Mesenchymal stem cells transplanted with self-assembling scaffolds differentiated to regenerate nucleus pulposus in an ex vivo model of degenerative disc disease

Intervertebral disc (IVD) degeneration is one of the major causes of chronic severe low back pain (LBP). IVD degeneration is characterized by changes in cell populations, and the subsequent loss of extracellular matrix (ECM) of the nucleus pulposus (NP) causing degenerative disc disease (DDD). Current treatments only address symptomatic pain rather than repairing the damaged IVDs. Therefore, there is a need for regenerative therapies that restore native tissue structure, cellularity and mechanical function to treat DDD. However, the development of cell therapeutic approaches is hindered due to the lack of availability of high quality and quantity of cells. Other problems include poor growth and differentiation of transplanted cells, and cell leakage from the site of injection in the damaged IVD. In this study, we investigated a novel strategy by combining biocompatible biomaterials and cell therapy to regenerate NP through in situ differentiation of transplanted cells using an ex vivo rabbit disc model of DDD. Results indicated that hydrogel scaffolds composed of self-assembling polyethylene glycol (PEG) functionalized with acrylate and thiol end groups promoted differentiation of human umbilical cord mesenchymal stem cells (MSCs) into NP-like cells (NPCs) in vitro. Upon transplantation into degenerated IVDs, PEG scaffolds limited leakage and retained the cells in the NP region of IVD transplants. Both the scaffold and the ex vivo disc environment promoted differentiation of MSCs into cell types capable of producing ECM including sulfated glycoaminoglycans at levels higher than when MSCs were injected into IVD explants alone. Transplanted cells using self-assembling scaffolds also expressed chondrogenic markers, SOX9, COL2, and ACAN, as well as putative NP markers, FOXF1, K19, and VIMENTIN both at transcriptional and translational levels as determined by quantitative real-time PCR and immunostaining. Overall, this study demonstrates the potential of MSCs for regeneration of the NP using a combined strategy of biomaterials and cell therapy.