Crystal structure of RANK ligand involved in bone metabolism.

Bone remodeling involves the resorption of bone by osteoclasts and the synthesis of bone matrix by osteoblasts. Recently, an essential cytokine system for osteoclast biology has been identified and extensively characterized. This system consists of a ligand, receptor activator of NF-kappaB ligand (RANKL), a receptor, RANK, and its soluble decoy receptor, osteoprotegerin (OPG). RANKL, a member of the tumor necrosis factor (TNF) family, triggers osteoclastogenesis by forming a complex with RANK, a member of the TNF receptor family. Because members of the TNF family have the same topology and the extracellular domains of the TNF receptor family members also adopt the same structural scaffold, in addition to their rapid increase in the number, this poses an intriguing question of how recognition between cognate ligands and receptors is achieved in a highly specific manner. Structural studies on the mouse RANKL extracellular domain showed that the RANKL is trimeric, and each subunit has a beta-strand jellyroll topology like the other members of the TNF family. A comparison of RANKL with TNF-beta and TNF-related apoptosis-inducing ligand (TRAIL), whose structures were determined to be in the complex form with their respective receptor, revealed conserved and specific features of RANKL in the TNF superfamily. Residues important for receptor binding and activation have also been confirmed by mutagenesis experiments. Further structural and mutational studies on the RANKL/RANK/OPG system will provide useful information for developing drug candidates that inhibit osteoclastogenesis and mediate problems of bone metabolism.

In situ observation of the four-step dehydration process of the 1 beta-methylcarbapenem antibiotic CS-834 crystal by X-rays.

The 1 beta-methylcarbapenem antibiotic CS-834 takes six crystalline forms depending on ambient conditions. The X-ray powder diffraction revealed that the dihydrate crystal (B2-form) was changed to the monohydrate (B1-form) through the intermediate form (B2'-form). The monohydrate form was then changed to the dehydrate (B0-form) through the intermediate B1'-form. The progress of the dehydration along the needle axis (c-axis) was observed under a microscope. When a single crystal of the B2-form was mounted on a diffractometer and the humidity was reduced, the crystal was gradually changed to the various dehydration forms with retention of the single crystal. The crystals of B2- to B0-forms form isostructures to each other except the solvent water molecules. In the crystal structure of the B1-form, the pivaloyloxymethyl moiety is disordered. One is nearly similar to that of the B2-form, while another is similar to that of the B0-form. Each crystal structure consists of a columnar arrangement of CS-834 along the c-axis, and the water molecules are located between the columns and form a characteristic hydrogen bond network. When the water molecules leave the crystal, the columns slide slightly following the slight conformational change in the pivaloyloxymethyl groups and are connected by another type of hydrogen bond network. Such a rearrangement of the hydrogen bond network should be a motive force of the phase change to the next step due to the dehydration. Since the hydrogen bond network extends along the c-axis, the dehydration proceeds along the c-axis as observed microscopically.

Physicochemical and crystal structure analyses of the antidiabetic agent troglitazone.

The antidiabetic agent troglitazone has two asymmetric carbons located at the chroman ring and the thiazolidine ring and is produced as a mixture of equal amounts of four optical isomers, 2R-5S, 2S-5R, 2R-5R, and 2S-5S. The crystalline powdered drug substance consists of two diastereomer pairs, 2R-5R/2S-5S and 2R-5S/2S-5R. There are many types of crystals obtained from various crystallization conditions. The X-ray structure analysis and the physicochemical analyses of troglitazone were performed. The solvated crystals of the 2R-5R/2S-5S pair were crystallized from several solutions: methanol, ethanol, acetonitrile, and dichloromethane. The ratio of solvent and troglitazone was 1 : 2 (L1/2-form). The monohydrate crystals were obtained from aqueous acetone solution (L1-form). On the other hand, only an anhydrate crystal of the 2R-5S/2S-5R pair was crystallized from various solutions (H0-form). The dihydrous mixed crystal (MA2-form) was obtained from a mixture of the two diastereomer pairs of 2R-5R/2S-5S and 2R-5S/2S-5R in equal amounts by the slow evaporation of aqueous acetone solution. The crystal structure of the MA2-form is similar to the H0-form. When the MA2 crystal was kept under low humidity, it was converted into the dehydrated form (MA0-form) with retention of the single crystal form. The structure of the MA0-form is isomorphous to the H0-form. The MA2-form was converted into the MA0-form and vice versa with retention of the single crystal under low and high humidity, respectively. The crystallization and storage conditions of the drug substances were successfully analyzed.

Physicochemical analyses of phase transition and dehydration processes of a new oral 1beta-methylcarbapenem antibiotic agent, CS-834.

The characterizations of the anhydrate (A-form), monohydrate (B1-form), and dihydrate (B2-form) of CS-834 were investigated by powder X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetry-differential thermal analysis (TG-DTA), infrared spectroscopy, and Karl Fischer moisture titration. The typical DSC curve of the B2-form showed five endothermic peaks at 35.0, 46.4, 56.2, 99.2, and 190.4 degrees C and an exothermic peak at 123.4 degrees C. In TG-DTA analysis, the three peaks at 35.0, 46.4, and 56.2 degrees C had a total weight loss of 7.3%, corresponding to the release of two water molecules. From morphological observation under thermomicroscopy, the endothermic peak at 99.2 degrees C was attributed to the melting of the dehydrous crystals (B0-form) and the exothermic peak at 123.4 degrees C to the recrystallization to the A-form crystals. The endothermic peak at 190.4 degrees C was due to the melting of the A-form crystals. After incubation for 6.0 h at 35, 50, 60, and 80 degrees C, the powder X-ray diffraction patterns of the B2-form indicated that it was converted into the A-form via the B1-form and B0-form. Thus CS-834 exists in homologous hydrous crystal forms in multiple-phase transformations with the dehydration of two water molecules.

Humanization of the mouse anti-Fas antibody HFE7A and crystal structure of the humanized HFE7A Fab fragment.

Binding of Fas ligand to Fas induces apoptosis. The Fas-Fas ligand system plays important roles in many biological processes, including the elimination of autoreactive lymphoid cells. We have previously obtained the mouse anti-Fas antibody HFE7A (m-HFE7A), which specifically induces apoptosis in inflammatory cells. In order to apply m-HFE7A for human therapy, we performed antibody humanization of m-HFE7A by grafting the mouse complementarity-determining regions (CDRs) to a human antibody. Five versions of humanized HFE7A (h-HFE7A) demonstrated the same antigen-binding affinity and same competition-binding activity against Fas as the chimeric HFE7A. Furthermore, these h-HFE7As induced the same degree of apoptosis in WR19L12a cells that express human Fas on their surface as chimeric HFE7A does. To further probe the structural basis for antibody humanization, we determined the three-dimensional structure of the h-HFE7A antigen-binding fragment (Fab) by X-ray crystallography and compared it with the crystal structure of the parent m-HFE7A Fab previously determined. The main-chain conformation in each h-HFE7A CDR is almost identical to that in m-HFE7A with root mean square (rms) deviations of 0.14-0.77 A. However, a significant segmental shift was observed in the CDR-L1 loop. Together with the high temperature factors of the CDR-L1 residues, both the loops are flexible, suggesting that the CDR-L1 loop would undergo conformational change upon binding to the antigen. Our results indicate that the humanization of m-HFE7A succeeded in maintaining the main-chain conformation as well as the flexibility of the CDR loop.

Crystallization and preliminary x-ray studies of the Fab fragment from a humanized version of the mouse anti-human fas antibody HFE7a.

A humanized version of the apoptosis-inducing mouse anti-human Fas monoclonal antibody, HFE7A, is under further development for the treatment of autoimmune diseases such as rheumatoid arthritis. We have crystallized the antigen-binding fragment (Fab) of the humanized HFE7A. The crystals belong to the orthorhombic space group P2(1)2(1)2(1) with cell dimensions a = 54.4 A, b = 82.7 A, c = 104.9 A and contain one Fab molecule in the asymmetric unit. X-ray diffraction data were collected to 2.8 A resolution.

Crystal structure of the antigen-binding fragment of apoptosis-inducing mouse anti-human Fas monoclonal antibody HFE7A.

Binding of Fas ligand to Fas induces apoptosis. The Fas-Fas ligand system plays important roles in many biological processes, including the elimination of autoreactive lymphoid cells. The mouse anti-human Fas monoclonal antibody HFE7A (m-HFE7A), which induces apoptosis, has been humanized based on a structure predicted by homology modeling. A version of humanized HFE7A is currently under development for the treatment of autoimmune diseases such as rheumatoid arthritis. For a deeper understanding of the protein engineering aspect of antibody humanization, for which information on the three-dimensional structure is essential, we determined the crystal structure of the m-HFE7A antigen-binding fragment (Fab) by X-ray crystallography at 2.5 A resolution. The main-chain conformation of the five loops in the six complementarity-determining regions (CDRs) was correctly predicted with root-mean-square deviations of 0.30-1.04 A based on a comparison of the crystal structure with the predicted structure. The CDR-H3 conformation of the crystal structure, which was not classified as one of the canonical structures, was completely different from that of the predicted structure but adopted the conformation which followed the "H3-rules." The results of charge distribution analysis of the antigen-binding site suggest that electrostatic interactions may be important for its binding to Fas.

Crystal structure of the extracellular domain of mouse RANK ligand at 2.2-A resolution.

Bone remodeling involves the resorption of bone by osteoclasts and the synthesis of bone matrix by osteoblasts. Receptor activator of NF-kappa B ligand (RANKL, also known as ODF and OPGL), a member of the tumor necrosis factor (TNF) family, triggers osteoclastogenesis by forming a complex with its receptor, RANK. We have determined the crystal structure of the extracellular domain of mouse RANKL at 2.2-A resolution. The structure reveals that the RANKL extracellular domain is trimeric, which was also shown by analytical ultracentrifugation, and each subunit has a beta-strand jellyroll topology like the other members of the TNF family. A comparison of RANKL with TNF beta and TNF-related apoptosis-inducing ligand (TRAIL), whose structures were determined to be in the complex form with their respective receptor, reveals conserved and specific features of RANKL in the TNF superfamily and suggests the presence of key residues of RANKL for receptor binding.