Structures of the interleukin 11 signalling complex reveal gp130 dynamics and the inhibitory mechanism of a cytokine variant.

Interleukin (IL-)11, an IL-6 family cytokine, has pivotal roles in autoimmune diseases, fibrotic complications, and solid cancers. Despite intense therapeutic targeting efforts, structural understanding of IL-11 signalling and mechanistic insights into current inhibitors are lacking. Here we present cryo-EM and crystal structures of the human IL-11 signalling complex, including the complex containing the complete extracellular domains of the shared IL-6 family ß-receptor, gp130. We show that complex formation requires conformational reorganisation of IL-11 and that the membrane-proximal domains of gp130 are dynamic. We demonstrate that the cytokine mutant, IL-11 Mutein, competitively inhibits signalling in human cell lines. Structural shifts in IL-11 Mutein underlie inhibition by altering cytokine binding interactions at all three receptor-engaging sites and abrogating the final gp130 binding step. Our results reveal the structural basis of IL-11 signalling, define the molecular mechanisms of an inhibitor, and advance understanding of gp130-containing receptor complexes, with potential applications in therapeutic development.

The Monomeric α-Crystallin Domain of the Small Heat-shock Proteins αB-crystallin and Hsp27 Binds Amyloid Fibril Ends.

Small heat-shock proteins (sHSPs) are ubiquitously expressed molecular chaperones present in all kingdoms of life that inhibit protein misfolding and aggregation. Despite their importance in proteostasis, the structure-function relationships of sHSPs remain elusive. Human sHSPs are characterised by a central, highly conserved α-crystallin domain (ACD) and variable-length N- and C-terminal regions. The ACD forms antiparallel homodimers via an extended ß-strand, creating a shared ß-sheet at the dimer interface. The N- and C-terminal regions mediate formation of higher order oligomers that are thought to act as storage forms for chaperone-active dimers. We investigated the interactions of the ACD of two human sHSPs, αB-crystallin (αB-C) and Hsp27, with apolipoprotein C-II amyloid fibrils using analytical ultracentrifugation and nuclear magnetic resonance spectroscopy. The ACD was found to interact transiently with amyloid fibrils to inhibit fibril elongation and naturally occurring fibril end-to-end joining. This interaction was sensitive to the concentration of fibril ends indicating a 'fibril-capping' interaction. Furthermore, resonances arising from the ACD monomer were attenuated to a greater extent than those of the ACD dimer in the presence of fibrils, suggesting that the monomer may bind fibrils. This hypothesis was supported by mutagenesis studies in which disulfide cross-linked ACD dimers formed by both αB-C and Hsp27 were less effective at inhibiting amyloid fibril elongation and fibril end-to-end joining than ACD constructs lacking disulfide cross-linking. Our results indicate that sHSP monomers inhibit amyloid fibril elongation, highlighting the importance of the dynamic oligomeric nature of sHSPs for client binding.

N- and C-terminal regions of αB-crystallin and Hsp27 mediate inhibition of amyloid nucleation, fibril binding, and fibril disaggregation.

Small heat-shock proteins (sHSPs) are ubiquitously expressed molecular chaperones that inhibit amyloid fibril formation; however, their mechanisms of action remain poorly understood. sHSPs comprise a conserved α-crystallin domain flanked by variable N- and C-terminal regions. To investigate the functional contributions of these three regions, we compared the chaperone activities of various constructs of human αB-crystallin (HSPB5) and heat-shock 27-kDa protein (Hsp27, HSPB1) during amyloid formation by α-synuclein and apolipoprotein C-II. Using an array of approaches, including thioflavin T fluorescence assays and sedimentation analysis, we found that the N-terminal region of Hsp27 and the terminal regions of αB-crystallin are important for delaying amyloid fibril nucleation and for disaggregating mature apolipoprotein C-II fibrils. We further show that the terminal regions are required for stable fibril binding by both sHSPs and for mediating lateral fibril-fibril association, which sequesters preformed fibrils into large aggregates and is believed to have a cytoprotective function. We conclude that although the isolated α-crystallin domain retains some chaperone activity against amyloid formation, the flanking domains contribute additional and important chaperone activities, both in delaying amyloid formation and in mediating interactions of sHSPs with amyloid aggregates. Both these chaperone activities have significant implications for the pathogenesis and progression of diseases associated with amyloid deposition, such as Parkinson's and Alzheimer's diseases.

The structure of the extracellular domains of human interleukin 11α receptor reveals mechanisms of cytokine engagement.

Interleukin (IL) 11 activates multiple intracellular signaling pathways by forming a complex with its cell surface α-receptor, IL-11Rα, and the ß-subunit receptor, gp130. Dysregulated IL-11 signaling has been implicated in several diseases, including some cancers and fibrosis. Mutations in IL-11Rα that reduce signaling are also associated with hereditary cranial malformations. Here we present the first crystal structure of the extracellular domains of human IL-11Rα and a structure of human IL-11 that reveals previously unresolved detail. Disease-associated mutations in IL-11Rα are generally distal to putative ligand-binding sites. Molecular dynamics simulations showed that specific mutations destabilize IL-11Rα and may have indirect effects on the cytokine-binding region. We show that IL-11 and IL-11Rα form a 1:1 complex with nanomolar affinity and present a model of the complex. Our results suggest that the thermodynamic and structural mechanisms of complex formation between IL-11 and IL-11Rα differ substantially from those previously reported for similar cytokines. This work reveals key determinants of the engagement of IL-11 by IL-11Rα that may be exploited in the development of strategies to modulate formation of the IL-11-IL-11Rα complex.

Polymorphism in disease-related apolipoprotein C-II amyloid fibrils: a structural model for rod-like fibrils.

Human apolipoprotein (apo) C-II is one of several plasma apolipoproteins that form amyloid deposits in vivo and is an independent risk factor for cardiovascular disease. Lipid-free apoC-II readily self-assembles into twisted-ribbon amyloid fibrils but forms straight, rod-like amyloid fibrils in the presence of low concentrations of micellar phospholipids. Charge mutations exerted significantly different effects on rod-like fibril formation compared to their effects on twisted-ribbon fibril formation. For instance, the double mutant, K30D-D69K apoC-II, readily formed twisted-ribbon fibrils, while the rate of rod-like fibril formation in the presence of micellar phospholipid was negligible. Structural analysis of rod-like apoC-II fibrils, using hydrogen-deuterium exchange and NMR analysis showed exchange protection consistent with a core cross-ß structure comprising the C-terminal 58-76 region. Molecular dynamics simulations of fibril arrangements for this region favoured a parallel cross-ß structure. X-ray fibre diffraction data for aligned rod-like fibrils showed a major meridional spacing at 4.6 Å and equatorial spacings at 9.7, 23.8 and 46.6 Å. The latter two equatorial spacings are not observed for aligned twisted-ribbon fibrils and are predicted for a model involving two cross-ß fibrils in an off-set antiparallel structure with four apoC-II units per rise of the ß-sheet. This model is consistent with the mutational effects on rod-like apoC-II fibril formation. The lipid-dependent polymorphisms exhibited by apoC-II fibrils could determine the properties of apoC-II in renal amyloid deposits and their potential role in the development of cardiovascular disease.

Intra- and Intersubunit Ion-Pair Interactions Determine the Ability of Apolipoprotein C-II Mutants To Form Hybrid Amyloid Fibrils.

The apolipoprotein family is structurally defined by amphipathic α-helical regions that interact with lipid surfaces. In the absence of lipid, human apolipoprotein (apo) C-II also forms well-defined amyloid fibrils with cross-ß structure. Formation of this ß-structure is accompanied by the burial of two charged residues, K30 and D69, that form an ion-pair within the amyloid fibril core. Molecular dynamics (MD) simulations indicate these buried residues form both intra- and intersubunit ion-pair interactions that stabilize the fibril. Mutations of the ion-pair (either K30D or D69K) reduce fibril stability and prevent fibril formation by K30D apoC-II under standard conditions. We investigated whether mixing K30D apoC-II with other mutants would overcome this loss of fibril forming ability. Co-incubation of equimolar mixtures of K30D apoC-II with wild-type, D69K, or double-mutant (K30D/D69K) apoC-II promoted the incorporation of K30D apoC-II into hybrid fibrils with increased stability. MD simulations showed an increase in the number of intersubunit ion-pair interactions accompanied the increased stability of the hybrid fibrils. These results demonstrate the important role of both intra- and intersubunit charge interactions in stabilizing apoC-II amyloid fibrils, a process that may be a key factor in determining the general ability of proteins to form amyloid fibrils.

Solution Conditions Affect the Ability of the K30D Mutation To Prevent Amyloid Fibril Formation by Apolipoprotein C-II: Insights from Experiments and Theoretical Simulations.

Apolipoproteins form amphipathic helical structures that bind lipid surfaces. Paradoxically, lipid-free apolipoproteins display a strong propensity to form cross-ß structure and self-associate into disease-related amyloid fibrils. Studies of apolipoprotein C-II (apoC-II) amyloid fibrils suggest that a K30-D69 ion pair accounts for the dual abilities to form helix and cross-ß structure. Consistent with this is the observation that a K30D mutation prevents fibril formation under standard fibril forming conditions. However, we found that fibril formation by K30D apoC-II proceeded readily at low pH and a higher salt or protein concentration. Structural analysis demonstrated that K30D apoC-II fibrils at pH 7 have a structure similar to that of the wild-type fibrils but are less stable. Molecular dynamics simulations of the wild-type apoC-II fibril model at pH 7 and 3 showed that the loss of charge on D69 at pH 3 leads to greater separation between residues K30 and D69 within the fibril with a corresponding reduction in ß-strand content around residue 30. In contrast, in simulations of the K30D mutant model at pH 7 and 3, residues D30 and D69 moved closer at pH 3, accompanied by an increase in ß-strand content around residue 30. The simulations also demonstrated a strong dominance of inter- over intramolecular contacts between ionic residues of apoC-II and suggested a cooperative mechanism for forming favorable interactions between the individual strands under different conditions. These observations demonstrate the important role of the buried K30-D69 ion pair in the stability and solution properties of apoC-II amyloid fibrils.

Hydrogen/Deuterium Exchange and Molecular Dynamics Analysis of Amyloid Fibrils Formed by a D69K Charge-Pair Mutant of Human Apolipoprotein C-II.

Plasma apolipoproteins form amphipathic α helices in lipid environments but in the lipid-free state show a high propensity to form ß structure and self-associate into amyloid fibrils. The widespread occurrence of apolipoproteins in amyloid plaques suggests disease-related roles, specifically in atherosclerosis. To reconcile the dual abilities of apolipoproteins to form either α helices or cross-ß sheet structures, we examined fibrils formed by human apolipoprotein C-II (apoC-II). A structural model for apoC-II fibrils shows a cross-ß core with parallel ß strands, including a buried K30-D69 charge pair. We investigated the effect of abolishing this charge pair in mutant D69K apoC-II. Fluorescence studies indicated more rapid fibril formation and less solvent accessibility of tryptophan (W26) in D69K apoC-II fibrils than in wild-type (WT) fibrils. X-ray diffraction data of aligned D69K apoC-II fibrils yielded a typical cross-ß structure with increased ß sheet spacing compared to that of WT fibrils. Hydrogen/deuterium (H/D) exchange patterns were similar for D69K apoC-II fibrils compared to WT fibrils, albeit with an overall reduction in the level of slow H/D exchange, particularly around residues 29-32. Molecular dynamics simulations indicated reduced ß strand content for a model D69K apoC-II tetramer compared to the WT tetramer and confirmed an expansion of the cross-ß spacing that contributed to the formation of a stable charge pair between K69 and E27. The results highlight the importance of charge-pair interactions within the apoC-II fibril core, which together with numerous salt bridges in the flexible connecting loop play a major role in the ability of lipid-free apoC-II to form stable cross-ß fibrils.

Fluphenazine·HCl and Epigallocatechin Gallate Modulate the Rate of Formation and Structural Properties of Apolipoprotein C-II Amyloid Fibrils.

Protein misfolding and aggregation, leading to amyloid fibril formation, are characteristic of many devastating and debilitating amyloid diseases. Accordingly, there is significant interest in the mechanisms underlying amyloid fibril formation and identification of possible intervention tools. Small molecule drug compounds approved for human use or for use in phase I-III clinical trials were investigated for their effects on amyloid formation by human apolipoprotein (apo) C-II. Several of these compounds modulated the rate of amyloid formation by apoC-II. Epigallocatechin gallate (EGCG), a green tea catechin, was an effective inhibitor of apoC-II fibril formation, and the antipsychotic drug, fluphenazine·HCl, was a potent activator. Both EGCG and fluphenazine·HCl exerted concentration-dependent effects on the rate of fibril formation, bound to apoC-II fibrils with high affinity, and competitively reduced thioflavin T binding. EGCG significantly altered the size distribution of fibrils, most likely by promoting the lateral association of fibrils and subsequent formation of large aggregates. Fluphenazine·HCl did not significantly alter the size distribution of fibrils, but it may induce the formation of a small population of rod-like fibrils that differ from the characteristic ribbon-like fibrils normally observed for apoC-II. The findings of this study emphasize the effects of small molecule drugs on the kinetics of amyloid fibril formation and their roles in determining fibril structure and aggregate size.

Charge and charge-pair mutations alter the rate of assembly and structural properties of apolipoprotein C-II amyloid fibrils.

The misfolding, aggregation, and accumulation of proteins as amyloid fibrils is a defining characteristic of several debilitating diseases. Human apolipoprotein C-II (apoC-II) amyloid fibrils are representative of the fibrils formed by a number of plasma apolipoproteins implicated in amyloid-related disease. Previous structural analyses identified a buried charge pair between residues K30 and D69 within apoC-II amyloid fibrils. We have investigated the effects of amino acid substitutions of these residues on apoC-II fibril formation. Two point mutations of apoC-II, D69K and K30D, as well as a reversed ion-pair mutant containing both mutations (KDDK) were generated. Fibril formation by the double mutant, apoC-II KDDK, and apoC-II D69K was enhanced compared to that of wild-type (WT) apoC-II, while apoC-II K30D lacked the ability to form fibrils under standard conditions. Structural analyses showed that WT apoC-II, apoC-II D69K, and apoC-II KDDK fibrils have similar secondary structures and morphologies. Size distribution analyses revealed that apoC-II D69K fibrils have a broader range of fibril sizes while apoC-II KDDK fibrils showed an increased frequency of closed fibrillar loops. ApoC-II D69K fibrils exhibited reduced thioflavin T binding capacity compared to that of fibrils formed by WT apoC-II and apoC-II KDDK. These results indicate that specific charge and charge-pair mutations within apoC-II significantly alter the ability to form fibrils and that position 69 within apoC-II plays a key role in the rate-limiting step of apoC-II fibril formation.