METTL3 Is Essential for Exercise Benefits in Diabetic Cardiomyopathy

Exercise improves functional outcomes in patients with diabetic cardiomyopathy (DiaCM). The molecular mechanism underlying cardiac benefits of exercise in DiaCM remains incompletely understood. N6-methyladenosine (m6A) is the most common form of messenger RNA modification in eukaryotes and has been implicated in cardiac development and disease. However, the role of m6A in DiaCM and in the mitigating effects of exercise on this disease are unclear.

Cardiomyocyte-specific N6-adenosine-methyltransferase-like 3 (METTL3, an m6A methyltransferase) knockout mice and their wild-type littermates were subjected to either chow diet or high-fat diet feeding and injection of streptozotocin to induce DiaCM, followed by an 8-week exercise training and assessment of cardiac function. Some of the mice were injected with adeno-associated viral vector encoding METTL3 to overexpress METTL3 in cardiomyocytes. Cardiac METTL3 expressions were assessed in patients with nonischemic primary dilated cardiomyopathies without or with diabetes. Potential METTL3 downstream effector YBX1 (Y-box binding protein 1) was identified through RNA sequencing. The functional role of YBX1 was examined through adeno-associated viral vector overexpression or knockdown in cardiomyocytes in DiaCM mice.

We showed that cardiac METTL3 protein expression and m6A level were downregulated in patient with dilated cardiomyopathy and further downregulated in patients with dilated cardiomyopathy and diabetes. Consistently, cardiac METTL3 and m6A were downregulated in mouse with DiaCM, whereas they were upregulated by exercise. Cardiomyocyte-specific METTL3 knockout eliminated the cardiac benefits of exercise on DiaCM. Conversely, cardiomyocyte-specific METTL3 overexpression improved systolic and diastolic function in 2 DiaCM mouse models. We demonstrated that exercise enhanced cardiac METTL3 expression in DiaCM through signal transducer and activator of transcription 3. Moreover, METTL3 attenuated DiaCM through m6A-depdendent YBX1 upregulation and the subsequent activation of Nrf2. Cardiomyocyte-specific YBX1 overexpression promoted Nrf2 activation and attenuated oxidative stress, resulting in an improvement in cardiac function in DiaCM. In contrast, cardiomyocyte-specific YBX1 gene knockdown abolished the effect of METTL3 on cardiac improvement in mice with DiaCM. Further, pharmacological activation of METTL3 using a small molecule attenuated cardiac dysfunction in DiaCM.

These studies reveal an essential role of METTL3 in the cardiac benefits of exercise and identify METTL3 and YBX1 as promising therapeutic targets for treating DiaCM.