Cysteine in cell culture media induces acidic IgG1 species by disrupting the disulfide bond network.

A high degree of charge heterogeneity is an unfavorable phenomenon commonly observed for therapeutic monoclonal antibodies (mAbs). Removal of these impurities during manufacturing often comes at the cost of impaired step yields. A wide spectrum of posttranslational and chemical modifications is known to modify mAb charge. However, a deeper understanding of underlying mechanisms triggering charged species would be beneficial for the control of mAb charge variants during bioprocessing. In this study, a comprehensive analytical investigation was carried out to define the root causes and mechanisms inducing acidic variants of an immunoglobulin G1-derived mAb. Characterization of differently charged species by liquid chromatography-mass spectrometry revealed the reduction of disulfide bonds in acidic variants, which is followed by cysteinylation and glutathionylation of cysteines. Importantly, biophysical stability and integrity of the mAb are not affected. By in vitro incubation of the mAb with the reducing agent cysteine, disulfide bond degradation was directly linked to an increase of numerous acidic species. Modifying the concentrations of cysteine during the fermentation of various mAbs illustrated that redox potential is a critical aspect to consider during bioprocess development with respect to charge variant control.

Structural Insight into IAPP-Derived Amyloid Inhibitors and Their Mechanism of Action.

Designed peptides derived from the islet amyloid polypeptide (IAPP) cross-amyloid interaction surface with Aß (termed interaction surface mimics or ISMs) have been shown to be highly potent inhibitors of Aß amyloid self-assembly. However, the molecular mechanism of their function is not well understood. Using solution-state and solid-state NMR spectroscopy in combination with ensemble-averaged dynamics simulations and other biophysical methods including TEM, fluorescence spectroscopy and microscopy, and DLS, we characterize ISM structural preferences and interactions. We find that the ISM peptide R3-GI is highly dynamic, can adopt a ß-like structure, and oligomerizes into colloid-like assemblies in a process that is reminiscent of liquid-liquid phase separation (LLPS). Our results suggest that such assemblies yield multivalent surfaces for interactions with Aß40. Sequestration of substrates into these colloid-like structures provides a mechanistic basis for ISM function and the design of novel potent anti-amyloid molecules.

A Minimal Functional Complex of Cytochrome P450 and FBD of Cytochrome P450 Reductase in Nanodiscs.

Structural interactions that enable electron transfer to cytochrome-P450 (CYP450) from its redox partner CYP450-reductase (CPR) are a vital prerequisite for its catalytic mechanism. The first structural model for the membrane-bound functional complex to reveal interactions between the full-length CYP450 and a minimal domain of CPR is now reported. The results suggest that anchorage of the proteins in a lipid bilayer is a minimal requirement for CYP450 catalytic function. Akin to cytochrome-b5 (cyt-b5 ), Arg 125 on the C-helix of CYP450s is found to be important for effective electron transfer, thus supporting the competitive behavior of redox partners for CYP450s. A general approach is presented to study protein-protein interactions combining the use of nanodiscs with NMR spectroscopy and SAXS. Linking structural details to the mechanism will help unravel the xenobiotic metabolism of diverse microsomal CYP450s in their native environment and facilitate the design of new drug entities.

Sulindac Sulfide Induces the Formation of Large Oligomeric Aggregates of the Alzheimer's Disease Amyloid-ß Peptide Which Exhibit Reduced Neurotoxicity.

Alzheimer's disease is characterized by deposition of the amyloid ß-peptide (Aß) in brain tissue of affected individuals. In recent years, many potential lead structures have been suggested that can potentially be used for diagnosis and therapy. However, the mode of action of these compounds is so far not understood. Among these small molecules, the nonsteroidal anti-inflammatory drug (NSAID) sulindac sulfide received a lot of attention. In this manuscript, we characterize the interaction between the monomeric Aß peptide and the NSAID sulindac sulfide. We find that sulindac sulfide efficiently depletes the pool of toxic oligomers by enhancing the rate of fibril formation. In vitro, sulindac sulfide forms colloidal particles which catalyze the formation of fibrils. Aggregation is immediate, presumably by perturbing the supersaturated Aß solution. We find that sulindac sulfide induced Aß aggregates are structurally homogeneous. The C-terminal part of the peptide adopts a ß-sheet structure, whereas the N-terminus is disordered. The salt bridge between D23 and K28 is present, similar as in wild type fibril structures. (13)C-(19)F transferred echo double resonance experiments suggest that sulindac sulfide colocalizes with the Aß peptide in the aggregate.

Structural Mechanism of the Interaction of Alzheimer Disease Aß Fibrils with the Non-steroidal Anti-inflammatory Drug (NSAID) Sulindac Sulfide.

Alzheimer disease is the most severe neurodegenerative disease worldwide. In the past years, a plethora of small molecules interfering with amyloid-ß (Aß) aggregation has been reported. However, their mode of interaction with amyloid fibers is not understood. Non-steroidal anti-inflammatory drugs (NSAIDs) are known γ-secretase modulators; they influence Aß populations. It has been suggested that NSAIDs are pleiotrophic and can interact with more than one pathomechanism. Here we present a magic angle spinning solid-state NMR study demonstrating that the NSAID sulindac sulfide interacts specifically with Alzheimer disease Aß fibrils. We find that sulindac sulfide does not induce drastic architectural changes in the fibrillar structure but intercalates between the two ß-strands of the amyloid fibril and binds to hydrophobic cavities, which are found consistently in all analyzed structures. The characteristic Asp(23)-Lys(28) salt bridge is not affected upon interacting with sulindac sulfide. The primary binding site is located in the vicinity of residue Gly(33), a residue involved in Met(35) oxidation. The results presented here will assist the search for pharmacologically active molecules that can potentially be employed as lead structures to guide the design of small molecules for the treatment of Alzheimer disease.

A Hot-Segment-Based Approach for the Design of Cross-Amyloid Interaction Surface Mimics as Inhibitors of Amyloid Self-Assembly.

The design of inhibitors of protein-protein interactions mediating amyloid self-assembly is a major challenge mainly due to the dynamic nature of the involved structures and interfaces. Interactions of amyloidogenic polypeptides with other proteins are important modulators of self-assembly. Here we present a hot-segment-linking approach to design a series of mimics of the IAPP cross-amyloid interaction surface with Aß (ISMs) as nanomolar inhibitors of amyloidogenesis and cytotoxicity of Aß, IAPP, or both polypeptides. The nature of the linker determines ISM structure and inhibitory function including both potency and target selectivity. Importantly, ISMs effectively suppress both self- and cross-seeded IAPP self-assembly. Our results provide a novel class of highly potent peptide leads for targeting protein aggregation in Alzheimer's disease, type 2 diabetes, or both diseases and a chemical approach to inhibit amyloid self-assembly and pathogenic interactions of other proteins as well.

Probing membrane enhanced protein-protein interactions in a minimal redox complex of cytochrome-P450 and P450-reductase.

Investigating the interplay in a minimal redox complex of cytochrome-P450 and its reductase is crucial for understanding cytochrome-P450's enzymatic activity. Probing the hotspots of dynamic structural interactions using NMR revealed the engagement of loop residues from P450-reductase to be responsible for the enhanced affinity of CYP450 towards its obligate redox partner.

MAK33 antibody light chain amyloid fibrils are similar to oligomeric precursors.

Little structural information is available so far on amyloid fibrils consisting of immunoglobulin light chains. It is not understood which features of the primary sequence of the protein result in fibril formation. We report here MAS solid-state NMR studies to identify the structured core of κ-type variable domain light chain fibrils. The core contains residues of the CDR2 and the ß-strands D, E, F and G of the native immunoglobulin fold. The assigned core region of the fibril is distinct in comparison to the core identified in a previous solid-state NMR study on AL-09 by Piehl at. al, suggesting that VL fibrils can adopt different topologies. In addition, we investigated a soluble oligomeric intermediate state, previously termed the alternatively folded state (AFS), using NMR and FTIR spectroscopy. The NMR oligomer spectra display a high degree of similarity when compared to the fibril spectra, indicating a high structural similarity of the two aggregation states. Based on comparison to the native state NMR chemical shifts, we suggest that fibril formation via domain-swapping seems unlikely. Moreover, we used our results to test the quality of different amyloid prediction algorithms.

Kinetic and Structural Characterization of the Effects of Membrane on the Complex of Cytochrome b 5 and Cytochrome c.

Cytochrome b 5 (cytb 5) is a membrane protein vital for the regulation of cytochrome P450 (cytP450) metabolism and is capable of electron transfer to many redox partners. Here, using cyt c as a surrogate for cytP450, we report the effect of membrane on the interaction between full-length cytb 5 and cyt c for the first time. As shown through stopped-flow kinetic experiments, electron transfer capable cytb 5 - cyt c complexes were formed in the presence of bicelles and nanodiscs. Experimentally measured NMR parameters were used to map the cytb 5-cyt c binding interface. Our experimental results identify differences in the binding epitope of cytb 5 in the presence and absence of membrane. Notably, in the presence of membrane, cytb 5 only engaged cyt c at its lower and upper clefts while the membrane-free cytb 5 also uses a distal region. Using restraints generated from both cytb 5 and cyt c, a complex structure was generated and a potential electron transfer pathway was identified. These results demonstrate the importance of studying protein-protein complex formation in membrane mimetic systems. Our results also demonstrate the successful preparation of novel peptide-based lipid nanodiscs, which are detergent-free and possesses size flexibility, and their use for NMR structural studies of membrane proteins.

The chaperone αB-crystallin uses different interfaces to capture an amorphous and an amyloid client.

Small heat-shock proteins, including αB-crystallin (αB), play an important part in protein homeostasis, because their ATP-independent chaperone activity inhibits uncontrolled protein aggregation. Mechanistic details of human αB, particularly in its client-bound state, have been elusive so far, owing to the high molecular weight and the heterogeneity of these complexes. Here we provide structural insights into this highly dynamic assembly and show, by using state-of-the-art NMR spectroscopy, that the αB complex is assembled from asymmetric building blocks. Interaction studies demonstrated that the fibril-forming Alzheimer's disease Aß1-40 peptide preferentially binds to a hydrophobic edge of the central ß-sandwich of αB. In contrast, the amorphously aggregating client lysozyme is captured by the partially disordered N-terminal domain of αB. We suggest that αB uses its inherent structural plasticity to expose distinct binding interfaces and thus interact with a wide range of structurally variable clients.

Bile acids down-regulate caveolin-1 in esophageal epithelial cells through sterol responsive element-binding protein.

Bile acids are synthesized from cholesterol and are major risk factors for Barrett adenocarcinoma (BAC) of the esophagus. Caveolin-1 (Cav1), a scaffold protein of membrane caveolae, is transcriptionally regulated by cholesterol via sterol-responsive element-binding protein-1 (SREBP1). Cav1 protects squamous epithelia by controlling cell growth and stabilizing cell junctions and matrix adhesion. Cav1 is frequently down-regulated in human cancers; however, the molecular mechanisms that lead to this event are unknown. We show that the basal layer of the nonneoplastic human esophageal squamous epithelium expressed Cav1 mainly at intercellular junctions. In contrast, Cav1 was lost in 95% of tissue specimens from BAC patients (n = 100). A strong cytoplasmic expression of Cav1 correlated with poor survival in a small subgroup (n = 5) of BAC patients, and stable expression of an oncogenic Cav1 variant (Cav1-P132L) in the human BAC cell line OE19 promoted proliferation. Cav1 was also detectable in immortalized human squamous epithelial, Barrett esophagus (CPC), and squamous cell carcinoma cells (OE21), but was low in BAC cell lines (OE19, OE33). Mechanistically, bile acids down-regulated Cav1 expression by inhibition of the proteolytic cleavage of 125-kDa pre-SREBP1 from the endoplasmic reticulum/Golgi apparatus and nuclear translocation of active 68-kDa SREBP1. This block in SREBP1's posttranslational processing impaired transcriptional activation of SREBP1 response elements in the proximal human Cav1 promoter. Cav1 was also down-regulated in esophagi from C57BL/6 mice on a diet enriched with 1% (wt/wt) chenodeoxycholic acid. Mice deficient for Cav1 or the nuclear bile acid receptor farnesoid X receptor showed hyperplasia and hyperkeratosis of the basal cell layer of esophageal epithelia, respectively. These data indicate that bile acid-mediated down-regulation of Cav1 marks early changes in the squamous epithelium, which may contribute to onset of Barrett esophagus metaplasia and progression to BAC.

The multifunctional enzyme CYP71B15 (PHYTOALEXIN DEFICIENT3) converts cysteine-indole-3-acetonitrile to camalexin in the indole-3-acetonitrile metabolic network of Arabidopsis thaliana.

Accumulation of camalexin, the characteristic phytoalexin of Arabidopsis thaliana, is induced by a great variety of plant pathogens. It is derived from Trp, which is converted to indole-3-acetonitrile (IAN) by successive action of the cytochrome P450 enzymes CYP79B2/B3 and CYP71A13. Extracts from wild-type plants and camalexin biosynthetic mutants, treated with silver nitrate or inoculated with Phytophthora infestans, were comprehensively analyzed by ultra-performance liquid chromatography electrospray ionization quadrupole time-of-flight mass spectrometry. This metabolomics approach was combined with precursor feeding experiments to characterize the IAN metabolic network and to identify novel biosynthetic intermediates and metabolites of camalexin. Indole-3-carbaldehyde and indole-3-carboxylic acid derivatives were shown to originate from IAN. IAN conjugates with glutathione, gamma-glutamylcysteine, and cysteine [Cys(IAN)] accumulated in challenged phytoalexin deficient3 (pad3) mutants. Cys(IAN) rescued the camalexin-deficient phenotype of cyp79b2 cyp79b3 and was itself converted to dihydrocamalexic acid (DHCA), the known substrate of CYP71B15 (PAD3), by microsomes isolated from silver nitrate-treated Arabidopsis leaves. Surprisingly, yeast-expressed CYP71B15 also catalyzed thiazoline ring closure, DHCA formation, and cyanide release with Cys(IAN) as substrate. In conclusion, in the camalexin biosynthetic pathway, IAN is derivatized to the intermediate Cys(IAN), which serves as substrate of the multifunctional cytochrome P450 enzyme CYP71B15.