Bow to your partner for signaling

LIF-dependent signaling: new pieces in the Lego

LIF, a member of the IL6 family of cytokine, displays pleiotropic effects on various cell types and organs. Its critical role in stem cell models (e.g.: murine ES, human mesenchymal cells) and its essential non redundant function during the implantation process of embryos, in eutherian mammals, put this cytokine at the core of many studies aiming to understand its mechanisms of action, which could benefit to medical applications. In addition, its conservation upon evolution raised the challenging question concerning the function of LIF in species in which there is no implantation. We present the recent knowledge about the established and potential functions of LIF in different stem cell models, (embryonic, hematopoietic, mesenchymal, muscle, neural stem cells and iPSC). We will also discuss EVO-DEVO aspects of this multifaceted cytokine.

A ligand peptide motif selected from a cancer patient is a receptor-interacting site within human interleukin-11

Interleukin-11 (IL-11) is a pleiotropic cytokine approved by the FDA against chemotherapy-induced thrombocytopenia. From a combinatorial selection in a cancer patient, we isolated an IL-11-like peptide mapping to domain I of the IL-11 (sequence CGRRAGGSC). Although this motif has ligand attributes, it is not within the previously characterized interacting sites. Here we design and validate in-tandem binding assays, site-directed mutagenesis and NMR spectroscopy to show (i) the peptide mimics a receptor-binding site within IL-11, (ii) the binding of CGRRAGGSC to the IL-11R alpha is functionally relevant, (iii) Arg4 and Ser8 are the key residues mediating the interaction, and (iv) the IL-11-like motif induces cell proliferation through STAT3 activation. These structural and functional results uncover an as yet unrecognized receptor-binding site in human IL-11. Given that IL-11R alpha has been proposed as a target in human cancer, our results provide clues for the rational design of targeted drugs.

Beyond dimerization: a membrane-dependent activation model for interleukin-4 receptor-mediated signalling

Class I cytokine receptors efficiently transfer activation signals from the extracellular space to the cytoplasm and play a dominant role in growth control and differentiation of human tissues. Although a significant body of literature is devoted to this topic, a consistent mechanistic picture for receptor activation in the membrane environment is still missing. Using the interleukin-4 receptor (IL-4R) as an example, we propose that the membrane-proximal stem-loop of the extracellular domains contains pivotal elements of a rotational switch. Interfacial energies of amino acid side-chains contained in the highly conserved WSXWS at the surface of the lipid bilayer suggest a new functional role for this motif. A generic activation mechanism for this receptor class is presented, which may impact the design of a new generation of biophysical assay systems.

STAT3: a potential therapeutic target in dendritic cells for the induction of transplant tolerance

Dendritic cells (DCs) control the segue from innate to adaptive immunity. Moreover, depending upon their milieu, DCs can either induce or inhibit immune responses. Whether DCs are immune stimulatory or tolerogenic apparently rests with whether or not the DCs express activated signal transducer and activator of transcription-3 (STAT3), the transcription factor induced by IL-6-like cytokines and IL-10. DCs expressing activated STAT3 produce less IL-12, which results in less effector T cell development. Moreover, DCs expressing activated STAT3 also express the tryptophan-catabolising enzyme indoleamine 2,3-dioxygenase. The kynurenine products of tryptophan catabolism induce T cell apoptosis; this area is of major interest to researchers working on tolerogenic DCs. In various disease models ranging from tumours to autoimmune diseases, administration of STAT3-activating cytokines resulted in attenuation of immune responses. Other corroborating evidence was obtained using conditional STAT3-deficient mice, or mice defective in cytokine signalling. Thus, persistently activating STAT3 in DCs may be a feasible strategy for controlling allograft rejection.

Identification of a basolateral sorting signal within the cytoplasmic domain of the interleukin-6 signal transducer gp130

Interleukin-6-type cytokine receptors are expressed in polarized cells such as hepatocytes and intestinal cells. For the interleukin-6-receptor gp80 and its signal transducer gp130, a preferential basolateral localization was demonstrated in Madin-Darby canine kidney (MDCK) cells and two basolateral sorting signals were identified within the cytoplasmic domain of gp80. The cytoplasmic tail of gp130 is responsible for signaling via the Janus kinase/signal transducer and activator of transcription pathway. In addition, it mediates the internalization of the receptor complex which is dependent on a di-leucine motif. Truncated gp130 lacking the cytoplasmic domain is sorted apically in MDCK cells. For identification of the basolateral sorting signal(s) of gp130, a series of deletion mutants in the cytoplasmic domain of gp130 have been generated and stably expressed in MDCK cells. Biotinylation analyses of these mutants show that a ten amino acids sequence between amino acids 782 and 792 which contains the di-leucine internalization motif is also essential for a basolateral sorting. Accordingly, we detect apical delivery of a gp130 mutant in which the di-leucine motif has been exchanged by two alanines (gp130LL/AA). These findings indicate that the di-leucine motif which directs the internalization of the IL-6 receptor complex also mediates the basolateral sorting of the signal transducer gp130.