LAMP-2 deficient mice show depressed cardiac contractile function without significant changes in calcium handling

Mutations in the highly glycosylated lysosome associated membrane protein-2 (LAMP-2) cause, as recently shown, familial Danon disease with mental retardation, mild myopathy and fatal cardiomyopathy. Extent and basis of the contractile dysfunction is not completely understood. In LAMP-2 deficient mice, we investigated cardiac function in vivo using Doppler-echocardiography and contractile function in vitro in isolated myocardial trabeculae. LAMP-2 deficient mice displayed reduced ejection fraction (EF) (58.9 +/- 3.4 vs. 80.7 +/- 5.1%, P < 0.05) and reduced cardiac output (8.3 +/- 3.1 vs. 14.7 +/- 3.6 ml/min, P < 0.05) as compared to wild-type controls. Isolated multicellular muscle preparations from LAMP-2 deficient mice confirmed depressed force development (3.2 +/- 0.6 vs. 8.4 +/- 0.9 mN/mm(2), P < 0.01). All groups showed similar force-frequency behaviour when normalised to baseline force. Post-rest potentiation was significantly depressed at intervals > 15 s in LAMP-2 deficient mice (P < 0.05). Although attenuated in absolute force development, the normalised inotropic response to increased calcium and beta-adrenoreceptor stimulation was unaltered. Electron microscopic analysis revealed autophagic vacuoles in LAMP-2 deficient cardiomyocytes. Protein analysis showed unaltered levels of SERCA2a, calsequestrin and phospholamban. Cardiac contractile function in LAMP-2 deficient mice as a model for Danon disease is significantly attenuated. The occurrence of autophagic vacuoles in LAMP-2 deficient myocytes is likely to be causal for the depressed contractile function resulting in an attenuated cardiac pump reserve.