Abstract
Sarcoplasmic Reticulum (SR) drives cardiomyocyte excitation-contraction coupling through the release and storage of Ca2+. Phasic fluctuations of Ca2+ require ATP. Mitochondria occupy ∼30% of the volume of cardiomyocytes and provide cardiomyocytes with ATP. Perturbations of mitochondria therefore have a detrimental effect on the heart. Mitochondrial homeostasis in mammals is regulated in part through fusion and fission by outer mitochondrial membrane proteins mitofusin 1 and 2 (Mfn1 and Mfn2) and in Drosophila by the homolog dMfn. Mutations in Mfn2 show impaired contractile function in humans (hMfn), mice (mMfn) and RNAi suppression in Drosophila (dMfn). Additionally Mfn2, independently of Mfn1, tethers the SR to mitochondria allowing for efficient transfer of Ca2+ between the organelles. It is currently not known whether mitochondria-SR tethering is required for proper Ca2+ cycling in cardiomyocytes. We monitored calcium transients in heart tubes of live flies, using the genetically encoded calcium indicator (GECI) GCaMP3.0 to study dynamic Ca2+ oscillations. Intensity signals from GCaMP3.0 in fly heart tubes are comparable to isolated cardiomyocytes in their rate cycling amplitude, and their response to L-type calcium channel antagonist, nifedipine. Additionally, non-calcium responsive GFP does not produce a signal that mimics the Ca2+ oscillations observed with GCaMP3.0. Comparisons between Wt Drosophila hearts and cardiomyopathic RNAi dMfn hearts expressing GCaMP3.0 have normal amplitude and time to decay constant Ca2+ signals. This study shows that suppression of dMfn with RNAi in the fly heart tube leads to cardiomyopathy. Monitoring intercellular Ca2+ levels with the (GECI) GCaMP3.0 demonstrates that Ca2+ signaling is not affected when mitochondria-SR tethering is disrupted.
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