Modulation of Mitofusin Activity to Treat Heart Disease

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Mitochondrial “connectivity” and mitochondrial-endoplasmic/sarcoplasmic reticulum (ER/SR) “proximity” each potentiate mitochondrial-mediated metabolism and necrosis through a variety of mechanisms. Mitofusins (MFN) 1 and 2 are large GTPases that play critical roles in mitochondrial connectivity and mitochondrial-ER/SR proximity. MFN1 and MFN2 reside in the outer mitochondrial membrane where they mediate mitochondrial fusion. MFN2, but not MFN1, also resides in the ER/SR membrane, where it tethers ER/SR to mitochondria through interactions with mitochondrial-localized MFN1 or MFN2. Deletion of MFN1 or MFN2 reduces myocardial infarct (MI) size during ischemia/reperfusion (I/R). Conversely, MFN1 and MFN2 overexpression augment metabolism. Given this information, therapeutic inhibition of MFNs would be expected to reduce infarct size during MI, while therapeutic activation of MFNs might attenuate heart failure (HF) by augmenting metabolism. The challenge has been to find a means to manipulate the activities of endogenous MFNs. In collaboration with others, we created the first peptides and small molecules that modulate conformations of MFN1 and MFN2 and delineated the underlying structural basis for these effects. We reported previously that MFN activators increase, while MFN inhibitors decrease, mitochondrial fusion. These are direct effects that require binding of these agents to either MFN1 or MFN2. We present here new data showing that MFN activators increase, while MFN inhibitors decrease, mitochondrial-ER/SR proximity and Ca2+ transfer to mitochondria. Moreover, we observed that MFN activators exacerbate infarct size during myocardial I/R, while MFN inhibitors reduce infarct size in both heart and brain I/R models. Interestingly, these effects of the activators are dependent on MFN2, but not MFN1, suggesting the importance of mitochondrial-ER/SR proximity but not excluding the possibility that MFN2- dependent changes in mitochondrial connectivity and shape also contribute. Additionally, MFN activators promote cardiomyocyte metabolism. The goals of this project are to understand the mechanisms by which MFN modulators impact cardiomyocyte death and metabolism and to test whether these agents might provide novel therapeutic strategies for MI and HF. We propose: 1. To correlate changes in MFN activation/inhibition with mitochondrial connectivity, mitochondrial-ER/SR proximity, Ca2+ transfer, cell death, and metabolism in adult cardiomyocytes. 2. To delineate the individual contributions of mitochondrial connectivity and mitochondrial- ER/SR proximity to cell death and metabolism in cardiomyocytes in vivo. 3. To assess whether mitofusin modulators provide novel therapeutic strategies for MI and HF. This project breaks new ground in defining the mechanisms by which MFN modulators impact cardiomyocyte death and metabolism and whether MFNs provide an actionable target for novel therapies directed against MI and HF.