Sirt5 regulates chondrocyte metabolism and osteoarthritis development through protein lysine malonylation
Chondrocyte metabolic dysfunction plays an important role in osteoarthritis (OA) development during aging and obesity. However, it is unknown how metabolic dysfunction occurs during these two conditions. Proteins post-translational modifications (PTMs) have recently emerged as an important regulator of cellular metabolism. We previously reported that one type of PTM, lysine malonylation (MaK) was increased in cartilage during obesity. Sirt5, the only known mammalian demalonylase, declines in cartilage during aging. To elucidate the interplay between Sirt5 deficiency and obesity in joint degeneration, we deploy systemic and cartilage-specific conditional knockout mouse models. Our findings reveal that the combination of Sirt5 deficiency and obesity exacerbates joint degeneration in mice. Through comprehensive proteomics analysis, we delineate the malonylome in chondrocytes, pinpointing MaK's predominant impact on various metabolic pathways such as carbon metabolism and glycolysis. Further biochemical investigation focuses on glyceraldehyde 3-phosphate dehydrogenase (GAPDH), a target of MaK, confirming a reduction in its enzymatic activity and functionality by MaK. Lastly, we identified a rare coding mutation in SIRT5 that dominantly segregates in a family with hand OA. The mutation results in substitution of an evolutionally invariant phenylalanine (Phe–F) to leucine (Leu–L) (F101L), which is located in the catalytic domain. F101L mutant results in higher MaK level and decreased expression of cartilage ECM genes (e.g., Acan and Col2a1) and upregulation of inflammation associated genes (e.g., Il6 and Nos2). In conclusion, we found that Sirt5 mediated MaK is an important regulator of chondrocyte cellular metabolism and dysregulation of Sirt5-MaK could be an important mechanism underlying aging and obesity associated OA development. Overall design: Primary mouse chondrocytes were transfected with either  Wildtype or SIRT5F101L mutant plasmids.