Strategies to Augment Mesenchymal Stem Cell Mediated Bone Regeneration in Fracture, Fusion, and Osteolytic Lesions

Abstract

Large bone lesions caused by disease or trauma are a fundamental challenge often requiring surgical stabilization supplemented with bone graft materials to restore function. A significant fraction of orthopedic procedures require complicated and potentially dangerous revisions, often caused by implant loosening. The goals of this dissertation were to establish a cell culture model for investigating malignant bone disease (MBD) and strategies for mitigating implant loosening. Otherwise relatively inert synthetic polymers were combined with human mesenchymal stem cell (hMSC)-derived osteogenic cell matrix (OCM) to mimic the osteogenic environment.

Osteolytic MBD is characterized by resorption of bone as the tumor progresses. The interplay between osteolytic bone tumors and osteoprogenitors was mimicked by co-culturing hMSCs with murine osteosarcoma (MOSJ) cells on OCM coated polystyrene beads in a rotating wall vessel (RWV) bioreactor. Bead aggregation yielded a 3D culture that downregulated the canonical Wnt (cWnt) pathway and downstream osteogenic gene expression in hMSCs. The presence OCM substantially increased osteosarcoma proliferation with associated loss of hMSCs. A screen of inhibitors acting on proteins within the cWnt and noncanonical Wnt (ncWnt) pathways identified targets for impacting tumor cell survival and expression of aldehyde dehydrogenase 1a1 (ALDH1A1), a tumor survival gene. Specifically, inhibition of signaling molecules proximal to AP-1 promoter regulation of ALDH1A1 is effective in reducing ALDH1A1 expression and tumor cell number.

A thermoplastic commonly used for bone implants, polyetheretherketone (PEEK), is inherently poor at integrating with surrounding bone and thus prone to loss of fixation. A strategy for covalently binding OCM to PEEK was developed that significantly increased cell attachment relative to unmodified PEEK and osteogenic modulation of hMSCs. Upon implantation in a murine femoral defect model, a remarkable increase in bone formation along the surface of OCM coated PEEK femoral fixation pins occurred without the need for exogenous cells.

This work demonstrates strategies to modify synthetic polymer systems to establish a co-culture model for MBD investigation and mitigation of implant failure through enhanced osseointegration.