circHIPK3 Sustains Ribosome Biogenesis and Protects Against Cardiac Aging by Stabilizing Ncl mRNA and Supporting its LLPS

To investigate the molecular mechanism of cardiac aging and identify novel regulatory targets, we focused on circular RNA HIPK3 (circHIPK3) and its role in regulating Nucleolin (Ncl)-mediated biological processes. circHIPK3 expression was progressively downregulated with age in mouse hearts. We generated cardiomyocyte-specific circHIPK3 knockout (CKO) mice, and found that these mice exhibited cardiac aging phenotypes, cardiac dysfunction, hypertrophy, and fibrosis. Mechanistically, circHIPK3 is directly bound to the 5' UTR of Ncl mRNA to enhance its stability, thereby preserving Ncl protein levels and supporting Ncl-mediated liquid-liquid phase separation (LLPS), a critical process for efficient ribosome biogenesis. circHIPK3 deficiency is accompanied by altered Ncl LLPS dynamics, reduced ribosomal RNA synthesis and ribosomal protein expression, and impaired de novo ribosome production. Conversely, circHIPK3 overexpression restored Ncl expression, rescued LLPS and ribosome biogenesis defects, and alleviated aging-related cardiac phenotypes in vitro and in vivo. In conclusion, we identify a novel "circHIPK3-Ncl-ribosome biogenesis" axis that protects against cardiac aging through post-transcriptional and biophysical regulation. circHIPK3 represents a potential biomarker and warrants further investigation as a therapeutic candidate for age-related cardiovascular dysfunction, providing new insights into the intersection of non-coding RNA function, ribosome biogenesis and organ aging.