Molecular basis of cytokine signalling - theme and variations

Interleukin-4 receptor signaling and its binding mechanism: A therapeutic insight from inhibitors tool box

Studies on Interlukin-4 (IL-4) disclosed great deal of information about its various physiological and pathological roles. All these roles depend upon its interaction and signaling through either type-I (IL-4Rα/common γ-chain) or type-II (IL-4Rα/IL-13Rα) receptors. Another cytokine, IL-13, shares some of the functions of IL-4, because both cytokines use a common receptor subunit, IL-4Rα. Here in this review, we discuss the structural details of IL-4 and IL-4Rα subunit and the structural similarities between IL-4 and IL-13. We also describe detailed chemistry of type-I and type-II receptor complexes and their signaling pathways. Furthermore, we elaborate the strength of type-II hetero dimer signals in response to IL-4 and IL-13. These cytokines are prime players in pathogenesis of allergic asthma, allergic hypersensitivity, different cancers, and HIV infection. Recent advances in the structural and binding chemistry of these cytokines various types of inhibitors were designed to block the interaction of IL-4 and IL-13 with their receptor, including several IL-4 mutant analogs and IL-4 antagonistic antibodies. Moreover, different targeted immunotoxins, which is a fusion of cytokine protein with a toxin or suicidal gene, are the new class of inhibitors to prevent cancer progression. In addition few small molecular inhibitors such as flavonoids have also been developed which are capable of binding with high affinity to IL-4Rα and, therefore, can be very effective in blocking IL-4-mediated responses.

IL-4 analogues with site-specific chemical modification at position 121 inhibit IL-4 and IL-13 biological activities

IL-4 signaling into a cell occurs via assembly of a receptor complex that consists of a high-affinity IL-4Rα chain and a low affinity chain, where the low-affinity chain is either γc or IL-13Rα1. It has been previously shown that mutational disruption of the low affinity interface in the IL-4DM (double mutein) yields an antagonist that inhibits IL-4 as well as IL-13-dependent responses. The present study reveals that new types of IL-4 antagonists can be generated by site-specific chemical modification. The chemically modified IL-4 analogues consist of (1) mixed disulfides created by refolding IL-4 cysteine muteins in the presence of different thiol compounds or (2) maleimide conjugates created by modifying cysteine muteins with maleimide derivatives. IL-4 analogues chemically modified at position 121 retain marginal binding affinity to γc or IL-13Rα1 receptor ectodomains during SPR interaction analysis. The biological activity of the analogues is strongly reduced in HEK-Blue IL-4/IL-13 cells as well as in Jurkat cells. Since the IL-4 analogues modified at position 121 have the ability to inhibit γc (IL-4)- and IL13Rα1 (IL-4/IL-13)-dependent responses in Jurkat and HEK-Blue cell lines, they effectively act as IL-4 antagonists. The results of our IL-4 study provide the first example of a cytokine that is transformed into a competitive inhibitor by site-specific chemical modification.

Protein-protein interactions: principles, techniques, and their potential role in new drug development

A vast network of genes is inter-linked through protein-protein interactions and is critical component of almost every biological process under physiological conditions. Any disruption of the biologically essential network leads to pathological conditions resulting into related diseases. Therefore, proper understanding of biological functions warrants a comprehensive knowledge of protein-protein interactions and the molecular mechanisms that govern such processes. The importance of protein-protein interaction process is highlighted by the fact that a number of powerful techniques/methods have been developed to understand how such interactions take place under various physiological and pathological conditions. Many of the key protein-protein interactions are known to participate in disease-associated signaling pathways, and represent novel targets for therapeutic intervention. Thus, controlling protein-protein interactions offers a rich dividend for the discovery of new drug targets. Availability of various tools to study and the knowledge of human genome have put us in a unique position to understand highly complex biological network, and the mechanisms involved therein. In this review article, we have summarized protein-protein interaction networks, techniques/methods of their binding/kinetic parameters, and the role of these interactions in the development of potential tools for drug designing.

Effects of clinically relevant MPL mutations in the transmembrane domain revealed at the atomic level through computational modeling

Background

Mutations in the thrombopoietin receptor (MPL) may activate relevant pathways and lead to chronic myeloproliferative neoplasms (MPNs). The mechanisms of MPL activation remain elusive because of a lack of experimental structures. Modern computational biology techniques were utilized to explore the mechanisms of MPL protein activation due to various mutations.

Results

Transmembrane (TM) domain predictions, homology modeling, ab initio protein structure prediction, and molecular dynamics (MD) simulations were used to build structural dynamic models of wild-type and four clinically observed mutants of MPL. The simulation results suggest that S505 and W515 are important in keeping the TM domain in its correct position within the membrane. Mutations at either of these two positions cause movement of the TM domain, altering the conformation of the nearby intracellular domain in unexpected ways, and may cause the unwanted constitutive activation of MPL's kinase partner, JAK2.

Conclusions

Our findings represent the first full-scale molecular dynamics simulations of the wild-type and clinically observed mutants of the MPL protein, a critical element of the MPL-JAK2-STAT signaling pathway. In contrast to usual explanations for the activation mechanism that are based on the relative translational movement between rigid domains of MPL, our results suggest that mutations within the TM region could result in conformational changes including tilt and rotation (azimuthal) angles along the membrane axis. Such changes may significantly alter the conformation of the adjacent and intrinsically flexible intracellular domain. Hence, caution should be exercised when interpreting experimental evidence based on rigid models of cytokine receptors or similar systems.

A synthetic peptide homologous to IL-10 functional domain induces monocyte differentiation to TGF-β+ tolerogenic dendritic cells

We have previously demonstrated that IT9302, a nonameric peptide homologous to the C-terminal domain of human IL-10, mimics several effects of the cytokine including down-regulation of the antigen presentation machinery and increased sensitivity of tumor cells to NK-mediated lysis. In the present report, we have explored a potential therapeutic utility for IT9302 related to the ex vivo production of tolerogenic dendritic cells (DCs). Our results indicate that IT9302 impedes human monocyte response to differentiation factors and reduces antigen presentation and co-stimulatory capacity by DCs. Additionally, peptide-treated DCs show impaired capacity to stimulate T-cell proliferation and IFN-γ production. IT9302 exerts its effect through mechanisms, in part, distinct from IL-10, involving STAT3 inactivation and NF-κB intracellular pathway. IT9302-treated DCs display increased expression of membrane-associated TGF-β, linked to a more effective induction of foxp3+ regulatory T cells. These results illustrate for the first time that a short synthetic peptide can promote monocytes differentiation to tolerogenic DCs with therapeutic potential for the treatment of autoimmune and transplantation-related immunopathologic disease.