Gene Expresion Profiling of Naturally Occuring Early Intervertebral Disc Degeneration Sugests a Crucial Role of Caveolin-1 in Notochordal Cel Preservation

Introduction
Intervertebral disc (IVD) degeneration and associated IVD disease are the most common causes for back problems in dogs. There has been an increasing interest in regenerative treatments for IVD disease. However, fundamental biomolecular events in the degenerative process have been largely unexplored. Early degeneration of the IVD involves the transistion from a nucleus pulposus (NP) rich in notochordal cells (NCs) to an NP rich in chondrocyte-like cells (CLCs). The aim of this study was to investigate the gene expression regulations involved in early IVD degeneration.

Material and Methods
A cRNA-microarray was performed to analyze gene expression regulations involved in early IVD degeneration. Relevant and significantly regulated biomolecular pathways were validated by investigating relevant candidate genes in healthy and degenerated IVD tissue from dogs and mice, and in cultured NCs using qPCR and immunohistochemistry / immunofluorescence.

Results
Early IVD degeneration involved significant regulations in numerous pathways, including extracellular matrix remodeling, Bone Morphogenic Protein- , and Wnt/β- catenin-signaling. With regard to Wnt/β-catenin, IVD degeneration involved significant downregulation of axin2 gene expression and caveolin-1 gene and protein expression. NCs in monolayer culture showed caveolin-1 gene and protein expression, which significantly increased during adherence of the NCs. The NP of 3 month-old wildtype mice were rich in viable NCs, whereas the NPs of 3-month caveolin-1 knock-out mice contained chondroid-like matrix with small, rounded cells, the majority of which showed signs of apoptosis.

Conclusions
Early IVD degeneration involves significant downregulation of canonical Wnt signaling and caveolin-1, which appears to be essential in the preservation of NCs and thus the healthy NP. This knowledge may be implemented in treatment strategies aimed at regenerating the degenerated IVD.