Mitochondrial fusion regulates the shape, distribution and function of the organelle and plays critical roles in protection from cell death and therefore offers a valid target for theurapeutic approaches to protect the heart from progression of disease or myocardial infarction events. Mitochondrial fusion is mediated by members of the dynamin related protein (DRP) family, large GTPases that, through their ability to self-assemble and hydrolyze GTP, control membrane remodeling events. In mammals, MFN1 and MFN2 function in place of a single outer membrane DRP in simple organisms, both are expressed ubiquitously but the relative expression level of each varies in a tissue dependent manner. Although MFN1 and MFN2 both function in mitochondrial fusion, they are not completely redundant, although the nature of the functional differences are not known. Our data indicate that the MFN1-MFN2 heterotypic trans complex is significantly more efficacious for fusion than either homotypic complex. From this, we predict that MFN1 and MFN2 have distinct molecular properties and that the functional significance of the two outer membrane DRPs in mammals is to provide a relatively simple regulatory fusion mechanism via their respective and relative expression levels. Further, data suggest that specific regulatory mechanisms exist, as we have shown that soluble Bax stimulates only MFN2 homotypic complexes. In order to understand the functional significance of two outer membrane fusion proteins and the regulatory mechanisms that govern their activity, we propose to characterize the biochemical properties of each DRP including GTP binding, GTP hydrolysis and complex assembly. These biochemical approaches together will determine the mechanistic basis for why MFN1 only, MFN2 only and MFN1/MFN2 mediated fusion efficiencies are distinct and will likely point to why and how they are distinctly regulated in cells. To explore what properties of MFN2 are modulated by the pro-apoptotic Bcl2 protein Bax, we will test the effect of Bax on the biochemical properties of MFN2, of particular significance in the heart where MFN2 is the predominantly expressed fusion DRP. We will further characterize the regulation of fusion by Bax through identification of interaction domains and exploring their functional significance in vitro and in cells. Bax also negatively effects mitochondrial fusion following activation by apoptotic signals and we will investigate whether the inhibition of fusion is direct or mediated through outer membrane permeabilization. As observed with the apoptotic pathway and mitochondrial dynamics, an interdependence also exists between mitochondrial fusion and mitochondrial motility, but the significance of this is relatively unexplored. We propose that mitochondrial tethering to microtubules and short range motility are both important regulators of mitochondrial fusion. We will determine the role that mitochondrial movement and tethering play in regulating mitochondrial fusion and investigate the molecular basis for the requirement of MFNs in these processes using an in vitro mitochondrial fusion-motility assay and complimentary biochemical approaches.
The experiments proposed in this grant will shed light on the fundamental mechanism of mitochondrial fusion, providing a foundation to further understand the physiological roles this process plays in cardiac function, under all circumstances including the healthy heart, in chronic heart disease and during myocardial infarction events. By probing the mechanistic link between mitochondrial fusion and apoptosis and the interdependence of mitochondrial fusion and mitochondrial tethering and motility, we will gain insight into the regulatory mechanisms that exist in cells. These approaches will contribute to the identification of novel targets for drug development and therapeutics for heart disease.
