ND15, subunit of the electron transport chain protein-complex I, age-dependently worsens cardiac performance in Drosophila melanogaster

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Abstract Introduction In the natural aging process of the heart, aside others, mitochondrial dysfunction is an essential driver for the development of reduced cardiac performance. Previous analysis of protein expression levels in cardiomyocytes of aged mice and have found aberrancies in subunits of the mitochondrial Complexes I – IV of the electron transport chain. Especially, the complex I subunit ND15 was highly downregulated. To elucidate the functional effects of this gene on the cardiac performance, we generated cardiomyocyte specific ND15 knock-downs in Drosophila melanogaster. Methods We generated Drosophila with heart-specific (+/+; Hand4.2-GAL4/UAS-ND15; tdtK/+) (ND15-KD)knockdown of ND15 using the UAS/GAL4 system. The flies' cardiomyocytes endogenously express tdTomato, facilitating fluorescence-based live imaging of the heart by high-resolution video fluorescence microscopy. The recorded data was processed using a R-Script, resulting in Results We compared wild type (WT) flies to ND15-KD flies at age 3 weeks(young) and 7 weeks(old). Knockdown of ND15 resulted in impaired cardiac function. Most parameters deteriorated in aged flies in comparison to young flies. The heart frequency was significantly reduced (WT:6 beats/s – KD:4 beats/s – p<0.0001) in the 7 weeks old group, end-diastolic diameter of the heart tube increased significantly in 3 weeks old ND15KD flies compared to controls(WT:65µm – KD:74µm p<0,0001), resulting in a dilated heart tube Interestingly enough, this dilation in ND15-KD disappeared in 7 weeks old flies. The fractional shortening was significantly decreased in young(WT:33,5% - KD:30% p<0,01) and deteriorated in old flies (WT:35% - KD:22% p<0,0001), indicating a reduced contraction capability. For both 3 weeks and 7 weeks old groups, the heart-rate adjusted interbeat interval variance (WT:22% -KD: 40% p<0,0001) as well as the time of no cardiac action (WT: 22% - KD: 35% p<0,0001) increased and showed significant irregularities. The duration of systole and diastole combined also exhibited arrhythmic patterns (WT:15% - KD:20.5% p<0,0001). A general arrhythmia index (AI) increased significantly (WT: 0.25 – KD:0.5 p<0,0001) These arrhythmia parameters emphasize the arrhythmogenic potential of the ND15-knockdown. The relative relaxation capabilities drastically worsened with age, displaying a diastolic dysfunction in the 7 weeks old group. At half-time relaxation, the WT was at 60% relaxation, ND15KD at 46%). Conclusion This study demonstrates that heart-specific deletion of ND15 results in a marked impairment of systolic and diastolic cardiac function in Drosophila accompanied by pro-arrhythmogenic actions. We could identify ND15 as a new mitochondrial molecular target for the maintenance of regular cardiac function and rhythmicity, especially with increasing age. Further mechanistic studies and translation to mammalian organisms are ongoing.