Characterization of the human nucleus pulposus cell phenotype and evaluation of novel marker gene expression to define adult stem cell differentiation

OBJECTIVE: Development of stem cell therapies for regenerating the nucleus pulposus (NP) are hindered by the lack of specific markers by which to distinguish NP cells from articular chondrocytes (ACs). The purpose of this study was to define the phenotype profile of human NP cells using gene expression profiling and to assess whether the identified markers could distinguish mesenchymal stem cell (MSC) differentiation to a correct NP cell phenotype. METHODS: Affymetrix MicroArray analyses were conducted on human NP cells and ACs, and differential expression levels for several positive (NP) and negative (AC) marker genes were validated by real-time quantitative polymerase chain reaction (PCR) analysis. Novel marker gene and protein expression was also assessed in human bone marrow-derived MSCs (BM-MSCs) and adipose tissue-derived MSCs (AD-MSCs) following differentiation in type I collagen gels. RESULTS: Analysis identified 12 NP-positive and 36-negative (AC) marker genes that were differentially expressed ≥20-fold, and for a subset of them (NP-positive genes PAX1, FOXF1, HBB, CA12, and OVOS2; AC-positive genes GDF10, CYTL1, IBSP, and FBLN1), differential expression was confirmed by real-time quantitative PCR. Differentiated BM-MSCs and AD-MSCs demonstrated significant increases in the novel NP markers PAX1 and FOXF1. AD-MSCs lacked expression of the AC markers IBSP and FBLN1, whereas BM-MSCs lacked expression of the AC marker IBSP but expressed FBLN1. CONCLUSION: This study is the first to use gene expression profiling to identify the human NP cell phenotype. Importantly, these markers can be used to determine the in vitro differentiation of MSCs to an NP-like, rather than an AC-like, phenotype. Interestingly, these results suggest that AD-MSCs may be a more appropriate cell type than BM-MSCs for use in engineering intervertebral disc tissue.