Myeloid cell leukemia-1 (Mcl-1) is a structurally and functionally unique anti-apoptotic Bcl-2 protein. While elevated levels of Mcl-1 contribute to tumor cell survival and drug resistance, loss of Mcl-1 in cardiac myocytes leads to rapid mitochondrial dysfunction and heart failure development. Although Mcl-1 is an anti-apoptotic protein, previous studies indicate that its functions extend beyond regulating apoptosis. Mcl-1 is localized to both the mitochondrial outer membrane and matrix. Here, we have identified that Mcl-1 in the outer mitochondrial membrane mediates mitochondrial fission, which is independent of its anti-apoptotic function. We demonstrate that Mcl-1 interacts with Drp1 to promote mitochondrial fission in response to various challenges known to perturb mitochondria morphology. Induction of fission by Mcl-1 reduces nutrient deprivation-induced cell death and the protection is independent of its BH3 domain. Finally, cardiac-specific overexpression of Mcl-1 OM , but not Mcl-1 Matrix , contributes to a shift in the balance towards fission and leads to reduced exercise capacity, suggesting that a pre-existing fragmented mitochondrial network leads to decreased ability to adapt to an acute increase in workload and energy demand. Overall, these findings highlight the importance of Mcl-1 in maintaining mitochondrial health in cells.

Myeloid cell leukemia-1 (Mcl-1) is a unique antiapoptotic Bcl-2 member that is critical for mitochondrial homeostasis. Recent studies have demonstrated that Mcl-1′s functions extend beyond its traditional role in preventing apoptotic cell death. Specifically, data suggest that Mcl-1 plays a regulatory role in autophagy, an essential degradation pathway involved in recycling and eliminating dysfunctional organelles. Here, we investigated whether Mcl-1 regulates autophagy in the heart. We found that cardiac-specific overexpression of Mcl-1 had little effect on baseline autophagic activity but strongly suppressed starvation-induced autophagy. In contrast, Mcl-1 did not inhibit activation of autophagy during myocardial infarction or mitochondrial depolarization. Instead, overexpression of Mcl-1 increased the clearance of depolarized mitochondria by mitophagy independent of Parkin. The increase in mitophagy was partially mediated via Mcl-1′s LC3-interacting regions and mutation of these sites significantly reduced Mcl-1-mediated mitochondrial clearance. We also found that Mcl-1 interacted with the mitophagy receptor Bnip3 and that the interaction was increased in response to mitochondrial stress. Overall, these findings suggest that Mcl-1 suppresses nonselective autophagy during nutrient limiting conditions, whereas it enhances selective autophagy of dysfunctional mitochondria by functioning as a mitophagy receptor.

Myeloid Cell Leukemia-1 (MCL-1) is an anti-apoptotic BCL-2 family protein that is necessary to maintain cardiac homeostasis in the adult heart. MCL-1 localizes to two distinct mitochondrial locations in myocytes, both on the outer mitochondrial membrane (MCL-1 OM ) and in the mitochondrial matrix (MCL-1 Matrix ). Our lab previously showed that cardiac-specific ablation of MCL-1 at both of these mitochondrial locations in mice led to severe contractile dysfunction and compromised mitochondrial function. Intriguingly, these defects were accompanied by signs of necrotic, rather than apoptotic, cell death. This indicates that MCL-1 has an alternate role in maintaining mitochondrial homeostasis in cardiac myocytes. Unexpectedly, we found that MCL-1 induces mitochondrial clearance in response to treatment with the chemical uncoupler FCCP in a Parkin-independent manner. Hypoxia is also known to induce mitochondrial clearance, and overexpression of MCL-1 further enhances hypoxia-mediated mitophagy. Fluorescence imaging identified MCL-1-positive mitochondria sequestered inside autophagosomes. MCL-1-mediated clearance is abrogated in autophagy deficient Atg5-/- cells, confirming that clearance is occurring via the autophagy pathway. Mutation of MCL-1’s BH3 domain (G198E D199A) does not affect its ability to induce clearance, suggesting that this role may be independent of its anti-apoptotic function. Also, replacing MCL-1’s BH domains with those of BCL-2, does not affect its ability to induce mitophagy. Next, we investigated whether MCL-1 functions as a mitophagy receptor and promotes removal of damaged mitochondria by binding to directly to LC3 through one or more of its three putative LC3-Interacting Region (LIR) motifs. Endogenous MCL-1 and LC3 co-immunoprecipitate in response to stress induced by FCCP. However, mutating each of MCL-1’s individual LIR motifs, as well as generating combined mutations in all three, does not affect MCL-1-mediated mitophagy. Instead, we found that MCL-1 interacts with the known mitophagy receptor BNIP3 both in vitro and in vivo . Thus, our data suggest that MCL-1 promotes elimination of dysfunctional mitochondria by positively regulating the mitophagy receptor BNIP3.