Anterograde trafficking of Toll-like receptors requires the cargo sorting adaptors TMED-2 and 7

Toll-Like Receptors (TLRs) play a pivotal role in immunity by recognising conserved structural features of pathogens and initiating the innate immune response. TLR signalling is subject to complex regulation that remains poorly understood. Here we show that two small type I transmembrane receptors, TMED2 and 7, that function as cargo sorting adaptors in the early secretory pathway are required for transport of TLRs from the ER to Golgi. Protein interaction studies reveal that TMED7 interacts with TLR2, TLR4 and TLR5 but not with TLR3 and TLR9. On the other hand, TMED2 interacts with TLR2, TLR4 and TLR3. Dominant negative forms of TMED7 suppress the export of cell surface TLRs from the ER to the Golgi. By contrast TMED2 is required for the ER-export of both plasma membrane and endosomal TLRs. Together, these findings suggest that association of TMED2 and TMED7 with TLRs facilitates anterograde transport from the ER to the Golgi.     

Role of the Spatzle Pro-domain in the generation of an active toll receptor ligand

The cytokine Sp?tzle is the ligand for Drosophila Toll, the prototype of an important family of membrane receptors that function in embryonic patterning and innate immunity. A dimeric precursor of Sp?tzle is processed by an endoprotease to produce a form (C-106) that cross-links Toll receptor ectodomains and establishes signaling. Here we show that before processing the pro-domain of Sp?tzle is required for correct biosynthesis and secretion. We mapped two loss-of-function mutations of Sp?tzle to a discrete site in the pro-domain and showed that the phenotype arises because of a defect in biosynthesis rather than signaling. We also report that the pro-domain and C-106 remain associated after cleavage and that this processed complex signals with the same characteristics as the C-terminal fragment. These results suggest that before activation the determinants on C-106 that bind specifically to Toll are sequestered by the pro-domain and that proteolytic processing causes conformational rearrangements that expose these determinants and enables binding to Toll. Furthermore, we show that the pro-domain is released when the Toll extracellular domain binds to the complex, a finding that has implications for the generation of a signaling-competent Toll dimer.     

Complex homogeneous and heterogeneous fluorescence anisotropy decays: enhancing analysis accuracy

In biological macromolecules, fluorophores often exhibit multiple depolarizing motions that require multiple lifetimes and rotational relaxation times to define fluorescence intensity and anisotropy decays. The related analysis of time-correlated single-photon counting data becomes uncertain due to the multitude of decay parameters and numerical sensitivity to deconvolution of the instrument response function (IRF) via discretization of integrals. By using simulations we show that improved discretizations based on quadratic and cubic local approximations of the IRF yield more accurate estimation of short rotational relaxation times and lifetimes than the commonly used Grinvald-Steinberg discretization, which in turn appears more reliable than two discretizations based on linear local approximations of the IRF. In addition, our simulation suggests that cubic approximation is the most advantageous in discriminating complex heterogeneous and homogeneous anisotropy decay. We show that among three different information criteria, the Akaike information criterion is best suited for detection of heterogeneity in rotational relaxation times. It is capable of detecting heterogeneity even when anisotropy decay appears homogeneous within statistical errors of estimation.     

Solution structure of the isolated Pelle death domain

The interaction between the death domains (DDs) of Tube and the protein kinase Pelle is an important component of the Toll pathway. Published crystallographic data suggests that the Pelle-Tube DD interface is plastic and implies that in addition to the two predominant Pelle-Tube interfaces, a third interaction is possible. We present the NMR solution structure of the isolated death domain of Pelle and a study of the interaction between the DDs of Pelle and Tube. Our data suggests the solution structure of the isolated Pelle DD is similar to that of Pelle DD in complex with Tube. Additionally, they suggest that the plasticity observed in the crystal structure may not be relevant in the functioning death domain complex.     

Structure of the glycosyltransferase EryCIII in complex with its activating P450 homologue EryCII

In the biosynthesis of the clinically important antibiotic erythromycin D, the glycosyltransferase (GT) EryCIII, in concert with its partner EryCII, attaches a nucleotide-activated sugar to the macrolide scaffold with high specificity. To understand the role of EryCII, we have determined the crystal structure of the EryCIII·EryCII complex at 3.1 Å resolution. The structure reveals a heterotetramer with a distinctive, elongated quaternary organization. The EryCIII subunits form an extensive self-complementary dimer interface at the center of the complex, and the EryCII subunits lie on the periphery. EryCII binds in the vicinity of the putative macrolide binding site of EryCIII but does not make direct interactions with this site. Our biophysical and enzymatic data support a model in which EryCII stabilizes EryCIII and also functions as an allosteric activator of the GT.     

An Oligomeric Signaling Platform Formed by the Toll-like Receptor Signal Transducers MyD88 and IRAK-4

Toll-like receptors (TLRs) mediate responses to pathogen-associated molecules as part of the vertebrate innate immune response to infection. Receptor dimerization is coupled to downstream signal transduction by the recruitment of a post-receptor complex containing the adaptor protein MyD88 and the IRAK protein kinases. In this work, we show that the death domains of human MyD88 and IRAK-4 assemble into closed complexes having unusual stoichiometries of 7:4 and 8:4, the Myddosome. Formation of the Myddosome is likely to be a key event for TLR4 signaling in vivo as we show here that pathway activation requires that the receptors cluster into lipid rafts. Taken together, these findings indicate that TLR activation causes the formation of a highly oligomeric signaling platform analogous to the death-inducing signaling complex of the Fas receptor pathway.In vertebrates, the initial responses of innate immunity are mediated by a family of pattern recognition receptors, which are able to sense the presence of a variety of microbial products such as lipids and non-self nucleic acid (1). One important family of pattern recognition receptors is the Toll-like receptors (TLRs)⁴ that are expressed by many immune system cell types such as macrophages and dendritic cells. TLRs are class one transmembrane receptors that are activated by a process of stimulus-induced dimerization of their extracellular domains. This in turn causes the cytoplasmic Toll/interleukin-1 (IL-1) domains (TIRs) to dimerize, forming a scaffold for the recruitment of downstream signaling components (2). TLRs use five signaling adaptor proteins to couple receptor activation to downstream signal transduction (3). All of these adaptors have TIRs and engage with the activated TLRs by TIR-TIR interactions.One of the adaptor proteins, MyD88, is of particular importance because it is used by all but one of the TLRs as well as by the IL-1 and interferon-γ receptors. MyD88-deficient mice have profoundly impaired innate immune responses and are susceptible to a wide range of infectious diseases. The MyD88 sequence is tripartite and is comprised of a death domain (DD) at the N terminus, a short (40-amino-acid) intermediate domain (ID) of unknown structure, and a C-terminal TIR. Evidence from yeast two-hybrid experiments suggests that MyD88 can self-associate with contacts in both the DD and the TIR (4). The current view of post-receptor signal transduction is that two MyD88 TIR domains bind to the activated TLR, and this enables the recruitment of the protein kinases IRAK-4 and IRAK-1 (5). These kinases have DDs at their N termini, and both are recruited into a complex with MyD88 after signal initiation. It appears that IRAK-4 is recruited first, and this binding requires the ID of MyD88 (6, 7). Thus MyD88s, a splice variant that lacks the ID, down-regulates TLR signaling and cannot recruit IRAK-4 into the post-receptor complex. In contrast, IRAK-1 interacts with MyD88s presumably by DD-DD rather than DD-ID interactions. The next step in the signaling process is for IRAK-4 to phosphorylate IRAK-1, causing activation of the latter and hyper-autophosphorylation. IRAK-1 then dissociates from the complex and interacts with the ubiquitin-protein isopeptide ligase (E3) TRAF6 (8, 9).DDs together with the structurally related caspase recruitment domains (CARDs) and death effector domains (DEDs) form the death domain superfamily (10). There are 215 proteins encoded by the human genome that are predicted to have this fold, and they are widely used in cellular signaling including the TLR and apoptotic pathways. Structurally, DDs contain six antiparallel α-helices, and they are predominantly involved in protein-protein interactions with other DDs. Three modes of DD-DD interaction, types 1, 2, and 3 (10), have been characterized and are illustrated by the structures of the Drosophila Tube-Pelle heterodimer (11), the Procaspase-9 homodimer (12), and most remarkably, by the PIDDosome (13). In the latter case, PIDD, RAIDD, and Caspase-2 form a complex, which results in the proximity-induced activation of Caspase-2 protease activity, which in turn leads to cytochrome c release and apoptotic cell death. The DDs of PIDD and RAIDD interact to produce a complex having a stoichiometry of 5:7, and the subunits are arranged in three layers with five PIDDs, five RAIDDs, and then two RAIDDs. The structure is stabilized by 25 DD-DD contacts of which six are type 2, nine are type 1, and 10 are type 3.In this study, we report that like PIDD and RAIDD, the DDs of human MyD88 and IRAK-4 assemble into defined structures having stoichiometries of 7:4 and 8:4. We propose that the structure has two layers with a ring of seven or eight MyD88 subunits and a second layer of four IRAK-4 subunits. The formation of these higher order assemblies provides insight into the complex regulation and cross-talk observed in the TLR signaling pathways.     

Autonomous folding of interdomain regions of a modular polyketide synthase

Domains within the multienzyme polyketide synthases are linked by noncatalytic sequences of variable length and unknown function. Recently, the crystal structure was reported of a portion of the linker between the acyltransferase (AT) and ketoreductase (KR) domains from module 1 of the erythromycin synthase (6-deoxyerythronolide B synthase), as a pseudodimer with the adjacent ketoreductase (KR). On the basis of this structure, the homologous linker region between the dehydratase (DH) and enoyl reductase (ER) domains in fully reducing modules has been proposed to occupy a position on the periphery of the polyketide synthases complex, as in porcine fatty acid synthase. We report here the expression and characterization of the same region of the 6-deoxyerythronolide B synthase module 1 AT-KR linker, without the adjacent KR domain (termed DeltaN AT1-KR1), as well as the corresponding section of the DH-ER linker. The linkers fold autonomously and are well structured. However, analytical gel filtration and ultracentrifugation analysis independently show that DeltaN AT1-KR1 is homodimeric in solution; site-directed mutagenesis further demonstrates that linker self-association is compatible with the formation of a linker-KR pseudodimer. Our data also strongly indicate that the DH-ER linker associates with the upstream DH domain. Both of these findings are incompatible with the proposed model for polyketide synthase architecture, suggesting that it is premature to allocate the linker regions to a position in the multienzymes based on the solved structure of animal fatty acid synthase.     

Optical spectroscopic characterization of single tryptophan mutants of chicken skeletal troponin C: evidence for interdomain interaction.

The effects of metal ion binding on the optical spectroscopic properties and temperature stability of two single tryptophan mutants of chicken skeletal TnC, F78W and F154W, have been examined. The absence of tyrosine and other tryptophan residues allowed the unambiguous assignment of the spectral signal from the introduced Trp residue. Changes in the molar ellipticity values in the far-UV CD spectra of the mutant proteins on metal ion binding were similar to those of wild-type TnC suggesting that the introduction of the Trp residue had no effect on the total secondary structure content. The fluorescence and near-UV absorbance data reveal that, in the apo state, Trp-78 is buried while Trp-154 is exposed to solvent. Additionally, the highly resolved (1)L(b) band of Trp-78 seen in the near-UV absorbance and CD spectra of the apo state of F78W suggest that this residue is likely in a rigid molecular environment. In the calcium-saturated state, Trp-154 becomes buried while the solvent accessibility of Trp-78 increases. The fluorescence emission and near-UV CD of Trp-78 in the N-terminal domain were sensitive to calcium binding at the C-terminal domain sites. Measurements of the temperature stability reveal that events occurring in the N-terminal domain affect the stability of the C-terminal domain and vice versa. This, coupled with the titration data, strongly suggests that there are interactions between the N- and C-terminal domains of TnC.     

A Randomized,Placebo Controlled Pilot Trial of Botulinum Toxin for Paratonic Rigidity in People with Advanced Cognitive Impairment

Objective

Evaluate safety and efficacy of Incobotulinumtoxin A in elderly patients with dementia and paratonia.

Setting

University-affiliated hospital, spasticity management Clinic.

Participants

Ten subjects were enrolled. Inclusion criteria: 1) severe cognitive impairment 2) diagnosis of Alzheimer’s disease, vascular dementia, or frontotemporal dementia, and 3) score >3 on the paratonic assessment instrument, with posture in an arm(s) interfering with provision of care. Exclusion criteria: 1) alternate etiologies for increased tone and 2) injection with botulinum toxin within the 6 months preceding the study.

Design

Single center, randomized, double blind, placebo-controlled, crossover trial with two treatment cycles of 16 weeks. Assessments occurred at 2, 6, 12 and16 weeks following injections. Subjects received up to 300 U of Incobotulinumtoxin A in arm(s).

Primary and Secondary Outcome Measures

Primary outcome measure was the modified caregiver burden scale (mCBS); exploratory secondary outcome measures were also performed. Analysis of variance and mixed modeling techniques were used to evaluate treatment effects.

Results

Incobotulinumtoxin A treatment produced significant improvement in mCBS total score −1.11 (–2.04 to −0.18) (Treatment effect and 95% CI), dressing sub-score −0.36 (–0.59 to 0.12), and cleaning under the left and right armpits sub-score −0.5 (–0.96 to −0.04), −0.41 (–0.79 to −0.04) respectively. PROM in the left and right elbow increased by 27.67 degrees (13.32–42.02) and 22.07 degrees (9.76–34.39) respectively. PROM in the left and right shoulder increased by 11.92 degrees (5.46–18.38) and 8.58 degrees (3.73–13.43) respectively. No significant treatment effect was found for GAS, VAS and PAINAD scales or change in time to perform care. No adverse drug reactions occurred.

Conclusions

Administration of Incobotulinumtoxin A in elderly people with advanced dementia and paratonia may be an efficacious and safe treatment to increase range of motion and reduce functional burden. Further studies are needed to confirm results.

Trial Registration

ClinicalTrials.Gov {"type":"clinical-trial","attrs":{"text":"NCT02212119","term_id":"NCT02212119"}}NCT02212119     

Structure-fluorescence correlations in a single tryptophan mutant of carp parvalbumin: solution structure, backbone and side-chain dynamics

Heterogeneous fluorescence intensity decays of tryptophan in proteins are often rationalized using a model which proposes that different rotameric states of the indole alanyl side-chain are responsible for the observed fluorescence lifetime heterogeneity. We present here the study of a mutant of carp parvalbumin bearing a single tryptophan residue at position 102 (F102W) whose fluorescence intensity decay is heterogeneous and assess the applicability of a rotamer model to describe the fluorescence decay data. We have determined the solution structure of F102W in the calcium ligated state using multi-dimensional nuclear magnetic resonance (NMR) and have used the minimum perturbation mapping technique to explore the possible existence of multiple conformations of the indole moiety of Trp102 of F102W and, for comparison, Trp48 of holo-azurin. The maps for parvalbumin suggest two potential conformations of the indole side-chain. The high energy barrier for rotational isomerization between these conformers implies that interwell rotation would occur on time-scales of milliseconds or greater and suggests a rotamer basis for the heterogeneous fluorescence. However, the absence of alternate Trp102 conformers in the NMR data (to within 3 % of the dominant species) suggests that the heterogeneous fluorescence of Trp102 may arise from mechanisms independent of rotameric states of the Trp side-chain. The map for holo-azurin has only one conformation, and suggests a rotamer model may not be required to explain its heterogeneous fluorescence intensity decay. The backbone and Trp102 side-chain dynamics at 30 degrees C of F102W has been characterized based on an analysis of (15)N NMR relaxation data which we have interpreted using the Lipari-Szabo formalism. High order parameter (S(2)) values were obtained for both the helical and loop regions. Additionally, the S(2) values imply that the calcium binding CD and EF loops are not strictly equivalent. The S(2) value for the indole side-chain of Trp102 obtained from the fluorescence, NMR relaxation and minimum perturbation data are consistent with a Trp moiety whose motion is restricted.