Modeling nucleus pulposus regeneration in vitro: mesenchymal stem cells, alginate beads, hypoxia, bone morphogenetic protein-2, and synthetic peptide B2A.

STUDY DESIGN: Using a low cell density, hypoxic, alginate-bead culture system, the effects of bone morphogenetic protein-2 (BMP-2) and synthetic peptide B2A on cell proliferation and extracellular matrix (ECM) synthesis were assessed at days 0, 3, 5, and 7, using nucleus pulposus (NP)-like differentiated mesenchymal stem cells (MSCs).

OBJECTIVE: This is a preliminary investigation into B2A's potential adjunctive role with MSCs and BMP-2, in NP regeneration.

SUMMARY OF BACKGROUND DATA: B2A analogs, alone and in combination with BMP-2, have been shown to promote proliferation and ECM production in chondrocytes and MSCs. Articular chondrocytes and NP cells often respond in a similar manner to growth factor treatments, thus suggesting a potential role for B2A in treating disc degeneration by NP regeneration.

METHODS: Using the NP regeneration in vitro model (low cell density, hypoxic, alginate bead culture), B2A and BMP-2 were evaluated alone and in combination, to determine effects on proliferation and ECM synthesis in the presence of transforming growth factor-beta 3 on NP-like differentiated MSCs.

RESULTS: B2A administration induced mild proliferation of NP-like differentiated MSCs and diminished an initial wave of low-dose BMP-2-prompted apoptosis. Individually and in combination, B2A and BMP-2 were found to inhibit transforming growth factor-beta 3-permitted collagen accumulation; levels remained similar in their presence. Both collagen I (Col I) and collagen II (Col II) were found in almost all specimens, but increased B2A levels favored Col II unlike BMP-2, which favored Col I. BMP-2 resulted in a minor reduction in aggrecan synthesis, which was unchanged by B2A.

CONCLUSION: Using this in vitro model, B2A induced proliferation, continuous aggrecan synthesis, and stabilized collagen accumulation favoring Col II. These characteristics are consistent with cells of the young, healthy NP, indicating potential use of the peptide early in an MSC-based NP-regeneration therapy; whereas, BMP-2 induced apoptosis, Col I accumulation, and aggrecan production hindrance, and was found untherapeutic.