Colloidal extracellular matrix gels for filling and remodeling osseous defects

Abstract

Event Abstract Back to Event Colloidal extracellular matrix gels for filling and remodeling osseous defects S Connor Dennis1, Jon Whitlow2, Michael S. Detamore1, 2 and Cory J. Berkland1, 2, 3 1 University of Kansas, Bioengineering, United States 2 University of Kansas, Chemical and Petroleum Engineering Department, United States 3 University of Kansas, Pharmaceutical Chemistry Department, United States Introduction: Injectable osteogenic gels present an appealing strategy for bone tissue engineering. Current grafting procedures offer surgeons a limited ability to reconstruct critical-sized bone defects[1]. Self-assembling glycosaminoglycan (GAG) and hydroxyapatite (HAP) colloids coupled with native extracellular matrix (ECM) microparticles introduce a platform of viscoelastic and osteoinductive gels. The present study not only characterizes the mechanical properties of the ECM-based colloidal gels that facilitate surgical placement, but also evaluates the in vivo osteogenic potential of the gels and their capability to deliver therapeutic doses of osteoinductive cytokines. Materials and Methods: Colloidal mixtures of nanoparticle HAP and hyaluronic acid (HA) polymers were formed under a variety of weight ratios. Decellularized cartilage (DCC) derived from porcine articular cartilage was cryogenically micronized and subsequently decellularized following a previously established protocol. Human demineralized bone matrix (DBM) was purchased from a supplier and cryogenically micronized. DCC and DBM microparticles were incorporated in HA-ECM suspensions and HA-HAP-ECM colloids. A controlled stress rheometer was used to characterize the rheological properties and viscoelastic recovery kinetics of HA-ECM suspensions and HA-HAP-ECM colloids in comparison to HA-HAP colloidal gels. In vitro cell viability assessments on the suspensions and colloidal gels were conducted with rBMSCs. Selected formulations were implanted in vivo in critical-sized rat calvarial defects. In a subsequent in vivo murine study, the same HA-HAP-ECM formulations were loaded with cytokines VEGF-165 and BMP-2 and implanted in critical-sized calvarial defects. Histological and microCT analysis (Fig. 3) were conducted to analyze the extent of endochondral (EC) ossification within defect sites. Results and Discussion: Rheological analysis revealed that HA-HAP colloids containing higher molecular weight HA polymer increased elastic properties of gels due to polymer-particle flocculation. Addition of ECM to these gels led to an unexpected increase in the storage modulus, viscoelastic recovery, and yield stress of the HA-HAP colloids (Fig. 1). In vitro studies (Fig. 2) of HA-HAP-ECM colloids evidence the activation of transcription factors indicative of EC ossification[2]. However, they only partially regenerated critical-sized defects in rat calvaria in vivo after 8 weeks. Further in vivo study confirms the ability of the HA-HAP-ECM gels to deliver both VEGF-165 and BMP-2, resulting in increased EC ossification within defects. Articular chondrocytes have a quiescent phenotype regulated by factors that inhibit proliferation and differentiation, and since EC ossification requires chondrocyte proliferation, DCC from hypertrophic growth plate cartilage rather than articular cartilage may further enable EC ossification[3]. Conclusion: The major finding of this study is the formulation of ECM-based colloidal gels that display viscoelastic features rendering them viable biocompatible bone tissue defect fillers. The colloidal gels prove to effectively induce EC ossification in vitro and in vivo, though complete regeneration of the defects was not observable within 8 weeks. The gels are also suitable vehicles for angiogenic and osteogenic cytokines that promote tissue regeneration. Future work to improve the regenerative ability of the implant include reformulation of the colloidal gel and the use DCC from alternative cartilage tissues such as hypertrophic cartilage. Dr. Cory Berkland; Dr. S. Connor Dennis; Dr. Michael Detamore