Bmal1 mediates nucleolin phase separation and prevents sepsis-induced myocardial dysfunction

Background Sepsis-induced myocardial dysfunction (SIMD) is a life-threatening complication of sepsis with high mortality, however, its underlying molecular mechanisms remain poorly understood. Brain and muscle ARNT-like protein 1 (Bmal1), a core circadian regulator, plays a well-established role in cardiovascular physiology, yet its function in SIMD has not been fully elucidated. Nucleolin (Ncl), a key nucleolar protein critical for ribosome biogenesis, exhibits liquid–liquid phase separation (LLPS) and may mediate cardiomyocyte stress responses. Methods SIMD models were established in mice by cecal ligation and puncture (CLP) and in H9C2 cardiomyocytes using lipopolysaccharide (LPS). Cardiac function was assessed via echocardiography. Molecular interactions were investigated using co-immunoprecipitation (Co-IP), molecular docking, and fluorescence recovery after photobleaching (FRAP). Ribosome biogenesis and nucleolar function were evaluated through AgNOR staining, sucrose gradient centrifugation, and Ribo-Halo assays. Results Bmal1 expression was significantly downregulated in both in vivo and in vitro SIMD models. Bmal1 deficiency exacerbated cardiac dysfunction, amplified inflammatory responses, and disrupted ribosome biogenesis. We identified a direct interaction between Bmal1 and Ncl and demonstrated that Bmal1 regulates Ncl expression and dynamics. Bmal1 silencing impaired Ncl’s LLPS, rRNA synthesis, and ribosome assembly. Furthermore, LPS-induced SIMD disrupted Ncl’s LLPS, while Bmal1 overexpression restored ribosome biogenesis. Conclusions Bmal1 deficiency aggravates SIMD by impairing Ncl’s LLPS and ribosomal biogenesis. Our findings reveal a novel Bmal1–Ncl’s LLPS axis that regulates ribosome biogenesis under septic stress, highlighting its potential as a therapeutic target for SIMD.