Impaired active myocardial relaxation is the earliest sign of anthracycline cardiotoxicity and reflects selective injury of the cardiomyocyte surface

Aims Anthracycline-induced cardiotoxicity is a leading cause of heart failure (HF) in cancer survivors and is usually detected only once systolic dysfunction is established, when myocardial injury may already be advanced. Diastolic dysfunction may occur earlier, but its onset and cellular basis are unknown. We investigated the temporal emergence of diastolic dysfunction during doxorubicin (DOX) exposure and its link to the selective vulnerability of crest/subsarcolemmal mitochondria (SSM) at the cardiomyocyte (CM) surface. Methods and results Using a clinically relevant chronic DOX protocol in adult mice, we performed longitudinal cardiac assessment from the first exposure, combining conventional echocardiography, tissue Doppler, and global longitudinal strain (GLS). Structural and molecular correlates were characterized by atomic force microscopy, transmission electron microscopy, and 3D-quantitative imaging of native cardiac tissue, spatially resolving the CM surface (SSM) from the interior (interfibrillar mitochondria, IFM) compartment. DOX impaired active myocardial relaxation from the first dose with prolonged isovolumic relaxation time and reduced mitral annular e′, whereas passive filling (transmitral filling, left atrial size) was preserved. Ejection fraction remained normal, with an early reduction in GLS. This early relaxation defect coincided with selective remodeling of the surface crest/SSM architecture, evident as crest flattening and progressive SSM loss and with depletion of the crest/SSM determinants Ephrin-B1 and Claudin-5, whereas IFM were affected only later, upon cumulative exposure. Surface SSM injury was accompanied by early, spatially ordered PINK1/Parkin activation, engaging SSM before IFM, and by loss of the sarcolemmal Na⁺/Ca²⁺ exchanger NCX1, likely impairing diastolic Ca 2+ removal. In CM-specific Ephrin-B1-deficient mice, which lack mature crest/SSM, DOX precipitated an accelerated transition to HF with reduced ejection fraction. Conclusions Impaired active relaxation is the earliest functional hallmark of anthracycline cardiotoxicity, underpinned by selective injury of the surface crest/SSM compartment and its Na⁺/Ca²⁺ exchanger NCX1, while IFM and passive filling are initially spared. Diastolic assessment from the first anthracycline cycles may improve early detection and prevention of HF progression.