A cannabinoid link between mitochondria and memory.

Cellular activity in the brain depends on the high energetic support provided by mitochondria, the cell organelles which use energy sources to generate ATP. Acute cannabinoid intoxication induces amnesia in humans and animals, and the activation of type-1 cannabinoid receptors present at brain mitochondria membranes (mtCB1) can directly alter mitochondrial energetic activity. Although the pathological impact of chronic mitochondrial dysfunctions in the brain is well established, the involvement of acute modulation of mitochondrial activity in high brain functions, including learning and memory, is unknown. Here, we show that acute cannabinoid-induced memory impairment in mice requires activation of hippocampal mtCB1 receptors. Genetic exclusion of CB1 receptors from hippocampal mitochondria prevents cannabinoid-induced reduction of mitochondrial mobility, synaptic transmission and memory formation. mtCB1 receptors signal through intra-mitochondrial Gαi protein activation and consequent inhibition of soluble-adenylyl cyclase (sAC). The resulting inhibition of protein kinase A (PKA)-dependent phosphorylation of specific subunits of the mitochondrial electron transport system eventually leads to decreased cellular respiration. Hippocampal inhibition of sAC activity or manipulation of intra-mitochondrial PKA signalling or phosphorylation of the Complex I subunit NDUFS2 inhibit bioenergetic and amnesic effects of cannabinoids. Thus, the G protein-coupled mtCB1 receptors regulate memory processes via modulation of mitochondrial energy metabolism. By directly linking mitochondrial activity to memory formation, these data reveal that bioenergetic processes are primary acute regulators of cognitive functions.

Mitochondrial morphology and cellular distribution are altered in SPG31 patients and are linked to DRP1 hyperphosphorylation.

Hereditary spastic paraplegia, SPG31, is a rare neurological disorder caused by mutations in REEP1 gene encoding the microtubule-interacting protein, REEP1. The mechanism by which REEP1-dependent processes are linked with the disease is unclear. REEP1 regulates the morphology and trafficking of various organelles via interaction with the microtubules. In this study, we collected primary fibroblasts from SPG31 patients to investigate their mitochondrial morphology. We observed that the mitochondrial morphology in patient cells was highly tubular compared with control cells. We provide evidence that these morphological alterations are caused by the inhibition of mitochondrial fission protein, DRP1, due to the hyperphosphorylation of its serine 637 residue. This hyperphosphorylation is caused by impaired interactions between REEP1 and mitochondrial phosphatase PGAM5. Genetically or pharmacologically induced decrease of DRP1-S637 phosphorylation restores mitochondrial morphology in patient cells. Furthermore, ectopic expression of REEP1 carrying pathological mutations in primary neuronal culture targets REEP1 to the mitochondria. Mutated REEP1 proteins sequester mitochondria to the perinuclear region of the neurons and therefore, hamper mitochondrial transport along the axon. Considering the established role of mitochondrial distribution and morphology in neuronal health, our results support the involvement of a mitochondrial dysfunction in SPG31 pathology.

CYP2U1 activity is altered by missense mutations in hereditary spastic paraplegia 56.

Hereditary spastic paraplegia (HSP) is an inherited disorder of the central nervous system mainly characterized by gradual spasticity and weakness of the lower limbs. SPG56 is a rare autosomal recessive early onset complicated form of HSP caused by mutations in CYP2U1. The CYP2U1 enzyme was shown to catalyze the hydroxylation of arachidonic acid. Here, we report two further SPG56 families carrying three novel CYP2U1 missense variants and the development of an in vitro biochemical assay to determine the pathogenicity of missense variants of uncertain clinical significance. We compared spectroscopic, enzymatic, and structural (from a 3D model) characteristics of the over expressed wild-type or mutated CYP2U1 in HEK293T cells. Our findings demonstrated that most of the tested missense variants in CYP2U1 were functionally inactive because of a loss of proper heme binding or destabilization of the protein structure. We also showed that functional data do not necessarily correlate with in silico predictions of variants pathogenicity, using different bioinformatic phenotype prediction tools. Our results therefore highlight the importance to use biological tools, such as the enzymatic test set up in this study, to evaluate the effects of newly identified variants in clinical settings.

High glucose repatterns human podocyte energy metabolism during differentiation and diabetic nephropathy.

Podocytes play a key role in diabetic nephropathy pathogenesis, but alteration of their metabolism remains unknown in human kidney. By using a conditionally differentiating human podocyte cell line, we addressed the functional and molecular changes in podocyte energetics during in vitro development or under high glucose conditions. In 5 mM glucose medium, we observed a stepwise activation of oxidative metabolism during cell differentiation that was characterized by peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α)-dependent stimulation of mitochondrial biogenesis and function, with concomitant reduction of the glycolytic enzyme content. Conversely, when podocytes were cultured in high glucose (20 mM), stepwise oxidative phosphorylation biogenesis was aborted, and a glycolytic switch occurred, with consecutive lactic acidosis. Expression of the master regulators of oxidative metabolism transcription factor A mitochondrial, PGC-1α, AMPK, and serine-threonine liver kinase B1 was altered by high glucose, as well as their downstream signaling networks. Focused transcriptomics revealed that myocyte-specific enhancer factor 2C (MEF2C) and myogenic factor 5 (MYF5) expression was inhibited by high glucose levels, and endoribonuclease-prepared small interfering RNA-mediated combined inhibition of those transcription factors phenocopied the glycolytic shift that was observed in high glucose conditions. Accordingly, a reduced expression of MEF2C, MYF5, and PGC-1α was found in kidney tissue sections that were obtained from patients with diabetic nephropathy. These findings obtained in human samples demonstrate that MEF2C-MYF5-dependent bioenergetic dedifferentiation occurs in podocytes that are confronted with a high-glucose milieu.-Imasawa, T., Obre, E., Bellance, N., Lavie, J., Imasawa, T., Rigothier, C., Delmas, Y., Combe, C., Lacombe, D., Benard, G., Claverol, S., Bonneu, M., Rossignol, R. High glucose repatterns human podocyte energy metabolism during differentiation and diabetic nephropathy.

Mitochondrial degradation and energy metabolism.

Mitochondria are intracellular power plants that feed most eukaryotic cells with the ATP produced by the oxidative phosphorylation (OXPHOS). Mitochondrial energy production is controlled by many regulatory mechanisms. The control of mitochondrial mass through both mitochondrial biogenesis and degradation has been proposed to be one of the most important regulatory mechanisms. Recently, autophagic degradation of mitochondria has emerged as an important mechanism involved in the regulation of mitochondrial quantity and quality. In this review, we highlight the intricate connections between mitochondrial energy metabolism and mitochondrial autophagic degradation by showing the importance of mitochondrial bioenergetics in this process and illustrating the role of mitophagy in mitochondrial patho-physiology. Furthermore, we discuss how energy metabolism could coordinate the biogenesis and degradation of this organelle.

New practical definitions for the diagnosis of autosomal recessive spastic ataxia of Charlevoix-Saguenay.

OBJECTIVE: Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is caused by mutations in the SACS gene. SACS encodes sacsin, a protein whose function remains unknown, despite the description of numerous protein domains and the recent focus on its potential role in the regulation of mitochondrial physiology. This study aimed to identify new mutations in a large population of ataxic patients and to functionally analyze their cellular effects in the mitochondrial compartment. METHODS: A total of 321 index patients with spastic ataxia selected from the SPATAX network were analyzed by direct sequencing of the SACS gene, and 156 patients from the ATAXIC project presenting with congenital ataxia were investigated either by targeted or whole exome sequencing. For functional analyses, primary cultures of fibroblasts were obtained from 11 patients carrying either mono- or biallelic variants, including 1 case harboring a large deletion encompassing the entire SACS gene. RESULTS: We identified biallelic SACS variants in 33 patients from SPATAX, and in 5 nonprogressive ataxia patients from ATAXIC. Moreover, a drastic and recurrent alteration of the mitochondrial network was observed in 10 of the 11 patients tested. INTERPRETATION: Our results permit extension of the clinical and mutational spectrum of ARSACS patients. Moreover, we suggest that the observed mitochondrial network anomalies could be used as a trait biomarker for the diagnosis of ARSACS when SACS molecular results are difficult to interpret (ie, missense variants and heterozygous truncating variant). Based on our findings, we propose new diagnostic definitions for ARSACS using clinical, genetic, and cellular criteria.

IBRDC2, an IBR-type E3 ubiquitin ligase, is a regulatory factor for Bax and apoptosis activation.

Bax, a pro-apoptotic protein from the Bcl-2 family, is central to apoptosis regulation. To suppress spontaneous apoptosis, Bax must be under stringent control that may include regulation of Bax conformation and expression levels. We report that IBRDC2, an IBR-type RING-finger E3 ubiquitin ligase, regulates the levels of Bax and protects cells from unprompted Bax activation and cell death. Downregulation of IBRDC2 induces increased cellular levels and accumulation of the active form of Bax. The ubiquitination-dependent regulation of Bax stability is suppressed by IBRDC2 downregulation and stimulated by IBRDC2 overexpression in both healthy and apoptotic cells. Although mostly cytosolic in healthy cells, upon induction of apoptosis, IBRDC2 accumulates in mitochondrial domains enriched with Bax. Mitochondrial accumulation of IBRDC2 occurs in parallel with Bax activation and also depends on the expression levels of Bcl-xL. Furthermore, IBRDC2 physically interacts with activated Bax. By applying Bax mutants in HCT116 Bax(-/-) cells, combined with the use of active Bax-specific antibodies, we have established that both mitochondrial localization and apoptotic activation of Bax are required for IBRDC2 translocation to the mitochondria.

Disruption of the histidine triad nucleotide-binding hint2 gene in mice affects glycemic control and mitochondrial function.

UNLABELLED: The histidine triad nucleotide-binding (HINT2) protein is a mitochondrial adenosine phosphoramidase expressed in the liver and pancreas. Its physiological function is unknown. To elucidate the role of HINT2 in liver physiology, the mouse Hint2 gene was deleted. Hint2(-/-) and Hint2(+/+) mice were generated in a mixed C57Bl6/J × 129Sv background. At 20 weeks, the phenotypic changes in Hint2(-/-) relative to Hint2(+/+) mice were an accumulation of hepatic triglycerides, decreased tolerance to glucose, a defective counter-regulatory response to insulin-provoked hypoglycemia, and an increase in plasma interprandial insulin but a decrease in glucose-stimulated insulin secretion and defective thermoregulation upon fasting. Leptin messenger RNA (mRNA) in adipose tissue and plasma leptin were elevated. In mitochondria from Hint2(-/-) hepatocytes, state 3 respiration was decreased, a finding confirmed in HepG2 cells where HINT2 mRNA was silenced. The linked complex II-III electron transfer was decreased in Hint2(-/-) mitochondria, which was accompanied by a lower content of coenzyme Q. Hypoxia-inducible factor-2α expression and the generation of reactive oxygen species were increased. Electron microscopy of mitochondria in Hint2(-/-) mice aged 12 months revealed clustered, fused organelles. The hepatic activities of 3-hydroxyacyl-coenzyme A dehydrogenase short chain and glutamate dehydrogenase (GDH) were decreased by 68% and 60%, respectively, without a change in protein expression. GDH activity was similarly decreased in HINT2-silenced HepG2 cells. When measured in the presence of purified sirtuin 3, latent GDH activity was recovered (126% in Hint2(-/-) versus 83% in Hint2(+/+) ). This suggests a greater extent of acetylation in Hint2(-/-) than in Hint2(+/+) . CONCLUSION: Hint2/HINT2 positively regulates mitochondrial lipid metabolism and respiration and glucose homeostasis. The absence of Hint2 provokes mitochondrial deformities and a change in the pattern of acetylation of selected proteins.

Metabolic consequences of NDUFS4 gene deletion in immortalized mouse embryonic fibroblasts.

Human mitochondrial complex I (CI) deficiency is associated with progressive neurological disorders. To better understand the CI pathomechanism, we here studied how deletion of the CI gene NDUFS4 affects cell metabolism. To this end we compared immortalized mouse embryonic fibroblasts (MEFs) derived from wildtype (wt) and whole-body NDUFS4 knockout (KO) mice. Mitochondria from KO cells lacked the NDUFS4 protein and mitoplasts displayed virtually no CI activity, moderately reduced CII, CIII and CIV activities and normal citrate synthase and CV (F(o)F(1)-ATPase) activity. Native electrophoresis of KO cell mitochondrial fractions revealed two distinct CI subcomplexes of ~830kDa (enzymatically inactive) and ~200kDa (active). The level of fully-assembled CII-CV was not affected by NDUFS4 gene deletion. KO cells exhibited a moderately reduced maximal and routine O(2) consumption, which was fully inhibited by acute application of the CI inhibitor rotenone. The aberrant CI assembly and reduced O(2) consumption in KO cells were fully normalized by NDUFS4 gene complementation. Cellular [NAD(+)]/[NADH] ratio, lactate production and mitochondrial tetramethyl rhodamine methyl ester (TMRM) accumulation were slightly increased in KO cells. In contrast, NDUFS4 gene deletion did not detectably alter [NADP(+)]/[NADPH] ratio, cellular glucose consumption, the protein levels of hexokinases (I and II) and phosphorylated pyruvate dehydrogenase (P-PDH), total cellular adenosine triphosphate (ATP) level, free cytosolic [ATP], cell growth rate, and reactive oxygen species (ROS) levels. We conclude that the NDUFS4 subunit is of key importance in CI stabilization and that, due to the metabolic properties of the immortalized MEFs, NDUFS4 gene deletion has only modest effects at the live cell level. This article is part of a special issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

The E3 ubiquitin ligase FBXL6 controls the quality of newly synthesized mitochondrial ribosomal proteins.

In mammals, about 99% of mitochondrial proteins are synthesized in the cytosol as precursors that are subsequently imported into the organelle. The mitochondrial health and functions rely on an accurate quality control of these imported proteins. Here, we show that the E3 ubiquitin ligase F box/leucine-rich-repeat protein 6 (FBXL6) regulates the quality of cytosolically translated mitochondrial proteins. Indeed, we found that FBXL6 substrates are newly synthesized mitochondrial ribosomal proteins. This E3 binds to chaperones involved in the folding and trafficking of newly synthesized peptide and to ribosomal-associated quality control proteins. Deletion of these interacting partners is sufficient to hamper interactions between FBXL6 and its substrate. Furthermore, we show that cells lacking FBXL6 fail to degrade specifically mistranslated mitochondrial ribosomal proteins. Finally, showing the role of FBXL6-dependent mechanism, FBXL6-knockout (KO) cells display mitochondrial ribosomal protein aggregations, altered mitochondrial metabolism, and inhibited cell cycle in oxidative conditions.

XPC silencing in normal human keratinocytes triggers metabolic alterations through NOX-1 activation-mediated reactive oxygen species.

Cancer cells utilize complex mechanisms to remodel their bioenergetic properties. We exploited the intrinsic genomic stability of xeroderma pigmentosum C (XPC) to understand the inter-relationships between genomic instability, reactive oxygen species (ROS) generation, and metabolic alterations during neoplastic transformation. We showed that knockdown of XPC (XPC(KD)) in normal human keratinocytes results in metabolism remodeling through NADPH oxidase-1 (NOX-1) activation, which in turn leads to increased ROS levels. While enforcing antioxidant defenses by overexpressing catalase, CuZnSOD, or MnSOD could not block the metabolism remodeling, impaired NOX-1 activation abrogates both alteration in ROS levels and modifications of energy metabolism. As NOX-1 activation is observed in human squamous cell carcinomas (SCCs), the blockade of NOX-1 could be a target for the prevention and the treatment of skin cancers.

REEP1 mutations in SPG31: frequency, mutational spectrum, and potential association with mitochondrial morpho-functional dysfunction.

Hereditary spastic paraplegias (HSP) constitute a heterogeneous group of neurodegenerative disorders characterized at least by slowly progressive spasticity of the lower limbs. Mutations in REEP1 were recently associated with a pure dominant HSP, SPG31. We sequenced all exons of REEP1 and searched for rearrangements by multiplex ligation-dependent probe amplification (MLPA) in a large panel of 175 unrelated HSP index patients from kindreds with dominant inheritance (AD-HSP), with either pure (n = 102) or complicated (n = 73) forms of the disease, after exclusion of other known HSP genes. We identified 12 different heterozygous mutations, including two exon deletions, associated with either a pure or a complex phenotype. The overall mutation rate in our clinically heterogeneous sample was 4.5% in French families with AD-HSP. The phenotype was restricted to pyramidal signs in the lower limbs in most patients but nine had a complex phenotype associating axonal peripheral neuropathy (= 5/11 patients) including a Silver-like syndrome in one patient, and less frequently cerebellar ataxia, tremor, dementia. Interestingly, we evidenced abnormal mitochondrial network organization in fibroblasts of one patient in addition to defective mitochondrial energy production in both fibroblasts and muscle, but whether these anomalies are directly or indirectly related to the mutations remains uncertain.

Ubiquitin-Dependent Degradation of Mitochondrial Proteins Regulates Energy Metabolism.

The ubiquitin proteasome system (UPS) regulates many cellular functions by degrading key proteins. Notably, the role of UPS in regulating mitochondrial metabolic functions is unclear. Here, we show that ubiquitination occurs in different mitochondrial compartments, including the inner mitochondrial membrane, and that turnover of several metabolic proteins is UPS dependent. We specifically detailed mitochondrial ubiquitination and subsequent UPS-dependent degradation of succinate dehydrogenase subunit A (SDHA), which occurred when SDHA was minimally involved in mitochondrial energy metabolism. We demonstrate that SDHA ubiquitination occurs inside the organelle. In addition, we show that the specific inhibition of SDHA degradation by UPS promotes SDHA-dependent oxygen consumption and increases ATP, malate, and citrate levels. These findings suggest that the mitochondrial metabolic machinery is also regulated by the UPS.

Complete loss of function of the ubiquitin ligase HERC2 causes a severe neurodevelopmental phenotype.

The ubiquitin-proteasome pathway is involved in the pathogenesis of several neurogenetic diseases. We describe a Mauritanian patient harboring a homozygous deletion restricted to two contiguous genes HERC2 and OCA2 and presenting with severe developmental abnormalities. The deletion causes the complete loss of HERC2 protein function, an E3-ubiquitin ligase. HERC2 is known to target XPA and BRCA1 for degradation and a mechanism whereby it is involved in DNA repair and cell cycle regulation. We showed that loss of HERC2 function leads to the accumulation of XPA and BRCA1 in the patient's fibroblasts and generates decreased sensitivity to apoptosis and increased level of DNA repair. Our data describe for the first time the phenotypic consequences, both at the clinical and cellular levels, of a complete loss of HERC2 function in a patient. They strongly suggest that profound ubiquitin ligase - associated dysfunction is responsible for the severe phenotype in this patient, and that dysfunction of this pathway may be involved in other patients with similar neurodevelopmental diseases.

Cannabinoid CB1 Receptors Are Localized in Striated Muscle Mitochondria and Regulate Mitochondrial Respiration.

The cannabinoid type 1 (CB1) receptor is widely distributed in the brain and peripheral organs where it regulates cellular functions and metabolism. In the brain, CB1 is mainly localized on presynaptic axon terminals but is also found on mitochondria (mtCB1), where it regulates cellular respiration and energy production. Likewise, CB1 is localized on muscle mitochondria, but very little is known about it. The aim of this study was to further investigate in detail the distribution and functional role of mtCB1 in three different striated muscles. Immunoelectron microscopy for CB1 was used in skeletal muscles (gastrocnemius and rectus abdominis) and myocardium from wild-type and CB1 -KO mice. Functional assessments were performed in mitochondria purified from the heart of the mice and the mitochondrial oxygen consumption upon application of different acute delta-9-tetrahydrocannabinol (Δ9-THC) concentrations (100 nM or 200 nM) was monitored. About 26% of the mitochondrial profiles in gastrocnemius, 22% in the rectus abdominis and 17% in the myocardium expressed CB1. Furthermore, the proportion of mtCB1 versus total CB1 immunoparticles was about 60% in the gastrocnemius, 55% in the rectus abdominis and 78% in the myocardium. Importantly, the CB1 immunolabeling pattern disappeared in muscles of CB1 -KO mice. Functionally, acute 100 nM or 200 nM THC treatment specifically decreased mitochondria coupled respiration between 12 and 15% in wild-type isolated mitochondria of myocardial muscles but no significant difference was noticed between THC treated and vehicle in mitochondria isolated from CB1 -KO heart. Furthermore, gene expression of key enzymes involved in pyruvate synthesis, tricarboxylic acid (TCA) cycle and mitochondrial respiratory chain was evaluated in the striated muscle of CB1 -WT and CB1 -KO. CB1 -KO showed an increase in the gene expression of Eno3, Pkm2, and Pdha1, suggesting an increased production of pyruvate. In contrast, no significant difference was observed in the Sdha and Cox4i1 expression, between CB1 -WT and CB1 -KO. In conclusion, CB1 receptors in skeletal and myocardial muscles are predominantly localized in mitochondria. The activation of mtCB1 receptors may participate in the mitochondrial regulation of the oxidative activity probably through the relevant enzymes implicated in the pyruvate metabolism, a main substrate for TCA activity.

TRIM4; a novel mitochondrial interacting RING E3 ligase, sensitizes the cells to hydrogen peroxide (H2O2) induced cell death.

The emerging evidences suggest that posttranslational modification of target protein by ubiquitin (Ub) not only regulate its turnover through ubiquitin proteasome system (UPS) but is a critical regulator of various signaling pathways. During ubiquitination, E3 ligase recognizes the target protein and determines the topology of ubiquitin chains. In current study, we studied the role of TRIM4, a member of the TRIM/RBCC protein family of RING E3 ligase, in regulation of hydrogen peroxide (H2O2) induced cell death. TRIM4 is expressed differentially in human tissues and expressed in most of the analyzed human cancer cell lines. The subcellular localization studies showed that TRIM4 forms distinct cytoplasmic speckle like structures which transiently interacts with mitochondria. The expression of TRIM4 induces mitochondrial aggregation and increased level of mitochondrial ROS in the presence of H2O2. It sensitizes the cells to H2O2 induced death whereas knockdown reversed the effect. TRIM4 potentiates the loss of mitochondrial transmembrane potential and cytochrome c release in the presence of H2O2. The analysis of TRIM4 interacting proteins showed its interaction with peroxiredoxin 1 (PRX1), including other proteins involved in regulation of mitochondrial and redox homeostasis. TRIM4 interaction with PRX1 is critical for the regulation of H2O2 induced cell death. Collectively, the evidences in the current study suggest the role of TRIM4 in regulation of oxidative stress induced cell death.

Cannabinoid control of brain bioenergetics: Exploring the subcellular localization of the CB1 receptor.

Brain mitochondrial activity is centrally involved in the central control of energy balance. When studying mitochondrial functions in the brain, however, discrepant results might be obtained, depending on the experimental approaches. For instance, immunostaining experiments and biochemical isolation of organelles expose investigators to risks of false positive and/or false negative results. As an example, the functional presence of cannabinoid type 1 (CB1) receptors on brain mitochondrial membranes (mtCB1) was recently reported and rapidly challenged, claiming that the original observation was likely due to artifact results. Here, we addressed this issue by directly comparing the procedures used in the two studies. Our results show that the use of appropriate controls and quantifications allows detecting mtCB1 receptor with CB1 receptor antibodies, and that, if mitochondrial fractions are enriched and purified, CB1 receptor agonists reliably decrease respiration in brain mitochondria. These data further underline the importance of adapted experimental procedures to study brain mitochondrial functions.

Mitoplasticity: adaptation biology of the mitochondrion to the cellular redox state in physiology and carcinogenesis.

Adaptation and transformation biology of the mitochondrion to redox status is an emerging domain of physiology and pathophysiology. Mitochondrial adaptations occur in response to accidental changes in cellular energy demand or supply while mitochondrial transformations are a part of greater program of cell metamorphosis. The possible role of mitochondrial adaptations and transformations in pathogenesis remains unexplored, and it has become critical to decipher the stimuli and the underlying molecular pathways. Immediate activation of mitochondrial function was described during acute exercise, respiratory chain injury, Endoplasmic Reticulum stress, genotoxic stress, or environmental toxic insults. Delayed adaptations of mitochondrial form, composition, and functions were evidenced for persistent changes in redox status as observed in endurance training, in fibroblasts grown in presence of respiratory chain inhibitors or in absence of glucose, in the smooth muscle of patients with severe asthma, or in the skeletal muscle of patients with a mitochondrial disease. Besides, mitochondrial transformations were observed in the course of human cell differentiation, during immune response activation, or in cells undergoing carcinogenesis. Little is known on the signals and downstream pathways that govern mitochondrial adaptations and transformations. Few adaptative loops, including redox sensors, kinases, and transcription factors were deciphered, but their implication in physiology and pathology remains elusive. Mitoplasticity could play a protective role against aging, diabetes, cancer, or neurodegenerative diseases. Research on adaptation and transformation could allow the design of innovative therapies, notably in cancer.

Adaptative capacity of mitochondrial biogenesis and of mitochondrial dynamics in response to pathogenic respiratory chain dysfunction.

AIMS: Cellular energy homeostasy relies on mitochondrial plasticity, the molecular determinants of which are multiple. Yet, the relative contribution of and possible cooperation between mitochondrial biogenesis and morphogenesis to cellular energy homeostasy remains elusive. Here we analyzed the adaptative capacity of mitochondrial content and dynamics in muscle biopsies of patients with a complex IV defect, and in skin fibroblasts challenged with complex IV inhibition. RESULTS: We observed a biphasic variation of the mitochondrial content upon complex IV inhibition in muscle biopsies and in skin fibroblasts. Adjustment of mitochondrial content for respiratory maintenance was blocked by using a dominant negative form of CREB (CREB-M1) and by L-NAME, a blocker of NO production. Accordingly, cells treated with KCN 6 µM showed higher levels of phospho-CREB, PGC1α mRNA, eNOS mRNA, and mtTFA mRNA. We also observed the increased expression of the fission protein DRP1 during fibroblasts adaptation, as well as mitochondrial ultrastructural defects indicative of increased fission in patients muscle micrographs. Accordingly, the expression of a dominant negative form of DRP1 (K38A mutant) reduced the biogenic response in fibroblasts challenged with 6 µM KCN. INNOVATION: Our findings indicate that mitochondrial biogenesis and mitochondrial fission cooperate to promote cellular adaptation to respiratory chain inhibition. CONCLUSIONS: Our data show for the first time that DRP1 intervenes during the initiation of the mitochondrial adaptative response to respiratory chain defects. The evidenced pathway of mitochondrial adaptation to respiratory chain deficiency provides a safety mechanism against mitochondrial dysfunction.