Mitofusin-2 (Mfn-2) is a dynamin-like protein that is involved in the rearrangement of the outer mitochondrial membrane. Research using various experimental systems has shown that Mfn-2 is a mediator of mitochondrial fusion, an evolutionarily conserved process responsible for the surveillance of mitochondrial homeostasis. Here, we find that cardiac myocyte mitochondria lacking Mfn-2 are pleiomorphic and have the propensity to become enlarged. Consistent with an underlying mild mitochondrial dysfunction, Mfn-2-deficient mice display modest cardiac hypertrophy accompanied by slight functional deterioration. The absence of Mfn-2 is associated with a marked delay in mitochondrial permeability transition downstream of Ca 2؉ stimulation or due to local generation of reactive oxygen species (ROS). Consequently, Mfn-2-deficient adult cardiomyocytes are protected from a number of cell death-inducing stimuli and Mfn-2 knockout hearts display better recovery following reperfusion injury. We conclude that in cardiac myocytes, Mfn-2 controls mitochondrial morphogenesis and serves to predispose cells to mitochondrial permeability transition and to trigger cell death.

Brown adipose tissue (BAT) is a highly vascularized organ with abundant mitochondria that produce heat through uncoupled respiration. Obesity is associated with a reduction of BAT function; however, it is unknown how obesity promotes dysfunctional BAT. Here, using a murine model of diet-induced obesity, we determined that obesity causes capillary rarefaction and functional hypoxia in BAT, leading to a BAT "whitening" phenotype that is characterized by mitochondrial dysfunction, lipid droplet accumulation, and decreased expression of Vegfa. Targeted deletion of Vegfa in adipose tissue of nonobese mice resulted in BAT whitening, supporting a role for decreased vascularity in obesity-associated BAT. Conversely, introduction of VEGF-A specifically into BAT of obese mice restored vascularity, ameliorated brown adipocyte dysfunction, and improved insulin sensitivity. The capillary rarefaction in BAT that was brought about by obesity or Vegfa ablation diminished β-adrenergic signaling, increased mitochondrial ROS production, and promoted mitophagy. These data indicate that overnutrition leads to the development of a hypoxic state in BAT, causing it to whiten through mitochondrial dysfunction and loss. Furthermore, these results link obesity-associated BAT whitening to impaired systemic glucose metabolism.

Rationale At birth, there is a switch from placental to pulmonary circulation and the heart commences its aerobic metabolism. In cardiac myocytes, this transition is marked by increased mitochondrial biogenesis and remodeling of the intracellular architecture. The mechanisms governing the formation of new mitochondria and their expansion within myocytes remain largely unknown. Mitofusins (Mfn-1 and Mfn-2) are known regulators of mitochondrial networks but their role during perinatal maturation of the heart has yet to be examined. Objective Determine the significance of mitofusins, during early postnatal cardiac development. Methods and Results We genetically inactivated Mfn-1 and Mfn-2 in mid-gestational and postnatal cardiac myocytes using a loxP/Myh6-cre approach. At birth, cardiac morphology and function of double-knockout (DKO) mice are normal. At that time, DKO mitochondria increase in numbers, appear to be spherical and heterogeneous in size but exhibit normal electron-density. By postnatal day 7, the mitochondrial numbers in DKO myocytes remain abnormally expanded and many lose matrix components and membrane organization. In this context, DKO mice develop dilated cardiomyopathy (DCM). This leads to a rapid decline in survival and all DKO mice perish before 16 days of age. Gene expression analysis of DKO hearts shows that mitochondria biogenesis genes are down regulated, the mitochondrial DNA is reduced and so are mitochondrially-encoded transcripts and proteins. Furthermore, mitochondrial turnover pathways are dysregulated. Conclusions Our findings establish that Mfn-1 and Mfn-2 are essential in mediating mitochondrial remodeling during postnatal cardiac development, a time of dramatic transitions in the bioenergetics and growth of the heart.

deletion of mitofusin-1 leads to mitochondrial fragmentation and improves tolerance to ROS-induced mitochondrial dysfunction and cell death. Am J Physiol Heart Circ Physiol 302: H167-H179, 2012. First published October 28, 2011 doi:10.1152/ajpheart.00833.2011.-Molecular studies examining the impact of mitochondrial morphology on the mammalian heart have previously focused on dynamin related protein-1 (Drp-1) and mitofusin-2 (Mfn-2), while the role of the other mitofusin isoform, Mfn-1, has remained largely unexplored. In the present study, we report the generation and initial characterization of cardiomyocyte-specific Mfn-1 knockout (Mfn-1 KO) mice. Using electron microscopic analysis, we detect a greater prevalence of small, spherical mitochondria in Mfn-1 KO hearts, indicating that the absence of Mfn-1 causes a profound shift in the mitochondrial fusion/ fission balance. Nevertheless, Mfn-1 KO mice exhibit normal leftventricular function, and isolated Mfn-1 KO heart mitochondria display a normal respiratory repertoire. Mfn-1 KO myocytes are protected from mitochondrial depolarization and exhibit improved viability when challenged with reactive oxygen species (ROS) in the form of hydrogen peroxide (H2O2). Furthermore, in vitro studies detect a blunted response of KO mitochondria to undergo peroxideinduced mitochondrial permeability transition pore opening. These data suggest that Mfn-1 deletion confers protection against ROSinduced mitochondrial dysfunction. Collectively, we suggest that mitochondrial fragmentation in myocytes is not sufficient to induce heart dysfunction or trigger cardiomyocyte death. Additionally, our data suggest that endogenous levels of Mfn-1 can attenuate myocyte viability in the face of an imminent ROS overload, an effect that could be associated with the ability of Mfn-1 to remodel the outer mitochondrial membrane.