PROJECT SUMMARY/ABSTRACT Diabetic cardiomyopathy and heart failure are a leading cause of death in diabetic patients. However, effective approaches to preventing and managing this deadly disease are still lacking. The long-term goal of my research is to identify cellular and molecular mechanisms that mediate diabetic cardiac injury. Lysosomes play important roles in cytoplasmic quality control and cellular homeostasis. Recent studies have demonstrated an association between diabetic cardiac injury and a disturbed lysosome pathway. A prominent feature of lysosomal dysfunction is increased lysosomal membrane permeabilization (LMP) which triggers protease leakage and cell death. Our preliminary results showed that cardiac damage in diabetic mice was accompanied by elevated expression of cathepsin D (CTSD), a major lysosomal protease. High glucose induced LMP in cultured cardiomyocytes, leading to altered expression and distribution of CTSD. Importantly, CTSD overexpression exacerbated high glucose-induced cardiomyocyte death, while knocking down CTSD or inhibiting CTSD activity attenuated high glucose toxicity. Our hypothesis is that the increased LMP and the ensuing CTSD leakage and aberrant accumulation mediate diabetic cardiac injury; thus enhancing lysosomal quality control and minimizing the ectopic effects of CTSD will protect the diabetic heart. We will pursue two specific aims to test this hypothesis. Aim 1 will determine whether LMP and the ensuing CTSD leakage contributes to hyperglycemia-induced cardiomyocyte death. Aim 2 will investigate the pathological significance of LMP in diabetic heart injury using mouse models of diabetes. Pharmacological and genetic approaches will be used to enhance the lysosomal repair and to manipulate the expression and maturation of CTSD. Diabetes-induced lysosomal injury will be assessed with a novel LMP reporter mouse line. The effects of altered CTSD on hyperglycemic cardiotoxicity and diabetic cardiomyopathy will be determined with multiple approaches. Successful completion of this project will provide novel insight into the mechanisms that mediate diabetic cardiac injury, facilitating drug design for preventing or treating cardiomyopathy and heart failure in diabetes.