Variations on ligand-receptor complexes

Targeting IL-34 in inflammatory autoimmune diseases

Interleukin-34 (IL-34) shares a common receptor with macrophage colony-stimulating factor (M-CSF), and can bind to CSF-1R, induces lymphocytes differentiation, proliferation, and regulates the synthesis of inflammatory components. Recent findings reported aberrant expression of IL-34 in several autoimmune disorders, such as lupus, arthritis, systemic sclerosis, inflammatory bowel diseases. The functional analysis further demonstrated that IL-34 may perform significantly in these inflammatory autoimmune disorders. IL-34 might consider as a biomarker for these diseases. I hope this collection of the findings in this review will improve knowledge of the role of IL-34, and targeting IL-34 may give the potential for these autoimmune diseases.

Structural basis for the dual recognition of helical cytokines IL-34 and CSF-1 by CSF-1R

Lacking any discernible sequence similarity, interleukin-34 (IL-34) and colony stimulating factor 1 (CSF-1) signal through a common receptor CSF-1R on cells of mononuclear phagocyte lineage. Here, the crystal structure of dimeric IL-34 reveals a helical cytokine fold homologous to CSF-1, and we further show that the complex architecture of IL-34 bound to the N-terminal immunoglobulin domains of CSF-1R is similar to the CSF-1/CSF-1R assembly. However, unique conformational adaptations in the receptor domain geometry and intermolecular interface explain the cross-reactivity of CSF-1R for two such distantly related ligands. The docking adaptations of the IL-34 and CSF-1 quaternary complexes, when compared to the stem cell factor assembly, draw a common evolutionary theme for transmembrane signaling. In addition, the structure of IL-34 engaged by a Fab fragment reveals the mechanism of a neutralizing antibody that can help deconvolute IL-34 from CSF-1 biology, with implications for therapeutic intervention in diseases with myeloid pathogenic mechanisms.

Structure of IL-33 and its interaction with the ST2 and IL-1RAcP receptors--insight into heterotrimeric IL-1 signaling complexes

Members of the interleukin-1 (IL-1) family of cytokines play major roles in host defense and immune system regulation in infectious and inflammatory diseases. IL-1 cytokines trigger a biological response in effector cells by assembling a heterotrimeric signaling complex with two IL-1 receptor chains, a high-affinity primary receptor and a low-affinity coreceptor. To gain insights into the signaling mechanism of the novel IL-1-like cytokine IL-33, we first solved its solution structure and then performed a detailed biochemical and structural characterization of the interaction between IL-33, its primary receptor ST2, and the coreceptor IL-1RAcP. Using nuclear magnetic resonance data, we obtained a model of the IL-33/ST2 complex in solution that is validated by small-angle X-ray scattering (SAXS) data and is similar to the IL-1beta/IL-1R1 complex. We extended our SAXS analysis to the IL-33/ST2/IL-1RAcP and IL-1beta/IL-1R1/IL-1RAcP complexes and propose a general model of the molecular architecture of IL-1 ternary signaling complexes.

Identification of key sequence determinants for the inhibitory function of the prodomain of TACE

The TNFalpha converting enzyme (TACE) is a zinc metalloproteinase that mediates shedding of multiple cell surface proteins. Regulation of TACE enzymatic activity is ultimately mediated via proteolytic removal of its inhibitory prodomain. Sequence determinants for TACE prodomain inhibition of the catalytic domain are yet to be identified. Surprisingly, although TACE and ADAM 10 (closest homologue) share only 23% sequence identity at their prodomains, the latter in isolation inhibits TACE with the same potency as TACE own prodomain. In contrast, the prodomain of ADAM 9 inhibited TACE only weakly. Detailed analysis of ADAM prodomains revealed two short regions for which TACE and ADAM 10 depart dramatically from all other family members. We prepared TACE prodomain variants containing full or partial switches to ADAM 9 residues at those two regions and examined their functional properties. Variants containing ADAM 9 substitutions including amino acid residues 72-82 and 126-137 were fully inactive for TACE inhibition. A third variant comprising residues 114-125 was active but at lower potency relative to wild type. All inactive variants appeared to be correctly folded. Finally, the amino acid residue Phe72 and the motif Asp-Asp-Val-Ile137 were identified within those regions as key determinants for TACE prodomain inhibitory function. We conclude that TACE and ADAM 10 prodomains are functionally equivalent in a way that separates them from the rest of the ADAM family.

SAM domains can utilize similar surfaces for the formation of polymers and closed oligomers

The mitogen-activated protein kinase (MAPK) Byr2 and its activator Ste4 are involved in the mating pheromone response pathway of Schizosaccharomyces pombe and interact via their SAM domains. SAM domains can self-associate to form higher-order structures, including dimers, polymers and closed oligomers. Ste4-SAM is adjacent to a trimeric leucine zipper domain and we have shown previously that the two domains together (Ste4-LZ-SAM) bind to a monomeric Byr2-SAM with high affinity (Kd approximately 20 nM), forming a 3:1 complex. Here, we map the surfaces of Byr2-SAM and Ste4-SAM that is involved the interaction. A set of 38 mutants of Byr2-SAM and 33 mutants of Ste4-SAM were prepared, covering most of the protein surfaces. These mutants were purified and screened for binding, yielding a map of residues that are required for binding and a complementary map of residues that are not required. We find that the interface maps to regions of the SAM domains that are known to be important for the formation of SAM polymers. These results indicate that SAM domains can create a variety of oligomeric architectures utilizing common binding surfaces.

The structure and binding mode of interleukin-18

Interleukin-18 (IL-18), a cytokine formerly known as interferon-gamma- (IFN-gamma-) inducing factor, has pleiotropic immunoregulatory functions, including augmentation of IFN-gamma production, Fas-mediated cytotoxicity and developmental regulation of T-lymphocyte helper type I. We determined the solution structure of IL-18 as a first step toward understanding its receptor activation mechanism. It folds into a beta-trefoil structure that resembles that of IL-1. Extensive mutagenesis revealed the presence of three sites that are important for receptor activation: two serve as binding sites for IL-18 receptor alpha (IL-18Ralpha), located at positions similar to those of IL-1 for IL-1 receptor type I (IL-1RI), whereas the third site may be involved in IL-18 receptor beta (IL-18Rbeta) binding. The structure and mutagenesis data provide a basis for understanding the IL-18-induced heterodimerization of receptor subunits, which is necessary for receptor activation.

A direct interaction between transforming growth factor (TGF)-betas and amyloid-beta protein affects fibrillogenesis in a TGF-beta receptor-independent manner

Transforming growth factor-beta (TGF-beta) receptor-mediated signaling has been proposed to mediate both the beneficial and deleterious roles for this cytokine in amyloid-beta protein (Abeta) function. In order to assess receptor dependence of these events, we used PC12 cell cultures, which are devoid of TGF-beta receptors. Surprisingly, TGF-beta potentiated the neurotoxic effects of the 40-residue Abeta peptide, Abeta-(1-40), in this model suggesting that there may be a direct, receptor-independent interaction between TGF-beta and Abeta-(1-40). Surface plasmon resonance confirmed that TGF-beta binds with high affinity directly to Abeta-(1-40) and electron microscopy revealed that TGF-beta enhances Abeta-(1-40) oligomerization. Immunohistochemical examination of mouse brain revealed that hippocampal CA1 and dentate gyrus, two regions classically associated with Abeta-mediated pathology, lack TGF-beta Type I receptor immunoreactivity, thus indicating that TGF-beta receptor-mediated signaling would not be favored in these regions. Our observations not only provide for a unique, receptor-independent mechanism of action for TGF-beta, but also help to reconcile the literature interpreting the role of TGF-beta in Abeta function. These data support a critical etiological role for this mechanism in neuropathological amyloidoses.

The cystine knot promotes folding and not thermodynamic stability in vascular endothelial growth factor

Cystine knots consist of three intertwined disulfide bridges and are considered major determinants of protein stability in proteins in which they occur. We questioned this function and observed that removal of individual disulfide bridges in human vascular endothelial growth factor (VEGF) does not reduce its thermodynamic stability but reduces its unexpected high thermal stability of 108 degrees C by up to 40 degrees C. In wild-type VEGF (deltaG(u,25)(0) = 5.1 kcal.mol(-1)), the knot is responsible for a large entropic stabilization of TdeltaS(u,25)(0) = -39.3 kcal mol(-1), which is compensated for by a deltaH(u,25)(0) of -34.2 kcal mol(-1). In the disulfide-deficient mutants, this entropic stabilization disappears, but instead of a decrease, we observe an increase in the thermodynamic stability by about 2 kcal.mol(-1). A detailed crystallographic analysis of the mutant structures suggests a role of the cystine knot motif in protein folding rather than in the stabilization of the folded state. When assuming that the sequential order of the disulfide bridge formation is conserved between VEGF and glycoprotein alpha-subunit, the crystal structure of the mutant C61A-C104A, which deviates by a root mean square deviation of more than 2.2 A from wild-type VEGF, identifies a true folding intermediate of VEGF.