Development of 3D culture systems for nucleus pulposus cells

Abstract

Degenerative disc disease (DDD) is a common cause of low back pain and accounts for medical costs of over USD 100 billion in the United States every year. Current treatments can only relieve symptoms while some even cause drawbacks such as accelerating the degeneration. Better therapeutic strategies that can restore the IVD to a healthy state are needed. The disease is closely correlated with cellular and biochemical changes in the nucleus pulposus (NP) in the intervertebral disc (IVD). For better understanding of the etiology of the disease, an in vitro culture system for NP cells (NPCs) is required as a model platform. However, the native phenotype of NPCs cannot be maintained in conventional monolayer culture due to dedifferentiation. Also, monolayer culture lacks a rich extracellular matrix (ECM), may be important in the development of the disease.

In this study, the expression of bovine NPC (bNPC) markers in the native NP tissue was examined at protein level. This study investigated the survival and phenotype of bNPCs in collagen-based three-dimensional (3D) culture systems incorporated with additional ECM components including laminin and glycosaminoglycans (GAGs).

In the native bovine NP tissue, the expression of COL2, KRT8, SNAP25 and SOSTDC1 was confirmed at protein level. As demonstrated by the increase in gene expression of chondrogenic markers, GAG production and COL2 deposition, collagen microspheres can partially maintain the phenotype of bNPCs, indicating that it is a good template for developing 3D culture systems for bNPCs. Also, as demonstrated by the increase in protein expression of cellular markers upon incorporation of unconjugated laminin, incorporation of more ECM components as free forms by physical mixing with collagen may further enhance the phenotype of bNPCs. Furthermore, preliminary data from a pilot study showed the topological features presented by the matrix to the cell as a potentially important factor in controlling the bNPC phenotype.

To conclude, this work suggested collagen microspheres as a good template for developing 3D culture systems for bNPCs. Further optimizations include incorporation of more ECM components may enhance the phenotype of bNPCs, as demonstrated by the laminin experiment in this study. The topological features presented by the matrix would be another direction for optimizations. The resulted optimal 3D culture system would bring potential benefits to research related to the etiology of DDD and development of new therapies for DDD.