Antibody recognition of complement Factor H reveals a flexible loop involved in Atypical Hemolytic Uremic Syndrome pathogenesis

Improving the solubility of single domain antibodies using VH-like hallmark residues

Single domain antibodies (sdAbs) can be generated from variable regions of heavy-chain antibodies, which lack light chain and CH1 region. They have attracted attention due to their small size and molecular characteristics. Hydrophilic hallmark amino acids at framework region 2 (FR2) are key residues involved in the solubility of sdAbs. Nevertheless, previous studies reported that several sdAbs with human VH-like hydrophobic hallmark residues were soluble in a monomeric state and suggested that solubility also depends on the amino acid sequences in the complementarity-determining region. In this study, we obtained two sdAbs (sdAb A and B) with VH-like hallmark residues and low solubility from an alpaca immune library. We introduced VHH-like mutations (V37Y, G44E, L45R, W47L) into the hallmark residues in FR2 of both sdAb A and B. We were able to prepare sdAb A as a monomer without an additive in the buffer, but sdAb B was polydispersed when arginine was not added to the buffer. We also predicted the hydrophobicity of the sdAb B surface by spatial aggregation propensity calculations and identified W99 as the residue responsible for its low solubility. Subsequently, we obtained the sdAb B mutant as a monomer by introducing the W99A mutation. We characterized the engineered sdAbs using structural, physicochemical, and biophysical analyses and found that the solubility-improved sdAbs retained their functionality. Our findings can be applied to improving the solubility of sdAbs even in the absence of structural information.

Overview on the role of complement-specific autoantibodies in diseases

The complement system is recognized as a major pathogenic or contributing factor in an ever-growing number of diseases. In addition to inherited factors, autoantibodies to complement proteins have been detected in various systemic and organ-specific disorders. These include antibodies directed against complement components, regulators and receptors, but also protein complexes such as autoantibodies against complement convertases. In some cases, the autoantibodies are relatively well characterized and a pathogenic role is incurred and their detection has diagnostic value. In other cases, the relevance of the autoantibodies is rather unclear. This review summarizes what we know of complement specific autoantibodies in diseases and identifies unresolved questions regarding their functional effect and relevance.

Development of Antibody-like Proteins Targeting the Oncogenic Ser/Thr Protein Phosphatase PPM1D

PPM1D, a protein Ser/Thr phosphatase, is overexpressed in various cancers and functions as an oncogenic protein by inactivating the p53 pathway. Therefore, molecules that bind PPM1D are expected to be useful anti-cancer agents. In this study, we constructed a phage display library based on the antibody-like small molecule protein adnectin and screened for PPM1D-specific binding molecules. We identified two adnectins, PMDB-1 and PMD-24, that bind PPM1D specific B-loop and PPM1D430 as targets, respectively. Specificity analyses of these recombinant proteins using other Ser/Thr protein phosphatases showed that these molecules bind to only PPM1D. Expression of PMDB-1 in breast cancer-derived MCF-7 cells overexpressing endogenous PPM1D stabilized p53, indicating that PMDB-1 functions as an inhibitor of PPM1D. Furthermore, MTT assay exhibited that MCF-7 cells expressing PMDB-1 showed inhibition of cell proliferation. These data suggest that the adnectin PMDB-1 identified in this study can be used as a lead compound for anti-cancer drugs targeting intracellular PPM1D.