BMP Antagonists - A Possible Cause for Spinal Non-Fusion?

INTRODUCTION:Spinal fusion is a procedure where the intervertebral disc (IVD) is removed and two adjacent vertebrae are forced to fuse by compression. This procedure is the most commonly applied procedure to achieve spinal stability and relief of back pain. However, non-successful fusion leads to pseudo-athrosis and ongoing pain. There is increasing evidence that supraphysiological doses of BMP2 and burst-release of this cytokine did not generate satisfying results in clinical studies. Current hypothesis was raised that IVD cells and/or tissue seem to inhibit the action of BMP2. In this overview we summarize the current evidence that BMPs might be inhibited by the secretome of human IVD cells, i.e., nucleus pulposus cells (NPC), annulus fibrosus cells (AFC) and cartilaginous endplate (CEPC) cells. METHODS:We stimulated low-passage (2-3) human bonemarrow-derived mesenchymal stromal cells (MSCs) and femoral hip-derived osteoblasts (OBs) and co-cultured these with allogeneic IVD cells obtained from spinal surgery. We then stimulated MSCs and the OBs in monolayer and osteogenic medium, whereas IVD cells were kept in 3D alginate bead culture and separated by high density pore culture inserts (0.4 µm pore size). We quantified relative gene expression at bone-relevant genes, alkaline phosphatase (ALP) activity and Alizarin red (ALZR) staining after 21 days. Furthermore, to test the effect of a previously investigated BMP2 analog to block the inhibitors, cells were further stimulated with 100 ng/mL BMP2 and/or L51P. RESULTS:We found significant inhibitory effects of IVD cells onto MSCs undergoing differentiation in presence of NPC, AFC and CEPC as shown in reduced osteogenic gene expression, ALZR staining and ALP activity (N = 11 donors paired on each side). In the case of allogeneic human OBs only a trend towards inhibition could be demonstrated (N = 7 donors on each side). The addition of L51P to the coculture recovered ossification. On the side of the IVD cells BMP2 and/or L51P had a strong chondrogenic effect. DISCUSSION & CONCLUSIONS:Our data suggested evidence for inhibition for MSCs. However, OBs did not show the same inhibitory effects but showed a trend in presence of IVD’s secretome. This warrants for animal models where the donor variance can be better controlled. ACKNOWLEDGEMENTS:This work was supported by a start-up grant from the Center for Applied Biotechnology and Molecular Medicine (CABMM). Further funds were received from the Swiss Society of Orthopaedics (SGOT), the clinical trials unit (CTU) of Bern University Hospital, and by a Eurospine Task Force Research grant #2019_22