residues to methionine sulfoxides during oxidation, occurring in vivo with high inter-individual variability and resulting in significant structural and functional alterations, transforms HDL from anti-atherogenic into pro-atherogenic particles. Further development of this hypothesis led to understanding that oxidative damage to apo A-I can induce an immune response resulting in formation of autoantibodies to this protein, thus promoting autoimmune disorders. My long-term goal in this field is to understand in detail the molecular mechanisms linking oxidative damage to apo A-I and atherogenesis and autoimmune disease as well as to learn how to effectively reverse a transformation of HDL into pro-atherogenic species during oxidative stress. In order to achieve this goal, I am studying in detail the effect of naturally occurring apo A-I methionine oxidation on various HDL functions such as promotion of cholesterol efflux, uptake by macrophages, interaction with HDL SR-BI receptor, induction of immune response and others. Recently, it has been shown that oxidative damage to apo A-I is reversible and enzymatic reduction of oxidized apo A-I in HDL can result in restoration of structure and function of these lipoproteins. Importantly, dihydrolipoic acid (DHLA) acts as an effective cofactor for the enzymatic repair of oxidative damage to intact lipoproteins, thus providing the therapeutic benefits of DHLA, and suggesting a novel strategy in the treatment and prevention of atherosclerosis and related CVD. I also suggested that the plasma level of oxidized apo A-I can be a clinically important biomarker of oxidative stress and an independent risk factor of atherosclerosis and related CVD. Currently, I am exploring this hypothesis.
Office: S2 302
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modulate the immune response
55 Lake Avenue North Worcester, MA 01605