The mitochondrial matrix protease CLPP plays a central role in the activation of the mitochondrial unfolded protein response (UPRmt) in Caenorhabditis elegans. Far less is known about mammalian UPRmt ...signaling, although similar roles were assumed for central players, including CLPP. To better understand the mammalian UPRmt signaling, we deleted CLPP in hearts of DARS2‐deficient animals that show robust induction of UPRmt due to strong dysregulation of mitochondrial translation. Remarkably, our results clearly show that mammalian CLPP is neither required for, nor it regulates the UPRmt in mammals. Surprisingly, we demonstrate that a strong mitochondrial cardiomyopathy and diminished respiration due to DARS2 deficiency can be alleviated by the loss of CLPP, leading to an increased de novo synthesis of individual OXPHOS subunits. These results question our current understanding of the UPRmt signaling in mammals, while introducing CLPP as a possible novel target for therapeutic intervention in mitochondrial diseases.
Synopsis
Loss of mammalian ClpP does not affect the mitochondrial unfolded protein response (UPRmt) under proteotoxic stress. Instead, loss of ClpP alleviates mitochondrial cardiomyopathy by partially rescuing the mitochondrial translation defect and reducing the level of potentially toxic peptides.
Loss of ClpP alleviates progressive mitochondrial cardiomyopathy.
ClpP is not required for mammalian UPRmt signaling.
Loss of ClpP slows down mitochondrial translation, thereby increasing the level of functional OXPHOS complexes in DARS2‐deficient mice.
Loss of mammalian ClpP does not affect the mitochondrial unfolded protein response (UPRmt) under proteotoxic stress. Instead, loss of ClpP alleviates mitochondrial cardiomyopathy by partially rescuing the mitochondrial translation defect and reducing the level of potentially toxic peptides.
Adaptive stress responses activated upon mitochondrial dysfunction are assumed to arise in order to counteract respiratory chain deficiency. Here, we demonstrate that loss of DARS2 (mitochondrial ...aspartyl-tRNA synthetase) leads to the activation of various stress responses in a tissue-specific manner independently of respiratory chain deficiency. DARS2 depletion in heart and skeletal muscle leads to the severe deregulation of mitochondrial protein synthesis followed by a strong respiratory chain deficit in both tissues, yet the activation of adaptive responses is observed predominantly in cardiomyocytes. We show that the impairment of mitochondrial proteostasis in the heart activates the expression of mitokine FGF21, which acts as a signal for cell-autonomous and systemic metabolic changes. Conversely, skeletal muscle has an intrinsic mechanism relying on the slow turnover of mitochondrial transcripts and higher proteostatic buffering capacity. Our results show that mitochondrial dysfunction is sensed independently of respiratory chain deficiency, questioning the current view on the role of stress responses in mitochondrial diseases.
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•DARS2 deficiency activates stress responses exclusively in the heart•Mitochondrial stress responses arise independently of respiratory deficiency•Loss of mitochondrial proteostasis initiates cell-intrinsic and systemic changes•Skeletal muscles rely on high proteostatic capacity and low mitochondrial RNA turnover
Dogan et al. find that disruption of mitochondrial protein synthesis leads to severe respiratory chain deficiency in both heart and skeletal muscle but that the activation of adaptive stress responses occur predominantly in cardiomyocytes, arguing that tissue-specific adaptive stress responses arise independently of respiratory chain deficiency.
Cytosolic mitochondrial DNA (mtDNA) elicits a type I interferon response, but signals triggering the release of mtDNA from mitochondria remain enigmatic. Here, we show that mtDNA-dependent immune ...signalling via the cyclic GMP-AMP synthase‒stimulator of interferon genes‒TANK-binding kinase 1 (cGAS-STING-TBK1) pathway is under metabolic control and is induced by cellular pyrimidine deficiency. The mitochondrial protease YME1L preserves pyrimidine pools by supporting de novo nucleotide synthesis and by proteolysis of the pyrimidine nucleotide carrier SLC25A33. Deficiency of YME1L causes inflammation in mouse retinas and in cultured cells. It drives the release of mtDNA and a cGAS-STING-TBK1-dependent inflammatory response, which requires SLC25A33 and is suppressed upon replenishment of cellular pyrimidine pools. Overexpression of SLC25A33 is sufficient to induce immune signalling by mtDNA. Similarly, depletion of cytosolic nucleotides upon inhibition of de novo pyrimidine synthesis triggers mtDNA-dependent immune responses in wild-type cells. Our results thus identify mtDNA release and innate immune signalling as a metabolic response to cellular pyrimidine deficiencies.
Cardiomyopathy and heart failure are common manifestations in mitochondrial disease caused by deficiencies in the oxidative phosphorylation (OXPHOS) system of mitochondria. Here, we demonstrate that ...the cardiac-specific loss of the assembly factor Cox10 of the cytochrome c oxidase causes mitochondrial cardiomyopathy in mice, which is associated with OXPHOS deficiency, lysosomal defects, and an aberrant mitochondrial morphology. Activation of the mitochondrial peptidase Oma1 in Cox10−/− mice results in mitochondrial fragmentation and induction of the integrated stress response (ISR) along the Oma1-Dele1-Atf4 signaling axis. Ablation of Oma1 or Dele1 in Cox10−/− mice aggravates cardiomyopathy. ISR inhibition impairs the cardiac glutathione metabolism, limits the selenium-dependent accumulation of the glutathione peroxidase Gpx4, and increases lipid peroxidation in the heart, ultimately culminating in ferroptosis. Our results demonstrate a protective role of the Oma1-Dele1-mediated ISR in mitochondrial cardiomyopathy and link ferroptosis to OXPHOS deficiency and mitochondrial disease.
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•Oma1-Dele1-Atf4 signaling elicits the ISR in OXPHOS-deficient hearts•Loss of Oma1 or Dele1 aggravates mitochondrial cardiomyopathy•Oma1-Dele1-Atf4 signaling protects against ferroptosis in OXPHOS deficiency•Oma1-dependent ISR promotes selenium-dependent accumulation of Gpx4
Ahola et al. demonstrate that OXPHOS deficiency in the heart of Cox10−/− mice causes cardiomyopathy and elicits an integrated stress response along the Oma1-Dele1-Atf4 axis. Oma1-dependent stress signaling preserves the glutathione metabolism and Gpx4 accumulation to limit lipid peroxidation, suppress ferroptosis, and delay cardiomyopathy.
Mitochondria are fundamental for cellular metabolism as they are both a source and a target of nutrient intermediates originating from converging metabolic pathways, and their role in the regulation ...of systemic metabolism is increasingly recognized. Thus, maintenance of mitochondrial homeostasis is indispensable for a functional energy metabolism of the whole organism. Here, we report that loss of the mitochondrial matrix protease CLPP results in a lean phenotype with improved glucose homeostasis. Whole‐body CLPP‐deficient mice are protected from diet‐induced obesity and insulin resistance, which was not present in mouse models with either liver‐ or muscle‐specific depletion of CLPP. However, CLPP ablation also leads to a decline in brown adipocytes function leaving mice unable to cope with a cold‐induced stress due to non‐functional adaptive thermogenesis. These results demonstrate a critical role for CLPP in different metabolic stress conditions such as high‐fat diet feeding and cold exposure providing tools to understand pathologies with deregulated Clpp expression and novel insights into therapeutic approaches against metabolic dysfunctions linked to mitochondrial diseases.
Synopsis
Complete loss of mitochondrial CLPP protease makes mice leaner with increased ability for glucose handling and protects them from diet‐induced obesity. In the same time, CLPP ablation also leads to a decline in brown adipocytes function leaving mice unable to cope with a cold‐induced stress due to dysfunctional thermogenesis.
Loss of CLPP protects against HFD‐induced obesity and insulin resistance.
CLPP deficiency increases FAO in white adipose tissue.
Loss of CLPP impairs cold‐induced thermogenesis.
Complete loss of mitochondrial CLPP protease protects mice from diet‐induced obesity and insulin‐resistance but impairs cold‐induced thermogenesis due to a decline in brown adipocyte function.
Mitochondrial dysfunction causes neurodegeneration but whether impairment of mitochondrial homeostasis in astrocytes contributes to this pathological process remains largely unknown. The m‐AAA ...protease exerts quality control and regulatory functions crucial for mitochondrial homeostasis. AFG3L2, which encodes one of the subunits of the m‐AAA protease, is mutated in spinocerebellar ataxia SCA28 and in infantile syndromes characterized by spastic‐ataxia, epilepsy and premature death. Here, we investigate the role of Afg3l2 and its redundant homologue Afg3l1 in the Bergmann glia (BG), radial astrocytes of the cerebellum that have functional connections with Purkinje cells (PC) and regulate glutamate homeostasis. We show that astrocyte‐specific deletion of Afg3l2 in the mouse leads to late‐onset motor impairment and to degeneration of BG, which display aberrant morphology, altered expression of the glutamate transporter EAAT2, and a reactive inflammatory signature. The neurological and glial phenotypes are drastically exacerbated when astrocytes lack both Afg31l and Afg3l2, and therefore, are totally depleted of the m‐AAA protease. Moreover, mitochondrial stress responses and necroptotic markers are induced in the cerebellum. In both mouse models, targeted BG show a fragmented mitochondrial network and loss of mitochondrial cristae, but no signs of respiratory dysfunction. Importantly, astrocyte‐specific deficiency of Afg3l1 and Afg3l2 triggers secondary morphological degeneration and electrophysiological changes in PCs, thus demonstrating a non‐cell‐autonomous role of glia in neurodegeneration. We propose that astrocyte dysfunction amplifies both neuroinflammation and glutamate excitotoxicity in patients carrying mutations in AFG3L2, leading to a vicious circle that contributes to neuronal death.
Main Points
Bergmann glia lacking the m‐AAA protease become reactive, show mitochondrial fragmentation and metabolic remodeling, and degenerate
Bergmann glia deficiency of the m‐AAA protease causes secondary degeneration of Purkinje cells
Although mitochondria are ubiquitous, each mitochondrial disease has surprisingly distinctly different pattern of tissue and organ involvement. Congruently, mutations in genes encoding for different ...mitochondrial tRNA synthetases result in the development of a very flamboyant group of diseases. Mutations in some of these genes, including aspartyl-tRNA synthetase (DARS2), lead to the onset of a white matter disease-leukoencephalopathy with brainstem and spinal cord involvement, and lactate elevation (LBSL) characterized by progressive spastic ataxia and characteristic leukoencephalopathy signature with multiple long-tract involvements. Puzzled by the white matter disease phenotypes caused by DARS2 deficiency when numerous other mutations in the genes encoding proteins involved in mitochondrial translation have a detrimental effect predominantly on neurons, we generated transgenic mice in which DARS2 was specifically depleted in forebrain-hippocampal neurons or myelin-producing cells. Our results now provide the first evidence that loss of DARS2 in adult neurons leads to strong mitochondrial dysfunction and progressive loss of cells. In contrast, myelin-producing cells seem to be resistant to cell death induced by DARS2 depletion despite robust respiratory chain deficiency arguing that LBSL might originate from the primary neuronal and axonal defect. Remarkably, our results also suggest a role for early neuroinflammation in the disease progression, highlighting the possibility for therapeutic interventions of this process.
Mitochondrial fusion and fission accompany adaptive responses to stress and altered metabolic demands. Inner membrane fusion and cristae morphogenesis depends on optic atrophy 1 (Opa1), which is ...expressed in different isoforms and is cleaved from a membrane-bound, long to a soluble, short form. Here, we have analyzed the physiological role of Opa1 isoforms and Opa1 processing by generating mouse lines expressing only one cleavable Opa1 isoform or a non-cleavable variant thereof. Our results show that expression of a single cleavable or non-cleavable Opa1 isoform preserves embryonic development and the health of adult mice. Opa1 processing is dispensable under metabolic and thermal stress but prolongs life span and protects against mitochondrial cardiomyopathy in OXPHOS-deficient Cox10 −/− mice. Mechanistically, loss of Opa1 processing disturbs the balance between mitochondrial biogenesis and mitophagy, suppressing cardiac hypertrophic growth in Cox10 −/− hearts. Our results highlight the critical regulatory role of Opa1 processing, mitochondrial dynamics, and metabolism for cardiac hypertrophy.
Inhibition of Opa1 processing in mice allows normal development but exacerbates mitochondrial cardiomyopathy.
Abstract
The mitochondrial matrix protease
CLPP
plays a central role in the activation of the mitochondrial unfolded protein response (
UPR
mt
) in
Caenorhabditis elegans
. Far less is known about ...mammalian
UPR
mt
signaling, although similar roles were assumed for central players, including
CLPP
. To better understand the mammalian
UPR
mt
signaling, we deleted
CLPP
in hearts of
DARS
2‐deficient animals that show robust induction of
UPR
mt
due to strong dysregulation of mitochondrial translation. Remarkably, our results clearly show that mammalian
CLPP
is neither required for, nor it regulates the
UPR
mt
in mammals. Surprisingly, we demonstrate that a strong mitochondrial cardiomyopathy and diminished respiration due to
DARS
2 deficiency can be alleviated by the loss of
CLPP
, leading to an increased
de novo
synthesis of individual
OXPHOS
subunits. These results question our current understanding of the
UPR
mt
signaling in mammals, while introducing
CLPP
as a possible novel target for therapeutic intervention in mitochondrial diseases.
Synopsis
image
Loss of mammalian ClpP does not affect the mitochondrial unfolded protein response (
UPR
mt
) under proteotoxic stress. Instead, loss of ClpP alleviates mitochondrial cardiomyopathy by partially rescuing the mitochondrial translation defect and reducing the level of potentially toxic peptides.
Loss of ClpP alleviates progressive mitochondrial cardiomyopathy.
ClpP is not required for mammalian UPR
mt
signaling.
Loss of ClpP slows down mitochondrial translation, thereby increasing the level of functional OXPHOS complexes in DARS2‐deficient mice.
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