Carsten Merkwirth

SLP-2 is required for stress-induced mitochondrial hyperfusion

Mitochondria are dynamic organelles, the morphology of which results from an equilibrium between two opposing processes, fusion and fission. Mitochondrial fusion relies on dynamin‐related GTPases, the mitofusins (MFN1 and 2) in the outer mitochondrial membrane and OPA1 (optic atrophy 1) in the inner mitochondrial membrane. Apart from a role in the maintenance of mitochondrial DNA, little is known about the physiological role of mitochondrial fusion. Here we report that mitochondria hyperfuse and form a highly interconnected network in cells exposed to selective stresses. This process precedes mitochondrial fission when it is triggered by apoptotic stimuli such as UV irradiation or actinomycin D. Stress‐induced mitochondrial hyperfusion (SIMH) is independent of MFN2, BAX/BAK, and prohibitins, but requires L‐OPA1, MFN1, and the mitochondrial inner membrane protein SLP‐2. In the absence of SLP‐2, L‐OPA1 is lost and SIMH is prevented. SIMH is accompanied by increased mitochondrial ATP production and represents a novel adaptive pro‐survival response against stress.

Prohibitins control cell proliferation and apoptosis by regulating OPA1-dependent cristae morphogenesis in mitochondria

Prohibitins comprise an evolutionarily conserved and ubiquitously expressed family of membrane proteins with poorly described functions. Large assemblies of PHB1 and PHB2 subunits are localized in the inner membrane of mitochondria, but various roles in other cellular compartments have also been proposed for both proteins. Here, we used conditional gene targeting of murine Phb2 to define cellular activities of prohibitins. Our experiments restrict the function of prohibitins to mitochondria and identify the processing of the dynamin-like GTPase OPA1, an essential component of the mitochondrial fusion machinery, as the central cellular process controlled by prohibitins. Deletion of Phb2 leads to the selective loss of long isoforms of OPA1. This results in an aberrant cristae morphogenesis and an impaired cellular proliferation and resistance toward apoptosis. Expression of a long OPA1 isoform in PHB2-deficient cells suppresses these defects, identifying impaired OPA1 processing as the primary cellular defect in the absence of prohibitins. Our results therefore assign an essential function for the formation of mitochondrial cristae to prohibitins and suggest a coupling of cell proliferation to mitochondrial morphogenesis.

Prohibitin function within mitochondria: essential roles for cell proliferation and cristae morphogenesis.

Prohibitins comprise an evolutionary conserved and ubiquitously expressed family of membrane proteins. Various roles in different cellular compartments have been proposed for prohibitin proteins. Recent experiments, however, identify large assemblies of two homologous prohibitin subunits, PHB1 and PHB2, in the inner membrane of mitochondria as the physiologically active structure. Mitochondrial prohibitin complexes control cell proliferation, cristae morphogenesis and the functional integrity of mitochondria. The processing of the dynamin-like GTPase OPA1, a core component of the mitochondrial fusion machinery, has been defined as a key process affected by prohibitins. The molecular mechanism of prohibitin function, however, remained elusive. The ring-like assembly of prohibitins and their sequence similarity with lipid raft-associated SPFH-family members suggests a scaffolding function of prohibitins, which may lead to functional compartmentalization in the inner membrane of mitochondria.

Prohibitins and the functional compartmentalization of mitochondrial membranes.

Prohibitins constitute an evolutionarily conserved and ubiquitously expressed family of membrane proteins that are essential for cell proliferation and development in higher eukaryotes. Roles for prohibitins in cell signaling at the plasma membrane and in transcriptional regulation in the nucleus have been proposed, but pleiotropic defects associated with the loss of prohibitin genes can be largely attributed to a dysfunction of mitochondria. Two closely related proteins, prohibitin-1 (PHB1) and prohibitin-2 (PHB2), form large, multimeric ring complexes in the inner membrane of mitochondria. The absence of prohibitins leads to an increased generation of reactive oxygen species, disorganized mitochondrial nucleoids, abnormal cristae morphology and an increased sensitivity towards stimuli-elicited apoptosis. It has been found that the processing of the dynamin-like GTPase OPA1, which regulates mitochondrial fusion and cristae morphogenesis, is a key process regulated by prohibitins. Furthermore, genetic analyses in yeast have revealed an intimate functional link between prohibitin complexes and the membrane phospholipids cardiolipin and phosphatidylethanolamine. In light of these findings, it is emerging that prohibitin complexes can function as protein and lipid scaffolds that ensure the integrity and functionality of the mitochondrial inner membrane.

Increased proteasome activity in human embryonic stem cells is regulated by PSMD11

Embryonic stem cells can replicate continuously in the absence of senescence and, therefore, are immortal in culture. Although genome stability is essential for the survival of stem cells, proteome stability may have an equally important role in stem-cell identity and function. Furthermore, with the asymmetric divisions invoked by stem cells, the passage of damaged proteins to daughter cells could potentially destroy the resulting lineage of cells. Therefore, a firm understanding of how stem cells maintain their proteome is of central importance. Here we show that human embryonic stem cells (hESCs) exhibit high proteasome activity that is correlated with increased levels of the 19S proteasome subunit PSMD11 (known as RPN-6 in Caenorhabditis elegans) and a corresponding increased assembly of the 26S/30S proteasome. Ectopic expression of PSMD11 is sufficient to increase proteasome assembly and activity. FOXO4, an insulin/insulin-like growth factor-I (IGF-I) responsive transcription factor associated with long lifespan in invertebrates, regulates proteasome activity by modulating the expression of PSMD11 in hESCs. Proteasome inhibition in hESCs affects the expression of pluripotency markers and the levels of specific markers of the distinct germ layers. Our results suggest a new regulation of proteostasis in hESCs that links longevity and stress resistance in invertebrates to hESC function and identity.

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.

TRPV1 Pain Receptors Regulate Longevity and Metabolism by Neuropeptide Signaling

The sensation of pain is associated with increased mortality, but it is unknown whether pain perception can directly affect aging. We find that mice lacking TRPV1 pain receptors are long-lived, displaying a youthful metabolic profile at old age. Loss of TRPV1 inactivates a calcium-signaling cascade that ends in the nuclear exclusion of the CREB-regulated transcriptional coactivator CRTC1 within pain sensory neurons originating from the spinal cord. In long-lived TRPV1 knockout mice, CRTC1 nuclear exclusion decreases production of the neuropeptide CGRP from sensory endings innervating the pancreatic islets, subsequently promoting insulin secretion and metabolic health. In contrast, CGRP homeostasis is disrupted with age in wild-type mice, resulting in metabolic decline. We show that pharmacologic inactivation of CGRP receptors in old wild-type animals can restore metabolic health. These data suggest that ablation of select pain sensory receptors or the inhibition of CGRP are associated with increased metabolic health and control longevity.

Regulation of B cell homeostasis and activation by the tumor suppressor gene CYLD

B cell homeostasis is regulated by multiple signaling processes, including nuclear factor-κB (NF-κB), BAFF-, and B cell receptor signaling. Conditional disruption of genes involved in these pathways has shed light on the mechanisms governing signaling from the cell surface to the nucleus. We describe a novel mouse strain that expresses solely and excessively a naturally occurring splice variant of CYLD (CYLDex7/8 mice), which is a deubiquitinating enzyme that is integral to NF-κB signaling. This shorter CYLD protein lacks the TRAF2 and NEMO binding sites present in full-length CYLD. A dramatic expansion of mature B lymphocyte populations in all peripheral lymphoid organs occurs in this strain. The B lymphocytes themselves exhibit prolonged survival and manifest a variety of signaling disarrangements that do not occur in mice with a complete deletion of CYLD. Although both the full-length and the mutant CYLD are able to interact with Bcl-3, a predominant nuclear accumulation of Bcl-3 occurs in the CYLD mutant B cells. More dramatic, however, is the accumulation of the NF-κB proteins p100 and RelB in CYLDex7/8 B cells, which, presumably in combination with nuclear Bcl-3, results in increased levels of Bcl-2 expression. These findings suggest that CYLD can both positively and negatively regulate signal transduction and homeostasis of B cells in vivo, depending on the expression of CYLD splice variants.

Mitofusin 2 Builds a Bridge between ER and Mitochondria

Mutations in mitofusin 2 (MFN2), a dynamin-like GTPase required for mitochondrial fusion, cause the peripheral neuropathy Charcot-Marie-Tooth type 2A. In a recent report in Nature, de Brito and Scorrano (2008) demonstrate a new function of MFN2-tethering the endoplasmic reticulum and mitochondria to control the efficiency of mitochondrial uptake of Ca2+ ions.

Loss of Prohibitin Membrane Scaffolds Impairs Mitochondrial Architecture and Leads to Tau Hyperphosphorylation and Neurodegeneration

Fusion and fission of mitochondria maintain the functional integrity of mitochondria and protect against neurodegeneration, but how mitochondrial dysfunctions trigger neuronal loss remains ill-defined. Prohibitins form large ring complexes in the inner membrane that are composed of PHB1 and PHB2 subunits and are thought to function as membrane scaffolds. In Caenorhabditis elegans, prohibitin genes affect aging by moderating fat metabolism and energy production. Knockdown experiments in mammalian cells link the function of prohibitins to membrane fusion, as they were found to stabilize the dynamin-like GTPase OPA1 (optic atrophy 1), which mediates mitochondrial inner membrane fusion and cristae morphogenesis. Mutations in OPA1 are associated with dominant optic atrophy characterized by the progressive loss of retinal ganglion cells, highlighting the importance of OPA1 function in neurons. Here, we show that neuron-specific inactivation of Phb2 in the mouse forebrain causes extensive neurodegeneration associated with behavioral impairments and cognitive deficiencies. We observe early onset tau hyperphosphorylation and filament formation in the hippocampus, demonstrating a direct link between mitochondrial defects and tau pathology. Loss of PHB2 impairs the stability of OPA1, affects mitochondrial ultrastructure, and induces the perinuclear clustering of mitochondria in hippocampal neurons. A destabilization of the mitochondrial genome and respiratory deficiencies manifest in aged neurons only, while the appearance of mitochondrial morphology defects correlates with tau hyperphosphorylation in the absence of PHB2. These results establish an essential role of prohibitin complexes for neuronal survival in vivo and demonstrate that OPA1 stability, mitochondrial fusion, and the maintenance of the mitochondrial genome in neurons depend on these scaffolding proteins. Moreover, our findings establish prohibitin-deficient mice as a novel genetic model for tau pathologies caused by a dysfunction of mitochondria and raise the possibility that tau pathologies are associated with other neurodegenerative disorders caused by deficiencies in mitochondrial dynamics.