Tendon tissue engineering: Synergistic effects of mechanical stimulation and PDGF-nanoparticles on tenocyte differentiation of adipose-derived stem cells on aligned collagen scaffolds

Tendon injuries are common but have a limited capacity to heal. Although tendon autograft is the most successful repair option available, it has drawbacks including the requirement of a second surgical site. A synthetic alternative is desirable. Current clinical use of synthetic materials typically requires harvesting patient donor cells and culturing them with a synthetic graft for a number of weeks prior to implantation. An implantable, synthetic graft that creates an environment conducive to autologous progenitor cell migration, proliferation and differentiation into tenocytes is needed. This study examines the effects of an electrochemically aligned collagen scaffold with embedded poly(lactic-co-glycolic acid)-monomethoxy-poly(ethylene glycol) (PLGA-m-PEG) nanoparticles (NP) containing platelet-derived growth factor, BB-isoform (PDGF-BB) on proliferation and induction of tenocyte differentiation in adipose-derived stem cells (ASC). Mechanical strain has been shown to be critical in developing tendon. Therefore, in addition to the effects of nano-scale topography of the aligned collagen and the proliferative and chemotransductive effects of PDGF, the mechanotransductive effect of cyclic strain applied to the cultured collagen constructs is examined. Applied strains were within a physiologic range achievable by post-implant physical therapy. The aligned collagen scaffold with control NP increases adipose derived stem cell proliferation, with organization coincident with the topography and expression of tenocytic pathway markers Tenomodulin (TNMD), Scleraxis (Scx) and Tenascin-C (TenC) compared to unaligned controls at 1 and 3 days of culture. The addition of PDGF NP to the scaffold augments all responses over control NP. By day 7, TNMD remains elevated with Scx and TenC falling to day 1 levels. This expression pattern of the tenocyte markers is consistent with that reported in the literature for other scaffold constructs, although temporally, it occurs earlier. Applied cyclic strain (5%) initially decreases cell proliferation in scaffolds with the control NP. The decrease in proliferation does not occur in scaffolds with PDGF NP. The tenocyte marker, TNMD is significantly up-regulated by 5% cyclic strain. The effects of PDGF on TNMD expression in the constructs to which strain is applied is not as dramatic as that seen with unstrained constructs over time, suggesting that mechanical strain may target differentiation while PDGF targets proliferation. A synergistic influence of applied mechanical strain and PDGF on tenocytic gene expression was observed. Future studies engineering the aligned collagen scaffold containing PDGF NP to achieve an appropriate temporal balance between these mechanisms has the potential for developing a synthetic implant of enormous clinical benefit for the repair of tendon injuries.