Safeguarding nucleolar homeostasis by CBX4 alleviates senescence and osteoarthritis

Chromobox homolog 4 (CBX4), a component of polycomb repressive complexes 1 (PRC1), plays important roles in the maintenance of cell identity and organism development through epigenetic silencing. However, it remains unclear whether CBX4 regulates the homeostasis of human stem cells. Here, we demonstrate that CBX4 counteracts human mesenchymal stem cell (hMSC) aging via the maintenance of nucleolar homeostasis. CBX4 protein decreases in aged hMSCs, and targeted knockout of CBX4 in young hMSCs destabilizes nucleolar heterochromatin, increases ribosome biogenesis, and accelerates cellular senescence. CBX4 maintains nucleolar homeostasis by recruiting fibrillarin at nucleolar rDNA repeats, limiting excessive expression of rRNAs. Importantly, overexpression of CBX4 expression alleviates physiological hMSC aging as well as attenuates the development of post-traumatic osteoarthritis in mice. Taken together, our study uncovers a novel role of CBX4 in counteracting senescence by maintaining nucleolar homeostasis, providing a potential therapeutic target for aging-associated disorders. RNA-Seq and ChIP-Seq