To Örebro University

Brown bears conserve muscle and bone mass during six months of inactive hibernation. The molecular mechanisms underlying hibernation physiology may have translational relevance for human therapeutics. We hypothesize that protective mechanisms involve increased tissue availability of the insulin-like growth factors (IGFs). In subadult Scandinavian Brown Bears, we observed that mean plasma IGF-1 and IGF-2 during hibernation was reduced to 36±10% and 56±15%, respectively, compared to the active state (N=12). Western ligand blotting identified IGFBP-3 as the major IGF binding protein in the active state, while IGFBP-2 was co-dominant during hibernation. Acid labile subunit (ALS) levels in hibernation were 41±16% those of the active state (N=6). Analysis of available grizzly bear RNA sequencing data revealed unaltered liver mRNA IGF-1, IGFBP-2, and IGFBP-3 levels, whereas ALS was significantly reduced during hibernation (N=6). Reduced ALS synthesis and circulating levels during hibernation should prompt a shift from ternary IGF/IGFBP/ALS to smaller binary IGF/IGFBP complexes, thereby increasing IGF tissue availability. Indeed, Size Exclusion Chromatography of bear plasma, demonstrate a shift to lower molecular weight IGF-containing complexes in the hibernating versus the active state. Further, we note that the major IGF-2 mRNA isoform expressed in liver in both Scandinavian brown bears and grizzly bears was an alternative splice variant in which Ser29 was replaced with a tetrapeptide possessing a positively charged Arg residue. Homology modelling of the bear IGF-2/IGFBP-2 complex showed the tetrapeptide in proximity to the heparin binding domain involved in bone-specific targeting of this complex. In conclusion, this study provides data which suggest that increased IGF tissue availability combined with tissue-specific targeting contribute to tissue preservation in hibernating bears.