Parkin-mediated mitophagy directs perinatal cardiac metabolic maturation in mice

A change of heart (mitochondria) Mitochondria provide an essential source of energy to drive cellular processes and are particularly important in heart muscle cells (see the Perspective by Gottlieb and Bernstein). After birth, the availability of oxygen and nutrients to organs and tissues changes. This invokes changes in metabolism. Gong et al. studied the developmental transitions in mouse heart mitochondria soon after birth. Mitochondria were replaced wholesale via mitophagy in cardiomyocytes over the first 3 weeks after birth. Preventing this turnover by interfering with parkin-mediated mitophagy specifically in cardiomyocytes prevented the normal metabolic transition and caused heart failure. Thus, the heart has coopted a quality-control pathway to facilitate a major developmental transition after birth. Wai et al. examined the role of mitochondrial fission and fusion in mouse cardiomyocytes. Disruption of these processes led to “middle-aged” death from a form of dilated cardiomyopathy. Mice destined to develop cardiomyopathy were protected by feeding with a high-fat diet, which altered cardiac metabolism. Science, this issue p. 10.1126/science.aad2459, p. 10.1126/science.aad0116; see also p. 1162 Exclusion of Parkin from mitochondria of cardiomyocytes prevents the normal perinatal-to-adult transformation of heart metabolism. [Also see Perspective by Gottlieb and Bernstein] INTRODUCTION During heart development, increased oxygenation during the early perinatal period and a change in nutrient availability evokes a switch in mitochondrial substrate preference from carbohydrates to fatty acids. This metabolic switching is reversed in adult heart disease. Genetic “reprogramming” of mitochondria plays a role in developmental and disease-related metabolic transitioning, but how mitochondrial plasticity is governed is unclear. Here, we found that mitophagy induced by PINK1-mitofusin 2 (Mfn2) –Parkin signaling was central to perinatal switching from glycolytic to fatty acid metabolism in mouse hearts. The Mfn2-Parkin interaction provoked generalized mitophagic removal of fetal cardiomyocyte mitochondria during the first 3 weeks of life and was a prerequisite for introduction of mature cardiac mitochondria optimized for fatty acid metabolism. RATIONALE We considered that the highly ordered paracrystallar structure of ATP biosynthetic pathways makes it unlikely that mitochondria can behave as flexible fuel organelles, readily adjusting their metabolism to differing substrate availability. Rather, we posited that mitochondria optimized for a given metabolic milieu must be replaced when conditions change, as during the perinatal period. In support of this notion, late fetal and adult cardiomyocyte mitochondria have distinct morphologies as well as metabolic preferences. Because targeted autophagic elimination of individual damaged mitochondria (mitophagy) is mediated by the Parkinson’s disease factors PINK1 and Parkin, we examined the consequences of cardiac-specific Parkin loss-of-function on perinatal mitochondrial maturation and metabolic transitioning in mouse hearts. Whereas Parkin deletion from adult hearts had no discernible adverse effects, cardiomyocyte-specific Parkin ablation from the first day of life was lethal in most mice before 3 weeks of age; in surviving mice, mitochondrial maturation was arrested at the fetal stage. RESULTS To interrupt Parkin-mediated mitophagy with more precision than gene ablation, we expressed PINK1 T111 and S442 phosphorylation site Mfn2 mutants. In cultured fibroblasts, the glutamic acid (E) substituted phosphomimic mutant Mfn2 EE spontaneously recruited Parkin to mitochondria and promoted mitophagy, whereas alanine (A) substituted nonphosphorylatable Mfn2 AA prevented Parkin translocation and interrupted mitophagy stimulated by mitochondrial depolarization. We expressed wild-type Mfn2, Mfn2 EE, and Mfn2 AA in mouse hearts. Mfn2 AA, when expressed perinatally but not at or after weaning, provoked cardiomyopathy that was lethal by 7 to 8 weeks. Cardiomyocyte mitochondria of surviving young adult Mfn2 AA mice had an eccentric morphology and impaired palmitoylcarnitine use, which are typical features of fetal heart mitochondria. The transcriptional signature of juvenile Mfn2 AA hearts was distinguished from age-matched controls by depressed abundance of fatty acid and branched chain amino acid metabolism messenger RNAs, again resembling fetal hearts. Mitochondrial biogenesis was impaired, and metabolite profiling of young adult Mfn2 AA hearts revealed developmental metabolic arrest at the perinatal stage—that is, impaired fatty acid use and preserved glycolytic function. Thus, interrupting Parkin-mediated mitophagy in perinatal mouse hearts prevented normal maturational metabolic transitioning to fatty acids through retention of fetal cardiomyocyte mitochondria. Mitophagy was a prerequisite for mitochondrial biogenesis in this context. CONCLUSION Fetal cardiomyocyte mitochondria undergo perinatal PINK1-Mfn2-Parkin–mediated mitophagy and replacement by mature adult mitochondria, rather than transcriptional reprogramming. Mitophagic mitochondrial removal underlies developmental cardiomyocyte mitochondrial plasticity and metabolic transitioning. Facilitating developmentally programmed mitochondrial turnover is functionally distinct from canonical selective targeting and removal of damaged mitochondria by Parkin in other contexts. Mitochondrial maturation fails when mitophagy is interrupted. Normal perinatal mitochondrial maturation is shown on the left: Heart sections from neonatal and 5-week-old hearts are superimposed on their electron micrographs. To the right are similar images from hearts expressing the dominant negative mitochondrial Parkin receptor, Mfn2 AA. Retention of fetal cardiomyocyte mitochondria in mitophagically impaired hearts was lethal. In developing hearts, changes in the cardiac metabolic milieu during the perinatal period redirect mitochondrial substrate preference from carbohydrates to fatty acids. Mechanisms responsible for this mitochondrial plasticity are unknown. Here, we found that PINK1-Mfn2-Parkin–mediated mitophagy directs this metabolic transformation in mouse hearts. A mitofusin (Mfn) 2 mutant lacking PINK1 phosphorylation sites necessary for Parkin binding (Mfn2 AA) inhibited mitochondrial Parkin translocation, suppressing mitophagy without impairing mitochondrial fusion. Cardiac Parkin deletion or expression of Mfn2 AA from birth, but not after weaning, prevented postnatal mitochondrial maturation essential to survival. Five-week-old Mfn2 AA hearts retained a fetal mitochondrial transcriptional signature without normal increases in fatty acid metabolism and mitochondrial biogenesis genes. Myocardial fatty acylcarnitine levels and cardiomyocyte respiration induced by palmitoylcarnitine were concordantly depressed. Thus, instead of transcriptional reprogramming, fetal cardiomyocyte mitochondria undergo perinatal Parkin-mediated mitophagy and replacement by mature adult mitochondria. Mitophagic mitochondrial removal underlies developmental cardiomyocyte mitochondrial plasticity and metabolic transitioning of perinatal hearts.