Mammalian Mss51 is a skeletal muscle-specific gene modulating cellular metabolism

YTHDF2 regulates MSS51 expression contributing to mitochondria dysfunction of granulosa cells in polycystic ovarian syndrome patients

RESEARCH QUESTION

Granulosa cells (GCs) dysfunction plays a crucial role in the pathogenesis of polycystic ovary syndrome (PCOS). It is reported that YTH domain-containing family protein 2 (YTHDF2) is upregulated in mural GCs of PCOS patients. What effect does the differential expression of YTHDF2 have in PCOS patients?

DESIGN

Mural GCs and cumulus GCs from 15 patients with PCOS and 15 ovulatory controls and 4 cases of pathological sections in each group were collected. Real-time PCR, Western Blot, immunohistochemistry, and immunofluorescence experiments were conducted to detect gene and protein expression. RNA immunoprecipitation assay was performed to evaluate the binding relationship between YTHDF2 and MSS51. Mitochondrial morphology, cellular ATP and ROS levels and glycolysis-related gene expression were detected after YTHDF2 overexpression or MSS51 inhibition.

RESULTS

In the present study, we found that YTHDF2 was upregulated in GCs of PCOS patients while MSS51 was downregulated. YTHDF2 protein can bind to MSS51 mRNA and affect MSS51 expression. The reduction of MSS51 expression or the increase in YTHDF2 expression can lead to mitochondrial damage, reduced ATP levels, increased ROS levels and reduced expression of LDHA, PFKP and PKM.

CONCLUSIONS

YTHDF2 may regulate the expression of MSS51, affecting the structure and function of mitochondria in GCs and interfering with cellular glycolysis, which may disturb the normal biological processes of GCs and follicle development in PCOS patients.

Identification of myokines susceptible to improve glucose homeostasis after bariatric surgery

IMPORTANCE AND OBJECTIVE

The identification of myokines susceptible to improve glucose homeostasis following bariatric surgery could lead to new therapeutic approaches for type 2 diabetes.

METHODS

Changes in the homeostasis model assessment (HOMA) test were assessed in patients before and 3 months after bariatric surgery. Changes in myokines expression and circulating levels were assessed using real-time quantitative polymerase chain reaction (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA). Myokines known to regulate glucose homeostasis were identified using literature (targeted study) and putative myokines using RNA-sequencing (untargeted study). A linear regression analysis adjusted for age and sex was used to search for associations between changes in the HOMA test and changes in myokines.

RESULTS

In the targeted study, brain-derived neurotrophic factor (BDNF) expression was upregulated (+30%, P = .006) while BDNF circulating levels were decreased (-12%, P = .001). Upregulated BDNF expression was associated with decreased HOMA of insulin resistance (HOMA-IR) (adjusted estimate [95% confidence interval {CI}]: -0.51 [-0.88 to -0.13], P = .010). Decreased BDNF serum levels were associated with decreased HOMA of beta-cell function (HOMA-B) (adjusted estimate [95% CI] = 0.002 [0.00002-0.0031], P = .046). In the untargeted study, upregulated putative myokines included XYLT1 (+64%, P < .001), LGR5 (+57, P< .001), and SPINK5 (+46%, P < .001). Upregulated LGR5 was associated with decreased HOMA-IR (adjusted estimate [95% CI] = -0.50 [-0.86 to -0.13], P = .009). Upregulated XYLT1 and SPINK5 were associated with increased HOMA of insulin sensitivity (HOMA-S) (respectively, adjusted estimate [95% CI] = 109.1 [28.5-189.8], P = .009 and 16.5 [0.87-32.19], P = .039).

CONCLUSIONS

Improved glucose homeostasis following bariatric surgery is associated with changes in myokines expression and circulating levels. In particular, upregulation of BDNF, XYLT1, SPINK5, and LGR5 is associated with improved insulin sensitivity. These results suggest that these myokines could contribute to improved glucose homeostasis following bariatric surgery.

STUDY REGISTRATION

NCT03341793 on ClinicalTrials.gov (https://clinicaltrials.gov/).

Site-1 protease inhibits mitochondrial respiration by controlling the TGF-β target gene Mss51

The mitochondrial response to changes in cellular energy demand is necessary for cellular adaptation and organ function. Many genes are essential in orchestrating this response, including the transforming growth factor (TGF)-β1 target gene Mss51, an inhibitor of skeletal muscle mitochondrial respiration. Although Mss51 is implicated in the pathophysiology of obesity and musculoskeletal disease, how Mss51 is regulated is not entirely understood. Site-1 protease (S1P) is a key activator of several transcription factors required for cellular adaptation. However, the role of S1P in muscle is unknown. Here, we identify S1P as a negative regulator of muscle mass and mitochondrial respiration. S1P disruption in mouse skeletal muscle reduces Mss51 expression and increases muscle mass and mitochondrial respiration. The effects of S1P deficiency on mitochondrial activity are counteracted by overexpressing Mss51, suggesting that one way S1P inhibits respiration is by regulating Mss51. These discoveries expand our understanding of TGF-β signaling and S1P function.

Transcriptome-based analysis of early post-mortem formation of pale, soft, and exudative (PSE) pork

Pale, soft, and exudative (PSE) meat can cause consumer dissatisfaction and economic losses. This study determined meat quality, glycolytic enzyme activity, and differential gene expression in the longissimus lumborum (LL) and semimembranosus (SM) of normal and PSE pork carcasses. The SM did not result in PSE meat. Hexokinase, lactate dehydrogenase, and pyruvate kinase activities were lower in the SM of PSE carcasses than in the normal carcasses. Functional enrichment analysis revealed that immune, inflammatory, and muscle fibre genes were significantly enriched in PSE pork. More specifically, PPP1R3G and MSS51 may be key genes regulating pork quality in the SM. Meanwhile, the differential expression of PLVAB, ADIPOQ, LEP, MYH4, MYH7, MYL3, MYL6B, FOS, ATF3, and HSPA6 may induce PSE formation in the LL. These results may provide insights into PSE pork formation mechanisms and reveal candidate genes for improving meat quality after validation.

Inhibition of the immunoproteasome modulates innate immunity to ameliorate muscle pathology of dysferlin-deficient BlAJ mice

Muscle repair in dysferlinopathies is defective. Although macrophage (Mø)-rich infiltrates are prominent in damaged skeletal muscles of patients with dysferlinopathy, the contribution of the immune system to the disease pathology remains to be fully explored. Numbers of both pro-inflammatory M1 Mø and effector T cells are increased in muscle of dysferlin-deficient BlAJ mice. In addition, symptomatic BlAJ mice have increased muscle production of immunoproteasome. In vitro analyses using bone marrow-derived Mø of BlAJ mice show that immunoproteasome inhibition results in C3aR1 and C5aR1 downregulation and upregulation of M2-associated signaling. Administration of immunoproteasome inhibitor ONX-0914 to BlAJ mice rescues muscle function by reducing muscle infiltrates and fibro-adipogenesis. These findings reveal an important role of immunoproteasome in the progression of muscular dystrophy in BlAJ mouse and suggest that inhibition of immunoproteasome may produce therapeutic benefit in dysferlinopathy.

Balanced mitochondrial and cytosolic translatomes underlie the biogenesis of human respiratory complexes

BACKGROUND

Oxidative phosphorylation (OXPHOS) complexes consist of nuclear and mitochondrial DNA-encoded subunits. Their biogenesis requires cross-compartment gene regulation to mitigate the accumulation of disproportionate subunits. To determine how human cells coordinate mitochondrial and nuclear gene expression processes, we tailored ribosome profiling for the unique features of the human mitoribosome.

RESULTS

We resolve features of mitochondrial translation initiation and identify a small ORF in the 3' UTR of MT-ND5. Analysis of ribosome footprints in five cell types reveals that average mitochondrial synthesis levels correspond precisely to cytosolic levels across OXPHOS complexes, and these average rates reflect the relative abundances of the complexes. Balanced mitochondrial and cytosolic synthesis does not rely on rapid feedback between the two translation systems, and imbalance caused by mitochondrial translation deficiency is associated with the induction of proteotoxicity pathways.

CONCLUSIONS

Based on our findings, we propose that human OXPHOS complexes are synthesized proportionally to each other, with mitonuclear balance relying on the regulation of OXPHOS subunit translation across cellular compartments, which may represent a proteostasis vulnerability.

Resequencing the Yaroslavl cattle genomes reveals signatures of selection and a rare haplotype on BTA28 likely to be related to breed phenotypes

The genomes of local livestock could shed light on their genetic history, mechanisms of adaptations to environments and unique genetics. Herein we look into the genetics and adaptations of the Russian native dairy Yaroslavl cattle breed using 22 resequenced individuals and comparing them with two related breeds (Russian Kholmogory and Holstein), and to the taurine set of the 1000 Bull Genomes Project (Run 9). HapFLK analysis with Kholmogory and Holstein breeds (using Yakut cattle as outgroup) resulted in 22 regions under selection (q‐value < 0.01) on 11 chromosomes assigned to Yaroslavl cattle, including a strong signature of selection in the region of the KIT gene on BTA6. The F_ST (fixation index) with the 1000 Bull Genomes Dataset showed 48 non‐overlapping top (0.1%) F_ST regions of which three overlapped HapFLK regions. We identified 1982 highly differentiated (F_ST > 0.40) missense mutations in the Yaroslavl genomes. These genes were enriched in the epidermal growth factor and calcium‐binding functional categories. The top F_ST intervals contained eight genes with allele frequencies quite different between the Yaroslavl and Kholmogory breeds and the rest of the 1000 Bull Genomes Dataset, including KAT6B, which had a nearly Yaroslavl breed‐specific deleterious missense mutation with the highest F_ST in our dataset (0.99). This gene is a part of a long haplotype containing other genes from F_ST and hapFLK analyses and with a negative association with weight and carcass traits according to the genotyping of 30 phenotyped Yaroslavl cattle individuals. Our work provides the industry with candidate genetic variants to be focused on in breed improvement efforts.

Yeast Translational Activator Mss51p and Human ZMYND17 - Two Proteins with a Common Origin, but Different Functions

Despite its similarity to protein biosynthesis in bacteria, translation in the mitochondria of modern eukaryotes has several unique features, such as the necessity for coordination of translation of mitochondrial mRNAs encoding proteins of the electron transport chain complexes with translation of other protein components of these complexes in the cytosol. In the mitochondria of baker's yeast Saccharomyces cerevisiae, this coordination is carried out by a system of translational activators that predominantly interact with the 5'-untranslated regions of mitochondrial mRNAs. No such system has been found in human mitochondria, except a single identified translational activator, TACO1. Here, we studied the role of the ZMYND17 gene, an ortholog of the yeast gene for the translational activator Mss51p, on the mitochondrial translation in human cells. Deletion of the ZMYND17 gene did not affect translation in the mitochondria, but led to the decrease in the cytochrome c oxidase activity and increase in the amount of free F1 subunit of ATP synthase. We also investigated the evolutionary history of Mss51p and ZMYND17 and suggested a possible mechanism for the divergence of functions of these orthologous proteins.

Metabolism: Evolution of dolphin sperm endurance

Mammalian sperm have long been known to use energy derived from the metabolism of sugars and fatty acids. A new study shows that sperm of dolphins and their relatives lost functionality of the glycolysis pathway and are fueled only by energy-rich fatty acids that are metabolized by extra-large mitochondria, giving them exceptional endurance.

Muscle transcriptome in mouse models of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder that is most often caused by mutations in collagen type I encoding genes. Even though bone fragility is the most conspicuous finding in OI, the muscle system is also affected. In the present study we explored the muscle phenotype related to collagen type I mutations on the transcriptome level. RNA sequencing was performed in gastrocnemius muscles of homozygous oim mice and of heterozygous Jrt mice, two models of severe OI. We found that oim and Jrt mice shared 27 differentially expressed genes, of which 11 were concordantly upregulated and 15 concordantly downregulated. Gene Set Enrichment Analysis revealed that in both oim and Jrt mice, genes involved in 'metabolism of lipids' were significantly enriched among upregulated genes. In addition, several genes coding for extracellular matrix components were upregulated in both oim and Jrt mice. Among downregulated genes, genes involved in 'muscle contraction' were enriched in both OI mouse models. These 'muscle contraction' genes coded for slow-twitch type I muscle fiber components. Another shared downregulated gene was Mss51, a metabolic stress-inducible factor that is found in mitochondria. These data show that two mouse models of severe OI share abnormalities in the expression of genes that code for extracellular matrix proteins, lipid and energy metabolism and structural proteins of type I muscle fibers. The muscle disturbances resulting from the collagen type I mutations in these mouse models could be viewed as a mild form of muscle dystrophy.

Mss51 deletion increases endurance and ameliorates histopathology in the mdx mouse model of Duchenne muscular dystrophy

Mitochondrial derangement is an important contributor to the pathophysiology of muscular dystrophies and may be among the earliest cellular deficits. We have previously shown that disruption of Mss51, a mammalian skeletal muscle protein that localizes to the mitochondria, results in enhanced muscle oxygen consumption rate, increased endurance capacity, and improved limb muscle strength in mice with wildtype background. Here, we investigate whether Mss51 deletion in the mdx murine model of Duchenne muscular dystrophy (mdx-Mss51 KO) counteracts the muscle pathology and mitochondrial irregularities observed in mdx mice. We found that mdx-Mss51 KO mice had increased myofiber oxygen consumption rates and an amelioration of muscle histopathology compared to mdx counterparts. This corresponded with greater treadmill endurance and less percent fatigue in muscle physiology, but no improvement in forelimb grip strength or limb muscle force production. These findings suggest that although Mss51 deletion ameliorates the skeletal muscle mitochondrial respiration defects in mdx and improves fatigue resistance in vivo, the lack of improvement in force production suggests that this target alone may be insufficient for a therapeutic effect.

Current and Future Therapeutic Strategies for Limb Girdle Muscular Dystrophy Type R1: Clinical and Experimental Approaches

Limb girdle muscular dystrophy type R1 disease is a progressive disease that is caused by mutations in the CAPN3 gene and involves the extremity muscles of the hip and shoulder girdle. The CAPN3 protein has proteolytic and non-proteolytic properties. The functions of the CAPN3 protein that have been determined so far can be listed as remodeling and combining contractile proteins in the sarcomere with the substrates with which it interacts, controlling the Ca²⁺ flow in and out through the sarcoplasmic reticulum, and regulation of membrane repair and muscle regeneration. Even though there are several gene therapies, cellular therapies, and drug therapies, such as glucocorticoid treatment, AAV- mediated therapy, CRISPR-Cas9, induced pluripotent stem cells, MYO-029, and AMBMP, which are either in preclinical or clinical phases, or have been completed, there is no final cure. Inhibitors and small molecules (tauroursodeoxycholic acid, salubrinal, rapamycin, CDN1163, dwarf open reading frame) targeting ER stress factors that are thought to be effective in muscle loss can be considered potential therapy strategies. At present, little can be done to treat the progressive muscle wasting, loss of function, and premature mortality of patients with LGMDR1, and there is a pressing need for more research to develop potential therapies.

Skeletal muscle transcriptome in healthy aging

Age-associated changes in gene expression in skeletal muscle of healthy individuals reflect accumulation of damage and compensatory adaptations to preserve tissue integrity. To characterize these changes, RNA was extracted and sequenced from muscle biopsies collected from 53 healthy individuals (22-83 years old) of the GESTALT study of the National Institute on Aging-NIH. Expression levels of 57,205 protein-coding and non-coding RNAs were studied as a function of aging by linear and negative binomial regression models. From both models, 1134 RNAs changed significantly with age. The most differentially abundant mRNAs encoded proteins implicated in several age-related processes, including cellular senescence, insulin signaling, and myogenesis. Specific mRNA isoforms that changed significantly with age in skeletal muscle were enriched for proteins involved in oxidative phosphorylation and adipogenesis. Our study establishes a detailed framework of the global transcriptome and mRNA isoforms that govern muscle damage and homeostasis with age.

The Failed Clinical Story of Myostatin Inhibitors against Duchenne Muscular Dystrophy: Exploring the Biology behind the Battle

Myostatin inhibition therapy has held much promise for the treatment of muscle wasting disorders. This is particularly true for the fatal myopathy, Duchenne Muscular Dystrophy (DMD). Following on from promising pre-clinical data in dystrophin-deficient mice and dogs, several clinical trials were initiated in DMD patients using different modality myostatin inhibition therapies. All failed to show modification of disease course as dictated by the primary and secondary outcome measures selected: the myostatin inhibition story, thus far, is a failed clinical story. These trials have recently been extensively reviewed and reasons why pre-clinical data collected in animal models have failed to translate into clinical benefit to patients have been purported. However, the biological mechanisms underlying translational failure need to be examined to ensure future myostatin inhibitor development endeavors do not meet with the same fate. Here, we explore the biology which could explain the failed translation of myostatin inhibitors in the treatment of DMD.

Metabolic shift underlies recovery in reversible infantile respiratory chain deficiency

Reversible infantile respiratory chain deficiency (RIRCD) is a rare mitochondrial myopathy leading to severe metabolic disturbances in infants, which recover spontaneously after 6-months of age. RIRCD is associated with the homoplasmic m.14674T>C mitochondrial DNA mutation; however, only ~ 1/100 carriers develop the disease. We studied 27 affected and 15 unaffected individuals from 19 families and found additional heterozygous mutations in nuclear genes interacting with mt-tRNAGlu including EARS2 and TRMU in the majority of affected individuals, but not in healthy carriers of m.14674T>C, supporting a digenic inheritance. Our transcriptomic and proteomic analysis of patient muscle suggests a stepwise mechanism where first, the integrated stress response associated with increased FGF21 and GDF15 expression enhances the metabolism modulated by serine biosynthesis, one carbon metabolism, TCA lipid oxidation and amino acid availability, while in the second step mTOR activation leads to increased mitochondrial biogenesis. Our data suggest that the spontaneous recovery in infants with digenic mutations may be modulated by the above described changes. Similar mechanisms may explain the variable penetrance and tissue specificity of other mtDNA mutations and highlight the potential role of amino acids in improving mitochondrial disease.

Distinct Roles of Zmynd17 and PGC1α in Mitochondrial Quality Control and Biogenesis in Skeletal Muscle

Maintaining skeletal muscle mitochondrial quality is important not only for muscle activity but also for systemic metabolism. Exercise has long been recognized to have a positive impact on muscle mitochondrial quality. Although exercise triggers various changes in the mitochondrial dynamics, its molecular basis remains to be elucidated. We have previously reported that inactivation of the muscle-specific protein, zinc finger MYND domain-containing protein 17 (Zmynd17), results in mitochondrial abnormalities. To investigate the link between Zmynd17 activity and exercise-induced mitochondrial maintenance, we observed the effect of consecutive exercise on the mitochondrial quality in Zmynd17-deficient muscles. Zmynd17-deficient mice displayed abnormal mitochondrial morphology in limb muscles, which remarkably improved upon voluntary exercise. Interestingly, morphological abnormalities in mitochondria were even more apparent when PGC1α, a regulator of exercise-induced mitochondrial biogenesis, was overexpressed in Zmynd17-KO limb muscle. These abnormalities were also ameliorated by voluntary exercise. Our results show that neither the effect of consecutive exercise on mitochondrial quality nor PGC1α-induced mitochondrial biogenesis are mediated through Zmynd17 activity, thereby suggesting the existence of a complex mechanism of mitochondrial quality control in muscles.

Mss51 deletion enhances muscle metabolism and glucose homeostasis in mice

Myostatin is a negative regulator of muscle growth and metabolism and its inhibition in mice improves insulin sensitivity, increases glucose uptake into skeletal muscle, and decreases total body fat. A recently described mammalian protein called MSS51 is significantly downregulated with myostatin inhibition. In vitro disruption of Mss51 results in increased levels of ATP, β-oxidation, glycolysis, and oxidative phosphorylation. To determine the in vivo biological function of Mss51 in mice, we disrupted the Mss51 gene by CRISPR/Cas9 and found that Mss51-KO mice have normal muscle weights and fiber-type distribution but reduced fat pads. Myofibers isolated from Mss51-KO mice showed an increased oxygen consumption rate compared with WT controls, indicating an accelerated rate of skeletal muscle metabolism. The expression of genes related to oxidative phosphorylation and fatty acid β-oxidation were enhanced in skeletal muscle of Mss51-KO mice compared with that of WT mice. We found that mice lacking Mss51 and challenged with a high-fat diet were resistant to diet-induced weight gain, had increased whole-body glucose turnover and glycolysis rate, and increased systemic insulin sensitivity and fatty acid β-oxidation. These findings demonstrate that MSS51 modulates skeletal muscle mitochondrial respiration and regulates whole-body glucose and fatty acid metabolism, making it a potential target for obesity and diabetes.

Genes lost during the transition from land to water in cetaceans highlight genomic changes associated with aquatic adaptations

The transition from land to water in whales and dolphins (cetaceans) was accompanied by remarkable adaptations. To reveal genomic changes that occurred during this transition, we screened for protein-coding genes that were inactivated in the ancestral cetacean lineage. We found 85 gene losses. Some of these were likely beneficial for cetaceans, for example, by reducing the risk of thrombus formation during diving (F12 and KLKB1), erroneous DNA damage repair (POLM), and oxidative stress-induced lung inflammation (MAP3K19). Additional gene losses may reflect other diving-related adaptations, such as enhanced vasoconstriction during the diving response (mediated by SLC6A18) and altered pulmonary surfactant composition (SEC14L3), while loss of SLC4A9 relates to a reduced need for saliva. Last, loss of melatonin synthesis and receptor genes (AANAT, ASMT, and MTNR1A/B) may have been a precondition for adopting unihemispheric sleep. Our findings suggest that some genes lost in ancestral cetaceans were likely involved in adapting to a fully aquatic lifestyle.

The Multiple Levels of Mitonuclear Coregulation

Together, the nuclear and mitochondrial genomes encode the oxidative phosphorylation (OXPHOS) complexes that reside in the mitochondrial inner membrane and enable aerobic life. Mitochondria maintain their own genome that is expressed and regulated by factors distinct from their nuclear counterparts. For optimal function, the cell must ensure proper stoichiometric production of OXPHOS subunits by coordinating two physically separated and evolutionarily distinct gene expression systems. Here, we review our current understanding of mitonuclear coregulation primarily at the levels of transcription and translation. Additionally, we discuss other levels of coregulation that may exist but remain largely unexplored, including mRNA modification and stability and posttranslational protein degradation.

Skeletal muscle in aged mice reveals extensive transformation of muscle gene expression

BACKGROUND

Aging leads to decreased skeletal muscle function in mammals and is associated with a progressive loss of muscle mass, quality and strength. Age-related muscle loss (sarcopenia) is an important health problem associated with the aged population.

RESULTS

We investigated the alteration of genome-wide transcription in mouse skeletal muscle tissue (rectus femoris muscle) during aging using a high-throughput sequencing technique. Analysis revealed significant transcriptional changes between skeletal muscles of mice at 3 (young group) and 24 (old group) months of age. Specifically, genes associated with energy metabolism, cell proliferation, muscle myosin isoforms, as well as immune functions were found to be altered. We observed several interesting gene expression changes in the elderly, many of which have not been reported before.

CONCLUSIONS

Those data expand our understanding of the various compensatory mechanisms that can occur with age, and further will assist in the development of methods to prevent and attenuate adverse outcomes of aging.

Zmynd17 controls muscle mitochondrial quality and whole-body metabolism

Muscle mitochondria are crucial for systemic metabolic function, yet their regulation remains unclear. The zinc finger MYND domain-containing protein 17 (Zmynd17) was recently identified as a muscle-specific gene in mammals. Here, we investigated the role of Zmynd17 in mice. We found Zmynd17 predominantly expressed in skeletal muscle, especially in fast glycolytic muscle. Genetic Zmynd17 inactivation led to morphologic and functional abnormalities in muscle mitochondria, resulting in decreased respiratory function. Metabolic stress induced by a high-fat diet upregulated Zmynd17 expression and further exacerbated muscle mitochondrial morphology in Zmynd17-deficient mice. Strikingly, Zmynd17 deficiency significantly aggravated metabolic stress-induced hepatic steatosis, glucose intolerance, and insulin resistance. Furthermore, middle-aged mice lacking Zmynd17 exhibited impaired aerobic exercise performance, glucose intolerance, and insulin resistance. Thus, our results indicate that Zmynd17 is a metabolic stress-inducible factor that maintains muscle mitochondrial integrity, with its deficiency profoundly affecting whole-body glucose metabolism.-Fujita, R., Yoshioka, K., Seko, D., Suematsu, T., Mitsuhashi, S., Senoo, N., Miura, S., Nishino, I., Ono, Y. Zmynd17 controls muscle mitochondrial quality and whole-body metabolism.

A CMC1-knockout reveals translation-independent control of human mitochondrial complex IV biogenesis

Defects in mitochondrial respiratory chain complex IV (CIV) frequently cause encephalocardiomyopathies. Human CIV assembly involves 14 subunits of dual genetic origin and multiple nucleus-encoded ancillary factors. Biogenesis of the mitochondrion-encoded copper/heme-containing COX1 subunit initiates the CIV assembly process. Here, we show that the intermembrane space twin CX₉C protein CMC1 forms an early CIV assembly intermediate with COX1 and two assembly factors, the cardiomyopathy proteins COA3 and COX14. A TALEN-mediated CMC1 knockout HEK293T cell line displayed normal COX1 synthesis but decreased CIV activity owing to the instability of newly synthetized COX1. We demonstrate that CMC1 stabilizes a COX1-COA3-COX14 complex before the incorporation of COX4 and COX5a subunits. Additionally, we show that CMC1 acts independently of CIV assembly factors relevant to COX1 metallation (COX10, COX11, and SURF1) or late stability (MITRAC7). Furthermore, whereas human COX14 and COA3 have been proposed to affect COX1 mRNA translation, our data indicate that CMC1 regulates turnover of newly synthesized COX1 prior to and during COX1 maturation, without affecting the rate of COX1 synthesis.

Ribosome recycling defects modify the balance between the synthesis and assembly of specific subunits of the oxidative phosphorylation complexes in yeast mitochondria

Mitochondria have their own translation machinery that produces key subunits of the OXPHOS complexes. This machinery relies on the coordinated action of nuclear-encoded factors of bacterial origin that are well conserved between humans and yeast. In humans, mutations in these factors can cause diseases; in yeast, mutations abolishing mitochondrial translation destabilize the mitochondrial DNA. We show that when the mitochondrial genome contains no introns, the loss of the yeast factors Mif3 and Rrf1 involved in ribosome recycling neither blocks translation nor destabilizes mitochondrial DNA. Rather, the absence of these factors increases the synthesis of the mitochondrially-encoded subunits Cox1, Cytb and Atp9, while strongly impairing the assembly of OXPHOS complexes IV and V. We further show that in the absence of Rrf1, the COX1 specific translation activator Mss51 accumulates in low molecular weight forms, thought to be the source of the translationally-active form, explaining the increased synthesis of Cox1. We propose that Rrf1 takes part in the coordination between translation and OXPHOS assembly in yeast mitochondria. These interactions between general and specific translation factors might reveal an evolutionary adaptation of the bacterial translation machinery to the set of integral membrane proteins that are translated within mitochondria.