DRP1-Mediated Mitophagy: Safeguarding Obese Hearts from Cardiomyopathy

Obesity and metabolic syndrome are commonly associated with an increased risk of cardiovascular disease. Obesity-induced cardiomyopathy is characterized by morphological, functional, and metabolic abnormalities in hearts of obese individuals, independent of other cardiovascular risk factors. Mitochondrial dysfunction is frequently associated with obesity-induced cardiomyopathy and is a major contributor to its pathogenesis¹.

Mitochondria are highly dynamic organelles that provide over 90% of the ATP required to sustain the metabolic needs of the heart. However, excess lipid uptake by cells as occurs with a high fat diet (HFD) is associated with mitochondrial impairment¹. When mitochondria become dysfunctional, they can generate excessive levels of reactive oxygen species and activate cell death². Accumulation of dysfunctional mitochondria poses a significant concern in terminally differentiated cardiomyocytes as they lack regenerative potential. Hence, an efficient mitochondrial quality control system is crucial for repairing or degrading damaged mitochondria to ensure maintenance of healthy mitochondria in the heart. While mitochondrial fusion allows mitochondria to maintain membrane potential by diluting damaged components², mitochondrial fission facilitates asymmetric segregation of unrecoverable mitochondria for clearance^2,3. The most well characterized pathway involved in mitochondria degradation is autophagy, an evolutionarily conserved process by which mitochondria are delivered to lysosomes in double-membraned autophagic vesicles². Mitochondrial autophagy (mitophagy) involves both labeling of the mitochondrion for degradation and de novo biogenesis of the autophagosome membrane, which is dependent on the autophagy-related 7 (Atg) conjugation system and LC3^2,4. In addition to canonical mitophagy, various non-canonical pathways exists that can also facilitate selective elimination of mitochondria⁴, such as Ulk1 (unc-51 like kinase 1)/Rab9 (Ras-related protein 9A)-dependent alternative autophagy (also called alternative mitophagy), microautophagy and Rab5-dependent endosomal degradation pathway⁴.

Multiple mitochondrial clearance pathways are often coordinately activated in disease or chronic conditions, such as obesity². Studies conducted by the Sadoshima laboratory have previously uncovered a time-dependent induction of mitophagy in HFD-induced obesity cardiomyopathy^5,6. During the early phase of HFD consumption, Atg7-dependent canonical mitophagy is activated as early as 3 weeks after the start of the HFD, reaching its peak at 6 weeks of feeding, and lasts for at least 2 months⁵. Deletion of Atg7 in cardiomyocytes attenuates HFD-induced mitophagy and worsens cardiomyopathy, highlighting the importance of Atg7-dependent mitophagy as a defense mechanism during the acute phase of HFD consumption⁵. Interestingly, activation of Atg7-dependent autophagy declines at 20 weeks of HFD consumption. During this chronic phase of HFD consumption, the Ulk1/Rab9-dependent alternative mitophagy pathway is activated and assumes the responsibility of clearing dysfunctional mitochondria⁶. Suppression of alternative mitophagy exacerbates the development of mitochondrial dysfunction and cardiomyopathy⁶. While canonical and alternative mitophagy take turns in clearing dysfunctional mitochondria in response to HFD consumption, it is of interest to investigate whether these two distinct mitophagy pathways share common regulatory components in their activation.

In this issue of Circulation Research, Tong and colleagues report on the role of Dynamin-related protein 1 (DRP1) in mediating both canonical and alternative mitophagy to protect hearts during HFD consumption (Figure)⁷. DRP1, a small GTPase responsible for mitochondrial fission, plays a role in baseline and stress-induced mitophagy in cardiomyocytes (CMs)^8–10. However, its role in HFD-induced mitophagy remains unclear. Here, Tong et. al challenged the tamoxifen-inducible cardiomyocyte-specific conditional Drp1 knockout mice (CKO) with HFD. Intriguingly, the authors discovered that Drp1 CKO mice are extremely sensitive to HFD and display lethality within 2–4 weeks after starting on a HFD. The formation of autophagosomes and mitophagy in response to HFD consumption is completely abrogated in hearts of Drp1 CKO mice. Consequently, severe mitochondrial dysfunction and heart failure are observed in Drp1 CKO mice. These results demonstrate that DRP1 is required for canonical mitophagy, which is essential for hearts to adapt to acute HFD-induced metabolic stress. The authors also examined mitophagy in cardiomyocyte specific Drp1 heterozygous (Drp1^+/−) mice and observed that HFD consumption leads to impaired mitophagy, along with mitochondrial dysfunction and decreased cardiac function. To determine the role of DRP1 in regulating mitophagy during the chronic phase of HFD, independent of its effects during the acute phase, the authors performed tamoxifen-induced deletion of Drp1 at 16 weeks after the start of HFD. Similar to the observations made with Drp1 deletion prior to HFD consumption, mitophagy is completely abolished and cardiac dysfunction is exacerbated in DRP1 CKO mice at 20 weeks of HFD consumption. These findings suggest that DRP1-mediated mitophagy is also critical in protecting the heart against excess lipids during the chronic phase of HFD consumption.

Figure. Role of DRP1 in obesity cardiomyopathy.

During the acute phase of high fat diet (HFD) consumption, DRP1 stimulates autophagosome formation and mitophagy by disrupting the Beclin 1/Bcl-2/Bcl-xL complex, promoting the release of Beclin 1. Deletion of DRP1 and abrogation of mitophagy prior to HFD causes rapid lethality. During the chronic phase, DRP1 undergoes phosphorylation at Ser616 and translocates to the mitochondria-associated endoplasmic reticulum membranes (MAMs), where it forms a complex with Rab9, MFF and Fis1 for the activation of alternative mitophagy. Deletion of DRP1 during chronic phase of HFD also abolishes mitophagy and results in cardiac dysfunction. WT, wildtype; CKO, tamoxifen-inducible cardiomyocyte-specific conditional knockout; ER, endoplasmic reticulum.

Interestingly, the authors discovered that DRP1 drives mitophagy during both the acute and chronic phases of obesity-induced cardiac dysfunction through distinct mechanisms (Figure). DRP1 phosphorylation at Ser616 and translocation to mitochondria are observed only in the chronic phase of HFD consumption. Previous finding from Sadoshima’s group revealed that DRP1 can directly stimulate autophagosome formation and mitophagy by disrupting the Beclin 1/Bcl-2/Bcl-xL complex, promoting the release of Beclin 1 and initiating autophagosome formation, without the need for DRP1 phosphorylation at Ser616⁸. Beclin 1 is a key regulator of mitophagy¹¹. Consistent with this model, the authors found that the interaction between Beclin 1 and Bcl-2/ Bcl-xL is more stable in hearts of Drp1 CKO mice compared to control mice during the acute phase of HFD consumption. In contrast, during the chronic phase of HFD consumption, when conventional autophagy is suppressed, DRP1 undergoes phosphorylation at Ser616 and translocates to the mitochondria-associated endoplasmic reticulum membranes (MAMs), where it forms a complex with Rab9, MFF and Fis1 for the activation of alternative mitophagy. Importantly, the authors also confirmed an increase in DRP1 phosphorylation in the hearts of human patients with obesity.

Tong et. al.’s work⁷ sheds light on the prominent role of endogenous DRP1 in protecting against HFD-induced cardiomyopathy. DRP1 is indispensable for the activation of mitophagy and the maintenance of cardiac function during both the acute and chronic phases of HFD consumption. Loss of DRP1 in cardiomyocytes is clearly detrimental for cardiac function in the setting of obesity. The rapid lethality and severe cardiac dysfunction observed in Drp1 CKO mice when deletion was induced one week before the start of HFD feeding raise the question of whether the observed phenotype is solely due to abrogated mitophagy. Since DRP1 carries mitophagy-independent functions¹², and even mitochondria-independent functions¹³, it is important to consider whether additional defects, apart from abrogated mitophagy, contribute to the dramatic phenotype in Drp1 CKO mice. On the other hand, inducible deletion of Drp1 at 16 weeks after the start of HFD leads to abrogated mitophagy and exacerbated cardiac dysfunction, but not rapid lethality as observed with Drp1 deletion prior to HFD. Comparing the effects of Drp1 deletion at these two different stages might suggest that the early-stage response to HFD is more important in maintaining survival. However, it is worth noting that Drp1 CKO mice started to die at 8 weeks and exhibited complete lethality by 13 weeks after Cre induction under baseline conditions. An important question arises as to whether the basal phenotype in Drp1 CKO mice contributes to the phenotype observed under HFD consumption.

Two distinct mechanisms by which DRP1 activates mitophagy in acute and chronic phases have been proposed in this study (Figure). During the acute HFD stress, DRP1 does not translocate to mitochondria, but activates autophagosome formation through release of Beclin 1. The question remains as to how DRP1 senses acute stress from HFD through a phosphorylation-independent mechanism and then disrupts the interaction between Beclin 1 and Bcl-2/Bcl-xL. Additionally, mitochondrial fission is believed to be an important process for segregating damaged mitochondria from healthy ones for mitophagy^2,3. The findings in this study suggests that mitophagy is independent of DRP1-mediated mitochondrial fission in the acute phase. Is it not necessary to segregate dysfunctional mitochondria from the healthy network during the acute phase or does an alternative mechanism exist? Another unanswered question is how the conventional mitophagy pathway transitions to alternative mitophagy during HFD consumption. Both forms of mitophagy achieve the same function of removing defective mitochondria, but why do cells require different mechanisms to perform the same function? Lastly, while Tong et. al demonstrate the essential role of DRP1 in maintaining mitochondrial and cardiac function in obese hearts, it is of great interest to explore whether targeting DRP1 could be a therapeutic approach.