Effects of reloading after simulated microgravity on proteoglycan metabolism in the nucleus pulposus and anulus fibrosus of the lumbar intervertebral disc

Study Design. An experiment to measure proteoglycan (PG) content and PG-related gene mRNA expressions in the lumbar intervertebral disc (IVD) of rats tail-suspended (TS) for up to 6 weeks with subsequent reloading. Objective. To assess the effects of reloading after simulated microgravity on PG metabolism in nucleus pulposus (NP) and anulus fibrosus (AF). Summary of Background Data. Although the PG content of rat lumbar IVD is reportedly decreased by low compressive force (due to so-called microgravity) during spaceflight, it is unknown whether it recovers completely on reloading and whether these effects differ between NP and AF. Methods. Eighty-five F344/N rats were divided as follows: caged control (C) or TS for either 3 or 6 weeks, with some TS rats reloaded for 1 or 2 days or 3 weeks after 3 weeks' suspension (TS+RL-1d, - 2d, or - 3w). The glycosaminoglycan content and mRNA levels for aggrecan, TIMP1, MMP3, and ADAMTS4 were measured in NP and AF. Results. The glycosaminoglycan contents of NP and AF were significantly decreased (by 27%-42%) in the TS groups, whereas in the TS+RL-3w group recovery was complete in NP, but incomplete in AF, without histologic degenerative changes at any time point. In NP, the aggrecan mRNA level was significantly downregulated in TS-3w, but recovered to control level on reloading (TS+RL-3w). In AF, the MMP3 mRNA level was significantly elevated in TS-6w. In the early (1-2 days) response of PG-related gene expressions to reloading, mRNA levels were significantly increased for aggrecan, TIMP1, and ADAMTS4 in NP and for MMP3 in AF, but significantly decreased for ADAMTS4 in AF (vs. the TS-3w group). Conclusion. Our results suggest that in IVD maintenance against the present type of mechanical stress, modulation of PG plays an important role and may be associated with molecular changes in PG-related genes.