Understanding the Aggregation Mechanism of and Developing Stabilization Strategies for Recombinant Fibroblast Growth...

Fibroblast Growth Factor 2 (FGF2) is a highly effective regulator of cell proliferation, differentiation, migration, and adhesion, suggesting a significant therapeutic potential as a tissue regeneration promoter both in acute and chronic tissue damage settings. Despite an extensive list of pathologies that lend themselves as viable targets for FGF2-based therapy (ranging from periodontics to burns to diabetic ulcers to coronary artery disease), the success record in the clinic remains modest, with no FDA approvals obtained so far. The inferior stability of this protein is frequently cited as the most significant factor behind its disappointing performance as a biotherapeutic. Multiple strategies have been designed and tested in an effort to ameliorate this problem, but the success remains elusive. We investigate the aggregation propensity of a recombinantly produced FGF2 using native mass spectrometry (MS) to identify conditions favoring formation of small soluble oligomers, which are considered precursors to larger aggregates. Tandem MS of proteolytic fragments produced by digestion of the oligomeric species allows the formation of external disulfide bonds to be identified as the process leading to oligomerization. Specifically, Cys-31 (one of the two unpaired cysteine residues in intact FGF2) appears to be a particularly active promoter of oligomerization by forming external disulfide bonds. As a high-pI protein, FGF2 readily associates with heparin, and molecular modeling identifies a positive charge basin proximal to Cys-31 as a potential heparin binding site, which can readily accommodate a synthetic heparin mimetic fondaparinux. Adding an equimolar amount of the latter to the FGF2 solution not only leads to formation of a stable protein/polyanion complex (as revealed by native MS), but also inhibits formation of FGF2 oligomers (presumably via a combination of steric hindrance and electrostatic repulsion). These findings advance our understanding of FGF2 stability, which will be invaluable for optimizing its formulation, storage, and administration.