LRP5-deficiency in OsxCreERT2 mice models intervertebral disc degeneration by aging and compression

Osterix is a critical transcription factor of mesenchymal stem cell fate, where its loss or loss of WNT signaling diverts differentiation to a chondrocytic lineage. Intervertebral disc (IVD) degeneration activates differentiation of prehypertrophic chondrocyte-like cells and inactivates WNT signaling, but its interacting role with osterix is unclear. First, compared to young-adult (5mo), mechanical compression of old (18mo) IVD induced greater IVD degeneration. Aging (5 vs 12mo) and/or compression reduced the transcription of osterix and notochordal marker T by 40-75%. Compression elevated transcription of hypertrophic chondrocyte marker MMP13 and pre-osterix transcription factor RUNX2, but less so in 12mo IVD. Next, using an Ai9/td reporter and immunohistochemistry, annulus fibrosus and nucleus pulposus cells of 5mo IVD expressed osterix, but aging and compression reduced its expression. Lastly, in vivo LRP5-deficiency in osterix-expressing cells degenerated the IVD, inactivated WNT signaling, reduced the biomechanical properties by 45-70%, and reduced transcription of osterix, notochordal markers and chondrocytic markers by 60-80%. Overall, these data indicate that age-related inactivation of WNT signaling in osterix-expressing cells may limit regeneration by depleting progenitors and attenuating the expansion of chondrocyte-like cells.