The effect of adsorbed serum proteins, RGD and proteoglycan-binding peptides on the adhesion of mesenchymal stem cells to hydroxyapatite

Prior studies from our laboratory have shown that RGD peptides increase the attachment of mesenchymal stem cells (MSCs) to hydroxyapatite (HA), however, RGD does not induce cell spreading when coupled to this type of biomaterial. In an effort to improve MSC spreading, and possibly cell attachment, proteoglycan-binding peptides (KRSR or FHRRIKA) were combined with RGD in the current study. It was found that the peptide combinations did not enhance MSC attachment relative to RGD alone, although a slight amount of spreading was elicited by both KRSR and FHRRIKA. Similar results were obtained with proteoglycan-binding peptides modified with a heptaglutamate domain, a motif that improves peptide tethering to HA. To determine whether differentiation status affected cell responses, MSCs were in vitro differentiated into osteoblasts, and evaluated as before. These experiments revealed that, like MSCs, osteoblasts did not adhere in greater numbers to the peptide combinations. Finally, none of the peptides or peptide combinations were able to stimulate the robust amount of cell adhesion and spreading elicited by serum-coated HA surfaces (of note, five different species of serum were tested). Given the propensity of HA to adsorb proadhesive proteins from blood/serum, we question the utility of functionalizing HA with RGD and/or proteoglycan-binding peptides.