Increased osmolarity and cell clustering preserve canine notochordal cell phenotype in culture

Degeneration of the intervertebral disc (IVD) is associated with a loss of notochordal cells (NCs) from the nucleus pulposus (NP) and their replacement by chondrocyte-like cells. NCs are known to maintain extracellular matrix quality and stimulate the chondrocyte-like NP cells, making NCs attractive for designing new tissue engineering approaches for IVD regeneration. However, optimal conditions, such as osmolarity and other characteristics of the culture media, for long-term culture of NCs are not known. The purpose of this study was to investigate the effects of different culture media and osmolarity on the physiology of NCs in vitro. NC clusters isolated from canine IVDs were suspended in alginate beads and cultured at 37°C under normoxic conditions for 28 days. Three different culture conditions were investigated; (1) Dulbecco's modified Eagle's medium (DMEM)/F12 (300 mOsm/L), (2) α-MEM (300 mOsm/L), and (3) α-MEM adjusted to 400 mOsm/L to mimic a hyperosmolar environment. NC morphology, expression of genes related to NC markers, matrix production and remodeling, and DNA- and glycosaminoglycan (GAG) analyses were performed on 1, 7, 14, and 28 days in culture. Large, vesicle-containing cells organized in clusters, characterized as NCs, remained present during 28 days for all culture conditions. However, the proportion of the NC clusters decreased over time, whereas the proportion of spindle-shaped cells increased. Gene expression profiling at 7, 14, and 28 days in culture compared to day 1 indicated a initial loss of NC phenotype followed by some recovery of brachyury and aggrecan gene expression after 28 days of culture supporting a potential recovery of NC phenotype. NCs cultured in α-MEM adjusted to 400 mOsm/L showed the highest gene expression of brachyury, cytokeratin 18, and aggrecan, the highest GAG production, and the lowest collagen 1α1 gene expression. In conclusion, NCs cultured in alginate in native cell clusters, partially retained their characteristic morphology and recovered their phenotype in long-term culture. The type of culture medium and medium osmolarity appear to be important factors for culturing NC clusters. These findings provide additional information concerning the maintenance of NCs in vitro that may aid further mechanistic inquiry into the biology of NCs.