Timothy Wai

The i-AAA protease YME1L and OMA1 cleave OPA1 to balance mitochondrial fusion and fission

OPA1 processing by YEM1L and OMA1 is dispensable for mitochondrial fusion and instead drives mitochondrial fragmentation, which is crucial for mitochondrial integrity and quality control.

Mitochondrial Dynamics and Metabolic Regulation.

Mitochondrial morphology varies tremendously across cell types and tissues, changing rapidly in response to external insults and metabolic cues, such as nutrient status. The many functions of mitochondria have been intimately linked to their morphology, which is shaped by ongoing events of fusion and fission of outer and inner membranes (OM and IM). Unopposed fission causes mitochondrial fragmentation, which is generally associated with metabolic dysfunction and disease. Unopposed fusion results in a hyperfused network and serves to counteract metabolic insults, preserve cellular integrity, and protect against autophagy. Here, we review the ways in which metabolic alterations convey changes in mitochondrial morphology and how disruption of mitochondrial morphology impacts cellular and organismal metabolism.

Imbalanced OPA1 processing and mitochondrial fragmentation cause heart failure 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 Mitochondrial fragmentation in cardiomyocytes causes heart failure in mice and can be rescued by metabolic intervention. [Also see Perspective by Gottlieb and Bernstein] INTRODUCTION Mitochondria are essential organelles whose form and function are inextricably linked. Balanced fusion and fission events shape mitochondria to meet metabolic demands and to ensure removal of damaged organelles. A fragmentation of the mitochondrial network occurs in response to cellular stress and is observed in a wide variety of disease conditions, including heart failure, neurodegenerative disorders, cancer, and obesity. However, the physiological relevance of stress-induced mitochondrial fragmentation remains unclear. RATIONALE Proteolytic processing of the dynamin-like guanosine triphosphatase (GTPase) OPA1 in the inner membrane of mitochondria is emerging as a critical regulatory step to balance mitochondrial fusion and fission. Two mitochondrial proteases, OMA1 and the AAA protease YME1L, cleave OPA1 from long (L-OPA1) to short (S-OPA1) forms. L-OPA1 is required for mitochondrial fusion, but S-OPA1 is not, although accumulation of S-OPA1 in excess accelerates fission. In cultured mammalian cells, stress conditions activate OMA1, which cleaves L-OPA1 and inhibits mitochondrial fusion resulting in mitochondrial fragmentation. In this study, we generated conditional mouse models for both YME1L and OMA1 and examined the role of OPA1 processing and mitochondrial fragmentation in the heart, a metabolically demanding organ that depends critically on mitochondrial functions. RESULTS Deletion of Yme1l in cardiomyocytes did not grossly affect mitochondrial respiration but induced the proteolytic cleavage of OPA1 by the stress-activated peptidase OMA1 and drove fragmentation of mitochondria in vivo. These mice suffered from dilated cardiomyopathy characterized by well-established features of heart failure that include necrotic cell death, fibrosis and ventricular remodelling, and a metabolic switch away from fatty acid oxidation and toward glucose use. We discovered that additional deletion of Oma1 in cardiomyocytes prevented OPA1 processing altogether and restored normal mitochondrial morphology and cardiac health. On the other hand, mice lacking YME1L in both skeletal muscle and cardiomyocytes exhibited normal cardiac function and life span despite mitochondrial fragmentation in cardiomyocytes. Imbalanced OPA1 processing in skeletal muscle, which is an insulin signaling tissue, induced systemic glucose intolerance and prevented cardiac glucose overload and cardiomyopathy. We observed a similar effect on cardiac metabolism upon feeding mice lacking Yme1l in cardiomyocytes a high-fat diet, which preserved heart function despite mitochondrial fragmentation. CONCLUSION Our work highlights the importance of balanced fusion and fission of mitochondria for cardiac function and unravels an intriguing link between mitochondrial dynamics and cardiac metabolism in the adult heart in vivo. Mitochondrial fusion mediated by L-OPA1 preserves cardiac function, whereas its stress-induced processing by OMA1 and mitochondrial fragmentation triggers dilated cardiomyopathy and heart failure. In contrast to previous genetic models of the mitochondrial fusion machinery, mice lacking Yme1l in cardiomyocytes do not show pleiotropic respiratory deficiencies and thus provide a tool to directly assess the physiological importance of mitochondrial dynamics. Preventing mitochondrial fragmentation by deleting Oma1 protects against cell death and heart failure. The identification of OMA1 as a critical regulator of mitochondrial morphology and cardiomyocyte survival holds promise for translational applications in cardiovascular medicine. Mitochondrial fragmentation induces a metabolic switch from fatty acid to glucose utilization in the heart. It turns out that reversing this switch and restoring normal cardiac metabolism is sufficient to preserve heart function despite mitochondrial fragmentation. These findings raise the intriguing possibility that the switch in fuel usage that occurs in the failing adult heart may, in fact, be maladaptive and could contribute to the pathogenesis of heart failure. Critical role of balanced mitochondrial fusion and fission for cardiac metabolism and heart function. Induced processing of the dynamin-like GTPase OPA1 in the inner membrane by the stress-activated peptidase OMA1 leads to mitochondrial fragmentation, cardiomyopathy, and heart failure, which is characterized by a switch in fuel utilization. Heart function can be preserved by reversing this metabolic switch without suppressing mitochondrial fragmentation. Mitochondrial morphology is shaped by fusion and division of their membranes. Here, we found that adult myocardial function depends on balanced mitochondrial fusion and fission, maintained by processing of the dynamin-like guanosine triphosphatase OPA1 by the mitochondrial peptidases YME1L and OMA1. Cardiac-specific ablation of Yme1l in mice activated OMA1 and accelerated OPA1 proteolysis, which triggered mitochondrial fragmentation and altered cardiac metabolism. This caused dilated cardiomyopathy and heart failure. Cardiac function and mitochondrial morphology were rescued by Oma1 deletion, which prevented OPA1 cleavage. Feeding mice a high-fat diet or ablating Yme1l in skeletal muscle restored cardiac metabolism and preserved heart function without suppressing mitochondrial fragmentation. Thus, unprocessed OPA1 is sufficient to maintain heart function, OMA1 is a critical regulator of cardiomyocyte survival, and mitochondrial morphology and cardiac metabolism are intimately linked.

TRIAP1/PRELI Complexes Prevent Apoptosis by Mediating Intramitochondrial Transport of Phosphatidic Acid

Cardiolipin (CL), a mitochondria-specific glycerophospholipid, is required for diverse mitochondrial processes and orchestrates the function of various death-inducing proteins during apoptosis. Here, we identify a complex of the p53-regulated protein TRIAP1 (p53CSV) and PRELI in the mitochondrial intermembrane space (IMS), which ensures the accumulation of CL in mitochondria. TRIAP1/PRELI complexes exert lipid transfer activity in vitro and supply phosphatidic acid (PA) for CL synthesis in the inner membrane. Loss of TRIAP1 or PRELI impairs the accumulation of CL, facilitates the release of cytochrome c, and renders cells vulnerable to apoptosis upon intrinsic and extrinsic stimulation. Survival of TRIAP1- and PRELI-deficient cells is conferred by an excess of exogenously provided phosphatidylglycerol. Our results reveal a p53-dependent cell-survival pathway and highlight the importance of the CL content of mitochondrial membranes in apoptosis.

Stomatin-Like Protein 2 Is Required for In Vivo Mitochondrial Respiratory Chain Supercomplex Formation and Optimal Cell Function

ABSTRACT Stomatin-like protein 2 (SLP-2) is a mainly mitochondrial protein that is widely expressed and is highly conserved across evolution. We have previously shown that SLP-2 binds the mitochondrial lipid cardiolipin and interacts with prohibitin-1 and -2 to form specialized membrane microdomains in the mitochondrial inner membrane, which are associated with optimal mitochondrial respiration. To determine how SLP-2 functions, we performed bioenergetic analysis of primary T cells from T cell-selective Slp-2 knockout mice under conditions that forced energy production to come almost exclusively from oxidative phosphorylation. These cells had a phenotype characterized by increased uncoupled mitochondrial respiration and decreased mitochondrial membrane potential. Since formation of mitochondrial respiratory chain supercomplexes (RCS) may correlate with more efficient electron transfer during oxidative phosphorylation, we hypothesized that the defect in mitochondrial respiration in SLP-2-deficient T cells was due to deficient RCS formation. We found that in the absence of SLP-2, T cells had decreased levels and activities of complex I-III2 and I-III2-IV1-3 RCS but no defects in assembly of individual respiratory complexes. Impaired RCS formation in SLP-2-deficient T cells correlated with significantly delayed T cell proliferation in response to activation under conditions of limiting glycolysis. Altogether, our findings identify SLP-2 as a key regulator of the formation of RCS in vivo and show that these supercomplexes are required for optimal cell function.

The membrane scaffold SLP2 anchors a proteolytic hub in mitochondria containing PARL and the i‐AAA protease YME1L

The SPFH (stomatin, prohibitin, flotillin, HflC/K) superfamily is composed of scaffold proteins that form ring‐like structures and locally specify the protein–lipid composition in a variety of cellular membranes. Stomatin‐like protein 2 (SLP2) is a member of this superfamily that localizes to the mitochondrial inner membrane (IM) where it acts as a membrane organizer. Here, we report that SLP2 anchors a large protease complex composed of the rhomboid protease PARL and the i‐AAA protease YME1L, which we term the SPY complex (for SLP2–PARL–YME1L). Association with SLP2 in the SPY complex regulates PARL‐mediated processing of PTEN‐induced kinase PINK1 and the phosphatase PGAM5 in mitochondria. Moreover, SLP2 inhibits the stress‐activated peptidase OMA1, which can bind to SLP2 and cleaves PGAM5 in depolarized mitochondria. SLP2 restricts OMA1‐mediated processing of the dynamin‐like GTPase OPA1 allowing stress‐induced mitochondrial hyperfusion under starvation conditions. Together, our results reveal an important role of SLP2 membrane scaffolds for the spatial organization of IM proteases regulating mitochondrial dynamics, quality control, and cell survival.

SLIRP Regulates the Rate of Mitochondrial Protein Synthesis and Protects LRPPRC from Degradation

We have studied the in vivo role of SLIRP in regulation of mitochondrial DNA (mtDNA) gene expression and show here that it stabilizes its interacting partner protein LRPPRC by protecting it from degradation. Although SLIRP is completely dependent on LRPPRC for its stability, reduced levels of LRPPRC persist in the absence of SLIRP in vivo. Surprisingly, Slirp knockout mice are apparently healthy and only display a minor weight loss, despite a 50–70% reduction in the steady-state levels of mtDNA-encoded mRNAs. In contrast to LRPPRC, SLIRP is dispensable for polyadenylation of mtDNA-encoded mRNAs. Instead, deep RNA sequencing (RNAseq) of mitochondrial ribosomal fractions and additional molecular analyses show that SLIRP is required for proper association of mRNAs to the mitochondrial ribosome and efficient translation. Our findings thus establish distinct functions for SLIRP and LRPPRC within the LRPPRC-SLIRP complex, with a novel role for SLIRP in mitochondrial translation. Very surprisingly, our results also demonstrate that mammalian mitochondria have a great excess of transcripts under basal physiological conditions in vivo.

Homozygous YME1L1 mutation causes mitochondriopathy with optic atrophy and mitochondrial network fragmentation

Mitochondriopathies often present clinically as multisystemic disorders of primarily high-energy consuming organs. Assembly, turnover, and surveillance of mitochondrial proteins are essential for mitochondrial function and a key task of AAA family members of metalloproteases. We identified a homozygous mutation in the nuclear encoded mitochondrial escape 1-like 1 gene YME1L1, member of the AAA protease family, as a cause of a novel mitochondriopathy in a consanguineous pedigree of Saudi Arabian descent. The homozygous missense mutation, located in a highly conserved region in the mitochondrial pre-sequence, inhibits cleavage of YME1L1 by the mitochondrial processing peptidase, which culminates in the rapid degradation of YME1L1 precursor protein. Impaired YME1L1 function causes a proliferation defect and mitochondrial network fragmentation due to abnormal processing of OPA1. Our results identify mutations in YME1L1 as a cause of a mitochondriopathy with optic nerve atrophy highlighting the importance of YME1L1 for mitochondrial functionality in humans. DOI: http://dx.doi.org/10.7554/eLife.16078.001

The Mitochondrial Metallochaperone SCO1 Is Required to Sustain Expression of the High-Affinity Copper Transporter CTR1 and Preserve Copper Homeostasis

Human SCO1 fulfills essential roles in cytochrome c oxidase (COX) assembly and the regulation of copper (Cu) homeostasis, yet it remains unclear why pathogenic mutations in this gene cause such clinically heterogeneous forms of disease. Here, we establish a Sco1 mouse model of human disease and show that ablation of Sco1 expression in the liver is lethal owing to severe COX and Cu deficiencies. We further demonstrate that the Cu deficiency is explained by a functional connection between SCO1 and CTR1, the high-affinity transporter that imports Cu into the cell. CTR1 is rapidly degraded in the absence of SCO1 protein, and we show that its levels are restored in Sco1-/- mouse embryonic fibroblasts upon inhibition of the proteasome. These data suggest that mitochondrial signaling through SCO1 provides a post-translational mechanism to regulate CTR1-dependent Cu import into the cell, and they further underpin the importance of mitochondria in cellular Cu homeostasis.

Meeting highlights from the 2013 European Society of Cardiology Heart Failure Association Winter Meeting on Translational Heart Failure Research

Meeting highlights from the 2013 European Society of Cardiology Heart Failure Association Winter Meeting on Translational Heart Failure Research