Bone regeneration is a complicated process which involved many cellular types such as osteoblasts, chondrocytes and mesenchymal stromal cells (MSC). MSC can differentiate toward chondrocytes and osteoblasts, suggesting their implication in bone regeneration processes. Bone reparation involved a complex coordination of growth factors. Among them, heparin affin regulatory peptide (HARP) is found in callus during regeneration. However, its role is poorly understood. The aim of this thesis was (1) to evaluate HARP effects on proliferation, migration and differentiation of MSC in vitro, (2) to evaluate HARP ability to promote bone regeneration or bone formation.Our results demonstrate that HARP has chemoattractive and proliferative properties on human MSC. Moreover, we show for the first time that HARP commits human MSC toward hypertrophy during chondrogenesis. This is of great interest since hypertrophic chondrocytes are of primary importance in the late stage of endochondral bone formation. We further tested the association of HARP to scaffolds in a model of bone regeneration in femoral defect in rat. HARP associated scaffolds showed an invasion of cartilage and bony tissues, mimicking endochondral bone formation, whereas scaffold alone was just filled with fibrous tissue.During regenerative processes glycosaminoglycans, polysaccharides sulfated chains, are known as major components of the extracellular matrix and modulate the effects of growth factors during regenerative processes. Recently, structurally and functionally mimetics of GAG had been developed. During my PhD thesis, I was associated to the work of a doctoral student of P. Albanese who showed that GAG mimetics induce osteoblastic differentiation of MSC without any other osteoinductive treatment. The ectopic implantation of mimetic associated scaffolds didn't show effects on osteoformation but induced an enhancement of vascularization and of osteoclastic activity, both related to tissue remodeling. These results validate that GAG mimetics are of great interest in bone regenerative field.This study demonstrates for the first time the chondroinductive potential of HARP through its ability to induce cartilage specific matrix production by MSC in vitro but also by inducing cartilage tissue synthesis in vivo. The effects of HARP observed on bone regeneration, by inducing an endochondral bone formation similar to that observed in normal bone regeneration, confirm that HARP could be a good candidate in orthopedic surgery. Moreover, scaffold covalently linked with GAG mimetics should be tested in association with HARP. This strategy could increase the half life, control the release and potentiate HARP properties in vivo.
