The use of injectable, thermosensitive poly(organophosphazene)–RGD conjugates for the enhancement of mesenchymal stem cell osteogenic differentiation

An injectable and thermosensitive poly(organophosphazene)–RGD conjugate to enhance functionality was synthesized by a covalent amide linkage between a cell adhesion peptide, GRGDS and carboxylic acid-terminated poly(organophosphazene). The aqueous solutions of synthesized poly(organophosphazene)–GRGDS conjugates existed in an injectable fluid state at room temperature and immediately formed a hydrogel at body temperature. The rabbit mesenchymal stem cells (rMSCs) on the polymer–GRGDS conjugate (conjugate 1-2, 0.05 mol fraction as GRGDS) hydrogel constructs using an injection method into a nude mouse were proved to express markers at mRNA level for all stages towards osteogenesis and mainly a sharp increase of osteocalcin (OCN, a typical late osteogenic differentiation marker) levels at 4th week post-induction indicated that the maturation process has started within this period. By histological and immunohistochemical evaluations, significantly high mineralization level by calcium contents was detected qualitatively and collagen type I (Col I), a major characteristic marker protein, was mainly and highly expressed by the rMSCs cultivated in the polymer–GRGDS conjugate hydrogel constructs formed into the nude mouse. The results suggest that the poly(organophosphazene)–GRGDS conjugate to enhance biofunctionality hold a promise for cell delivery material to induce osteogenic differentiation of MSC for enhancing ectopic bone formation.