A novel agonist with homobivalent single-domain antibodies that bind the FGF receptor 1 domain III functions as an FGF2 ligand

Fibroblast growth factor (FGF) is a multifunctional protein that exhibits a wide range of biological effects. Most commonly, it acts as a mitogen, but it also has regulatory, morphological, and endocrine effects. The four receptor subtypes of FGF are activated by more than 20 different FGF ligands. FGF2, one of the FGF ligands, is an essential factor for cell culture in stem cells for regenerative medicine; however, recombinant FGF2 is extremely unstable. Here, we successfully generated homobivalent agonistic single-domain antibodies (variable domain of heavy chain of heavy chain antibodies referred to as VHHs) that bind to domain III and induce activation of the FGF receptor 1 and thus transduce intracellular signaling. This agonistic VHH has similar biological activity (EC(50)) as the natural FGF2 ligand. Furthermore, we determined that the agonistic VHH could support the proliferation of human-induced pluripotent stem cells (PSCs) and human mesenchymal stem cells, which are PSCs for regenerative medicine. In addition, the agonistic VHH could maintain the ability of mesenchymal stem cells to differentiate into adipocytes or osteocytes, indicating that it could maintain the properties of PSCs. These results suggest that the VHH agonist may function as an FGF2 mimetic in cell preparation of stem cells for regenerative medicine with better cost effectiveness.