UCLA

Purpose: Spaceflight induces osteoporosis by mechanical unloading. Interestingly, recent studies report that microgravity also directly affects osteoblasts (OB) and osteoclasts (OC) in vitro, by upregulating OC activity and disrupting the OB cytoskeleton. Non-weight-bearing bones, such as those in the temporomandibular joint (TMJ), provide important insight into cellular-level effects of microgravity in vivo without confounding changes in load. NELL-1 is a novel osteogenic growth factor, which upon PEGylation (NELL-PEG) demonstrates enhanced pharmacokinetics and promising potential as a systemic therapy for osteoporosis in mice. To test NELL-PEG systemic therapy in spaceflight-induced osteoporosis, we have collaborated with CASIS and NASA through the Rodent Research 5 (RR-5) mission. This study is a branch of the RR-5 mission and, for the first time, focuses on the TMJ to investigate how space microgravity and NELL-1-based treatment affect non-load-bearing bones in vivo. Methods: To meet the mission’s technical demands, we conjugated NELL-PEG with bioinert bisphosphonate as a bone-targeting molecule (BP-NELL-PEG, or ‘BP-NP’) to enhance NELL-PEG’s pharmacokinetics and safety. Eight-month-old female BALB/c mice (n=10/group) were randomly assigned to 4 groups: (1) Flight + PBS, (2) Flight + BP-NP, (3) Ground + PBS, and (4) Ground + BP-NP. Flight and Ground groups were housed in the International Space Station and the Kennedy Space Center, FL, respectively, for 9 weeks. Mice received 10mg/kg BP-NP or PBS via intraperitoneal injection every 2 weeks. At 9 weeks, TMJ were analyzed with micro-CT and histology. Results: Micro-CT of subchondral bone in the mandibular condyle showed significantly increased BV/TV in Flight + PBS compared to Ground + PBS. BP-NP treatment showed an even greater increase in BV/TV, in both Ground and Flight groups. There were no significant changes in bone mineral density with either flight or BP-NP treatment. Histological analysis revealed pronounced changes in the condyle cartilaginous zones with BP-NP. Most strikingly, BP-NP treatment showed a 3-fold increase in thickness of the calcified hypertrophic zone in Ground and Flight groups. Conclusion: Spaceflight induces subchondral bone changes in the TMJ, a non-weight-bearing joint, suggesting that microgravity has direct effects on OB and OC and site-specific effects on bone. In addition, BP-NP systemic therapy results in striking chondrogenic and osteogenic changes in the TMJ.