Rapamycin Ameliorates Age‐Associated Intervertebral Disc Degeneration in Male Marmosets

Abstract

Introduction Age-associated intervertebral disc degeneration (IDD) is a major contributor to low back pain and physical disability in the US. Rapamycin has been shown in several animal models, including mammalian, to increase lifespan and decrease aging and age-associated pathologies, including cancer, cardiac and neurodegenerative diseases. Rapamycin is an FDA approved drug that inhibits mammalian target of rapamycin (mTOR) complex 1, which is a master regulator of cell growth and proliferation signals. It has not yet been determined if rapamycin treatment influences age-associated IDD. Hence, the goal of this study was to determine the effect of a one-year oral rapamycin treatment on IDD progression in old marmosets. Methods Old male marmosets were given rapamycin using a delivery system that microencapsulates rapamycin in a eudagrit coating and is mixed into yogurt for feeding via a syringe to accurately account for each daily dose. As control, old animals were given empty eudagrit capsules delivered the same way, for one year. Spine specimens were collected from young (<5 years old), old (> 10 years old) + vehicle, and old + rapamycin marmosets. Treatment was given for one year prior to sacrifice. Cellularity and histological structure were assessed using H&E staining. Disc matrix anabolism was assessed by aggrecan immunohistochemistry (IHC) and DMMB assay to quantify sulfated glycosaminoglycans. Disc matrix catabolism was assessed via WB for aggrecan fragments. Autophagy was measured via WB for established autophagy protein markers. Student's t-test was used for significance, n=3-4. Results Rapamycin treatment significantly decreased histological scoring for IDD characteristics compared to control old marmosets (Figure 1a-c.) Rapamycin treatment significantly increased older marmosets GAG levels and increased aggrecan IHC staining compared to control (Figure 2a, b). Similarly, rapamycin treatment decreased ADAMTS-mediated and MMP-mediated aggrecan fragmentation (Figure 2c, d). Rapamycin treatment did not significantly affect the autophagy markers LC3-II, Atg7, beclin-1, or Atg12-Atg5, but it did significantly decrease p62 expression. Discussion Overall, rapamycin treatment significantly improved disc catabolism, increasing aggrecan production, and histological score in old marmosets' discs. These results suggest that one-year daily rapamycin treatment delayed IDD progression in older male marmosets. Interestingly, autophagy related proteins were not significantly increased in older male marmosets treated with rapamycin, although rapamycin treatment has been shown to increase autophagy in other tissues, as it is a known inhibitor of mTOR. This implies either rapamycin given orally cannot diffuse into the disc, and systemic effects of rapamycin were observed in other assays, or autophagy is not regulated by rapamycin in the disc. This unique finding warrants further study. Significance This is the first study to investigate the in vivo effects of rapamycin treatment on age-associated IDD in a nonhuman primate model and suggests rapamycin might be a viable treatment to improve IDD characteristics in patients.