bims-mimbat 2023-12-24 papers

bims-mimbat

Biomed News

on Mitochondrial metabolism in brown adipose tissue

Issue of 2023‒12‒24
fifteen papers selected by
José Carlos de Lima-Júnior, Washington University

Adipose retinol saturase is regulated by β-adrenergic signaling and its deletion impairs lipolysis in adipocytes and acute cold tolerance in mice.
Mitochondrial metabolism and respiration adjustments following temperature acclimation in Daphnia magna.
Layered mechanisms regulating the human mitochondrial NAD+ transporter SLC25A51.
Complex II ambiguities-FADH2 in the electron transfer system.
Performance of TMRM and Mitotrackers in mitochondrial morphofunctional analysis of primary human skin fibroblasts.
Mtfp1 ablation enhances mitochondrial respiration and protects against hepatic steatosis.
Unmasking Crucial Residues in Adipose Triglyceride Lipase (ATGL) for Co-Activation with Comparative Gene Identification-58 (CGI-58).
Subcutaneous white adipose tissue independently regulates burn-induced hypermetabolism via immune-adipose crosstalk.
Lysophosphatidic acid triggers inflammation in the liver and white adipose tissue in rat models of 1-acyl-sn-glycerol-3-phosphate acyltransferase 2 deficiency and overnutrition.
Investigation of the Mechanism of Membrane Potential Generation by Heme-Copper Respiratory Oxidases in a Real Time Mode.
Regulation of lipolysis by 14-3-3 proteins on human adipocyte lipid droplets.
Mitochondrial dysfunction abrogates dietary lipid processing in enterocytes.
Temperature sensing: A potassium channel as cold sensor in the rain tree Samanea saman.
Uncoupled substrate binding underlies the evolutionary switch between Na + and H + -coupled prokaryotic aspartate transporters.
Mitochondrial quality control pathways sense mitochondrial protein import.

Mol Metab. 2023 Dec 19. pii: S2212-8778(23)00189-8. [Epub ahead of print] 101855

Adipose retinol saturase is regulated by β-adrenergic signaling and its deletion impairs lipolysis in adipocytes and acute cold tolerance in mice.

Chen Li, Marie F Kiefer, Sarah Dittrich, Roberto E Flores, Yueming Meng, Na Yang, Sascha Wulff, Sabrina Gohlke, Manuela Sommerfeld, Sylvia J Wowro, Konstantin M Petricek, Dominic Dürbeck, Leonard Spranger, Knut Mai, Holger Scholz, Tim J Schulz, Michael Schupp.

  OBJECTIVE: Retinol saturase (RetSat) is an endoplasmic reticulum-localized oxidoreductase highly expressed in organs involved in lipid metabolism such as white (WAT) and brown adipose tissue (BAT). Cold exposure was shown to increase RETSAT protein in BAT but its relevance for non-shivering thermogenesis, a process with beneficial effects on metabolic health, is unknown.METHODS: We analyzed the regulation of RetSat expression in white and brown adipocytes and different murine adipose tissue depots upon β-adrenergic stimulation and cold exposure. RetSat function during the differentiation and β-adrenergic stimulation of brown adipocytes was dissected by loss-of-function experiments. Mice with BAT-specific deletion of RetSat were generated and exposed to cold. Gene expression in human WAT was analyzed and the effect of RetSat depletion on adipocyte lipolysis investigated.
RESULTS: We show that cold exposure induces RetSat expression in both WAT and BAT of mice via β-adrenergic signaling. In brown adipocytes, RetSat has minor effects on differentiation but is required for maximal thermogenic gene and protein expression upon β-adrenergic stimulation and mitochondrial respiration. In mice, BAT-specific deletion of RetSat impaired acute but not long-term adaptation to cold exposure. RetSat expression in subcutaneous WAT of humans correlates with the expression of genes related to mitochondrial function. Mechanistically, we found that RetSat depletion impaired β-agonist-induced lipolysis, a major regulator of thermogenic gene expression in adipocytes.
CONCLUSIONS: Thus, RetSat expression is under β-adrenergic control and determines thermogenic capacity of brown adipocytes and acute cold tolerance in mice. Modulating RetSat activity may allow for therapeutic interventions towards pathologies with inadequate metabolic activity.

Keywords:  adipose tissue; lipolysis; mitochondria; retinol saturase; thermogenesis; β-adrenergic signaling

DOI:  https://doi.org/10.1016/j.molmet.2023.101855
J Therm Biol. 2023 Dec 12. pii: S0306-4565(23)00302-9. [Epub ahead of print]119 103761

Mitochondrial metabolism and respiration adjustments following temperature acclimation in Daphnia magna.

Nadine Hoffschröer, Cecilia Laspoumaderes, Bettina Zeis, Nelly Tremblay.

  Seasonal temperature changes and local variations in the water column challenge lentic zooplankton in their habitat. At the cellular level, exposure to varying temperatures affects the mitochondrial functional properties of poikilothermic organisms. Metabolic enzymes that supply reduced substrates to the electron transport chain and elements of the oxidative phosphorylation system must therefore adjust their activity and flux rates to the altered temperature conditions. In the present study, Daphnia magna respiration was analyzed in response to acute and chronic changes in ambient temperature. Oxygen consumption as well as substrate and electron flux rates were measured at the animals' acclimation temperature and at two additional acute temperatures. High activity of citrate synthase (CS) in cold-acclimated animals (10 °C) may have resulted from mitochondrial quantitative adjustments. However, thermal sensitivity of the functional properties of mitochondrial enzymes was greater in warm-acclimated animals (30 °C). In whole animals, temperature-induced changes were partly compensated by acclimation, but these changes were promoted by acclimation in the case of the mitochondrial electron transport chain. Thus, respiration realised in whole animals was limited by the provision of reduced substrates in the tricarboxylic acid cycle rather than by restrictions of the respiratory chain complexes. This may minimize production of reactive oxygen species and resulting damage and reduce waste of substrates from the animals' energy reserves. Still, the integrated biomarker response indicated increased defense against oxidative stress at elevated temperatures.

Keywords:  Daphnia; Metabolism; Mitochondria; OROBOROS; OXPHOS; Respiration; Temperature acclimation

DOI:  https://doi.org/10.1016/j.jtherbio.2023.103761
Biochem Soc Trans. 2023 Dec 20. 51(6): 1989-2004

Layered mechanisms regulating the human mitochondrial NAD+ transporter SLC25A51.

Shivansh Goyal, Xiaolu A Cambronne.

  SLC25A51 is the primary mitochondrial NAD+ transporter in humans and controls many local reactions by mediating the influx of oxidized NAD+. Intriguingly, SLC25A51 lacks several key features compared with other members in the mitochondrial carrier family, thus its molecular mechanism has been unclear. A deeper understanding would shed light on the control of cellular respiration, the citric acid cycle, and free NAD+ concentrations in mammalian mitochondria. This review discusses recent insights into the transport mechanism of SLC25A51, and in the process highlights a multitiered regulation that governs NAD+ transport. The aspects regulating SLC25A51 import activity can be categorized as contributions from (1) structural characteristics of the transporter itself, (2) its microenvironment, and (3) distinctive properties of the transported ligand. These unique mechanisms further evoke compelling new ideas for modulating the activity of this transporter, as well as new mechanistic models for the mitochondrial carrier family.

Keywords:  MCART1; NAD; SLC25A51; mitochondrial carrier family; mitochondrial transport

DOI:  https://doi.org/10.1042/BST20220318
J Biol Chem. 2023 Nov 22. pii: S0021-9258(23)02498-5. [Epub ahead of print]300(1): 105470

Complex II ambiguities-FADH2 in the electron transfer system.

Erich Gnaiger.

  The prevailing notion that reduced cofactors NADH and FADH2 transfer electrons from the tricarboxylic acid cycle to the mitochondrial electron transfer system creates ambiguities regarding respiratory Complex II (CII). CII is the only membrane-bound enzyme in the tricarboxylic acid cycle and is part of the electron transfer system of the mitochondrial inner membrane feeding electrons into the coenzyme Q-junction. The succinate dehydrogenase subunit SDHA of CII oxidizes succinate and reduces the covalently bound prosthetic group FAD to FADH2 in the canonical forward tricarboxylic acid cycle. However, several graphical representations of the electron transfer system depict FADH2 in the mitochondrial matrix as a substrate to be oxidized by CII. This leads to the false conclusion that FADH2 from the β-oxidation cycle in fatty acid oxidation feeds electrons into CII. In reality, dehydrogenases of fatty acid oxidation channel electrons to the Q-junction but not through CII. The ambiguities surrounding Complex II in the literature and educational resources call for quality control, to secure scientific standards in current communications of bioenergetics, and ultimately support adequate clinical applications. This review aims to raise awareness of the inherent ambiguity crisis, complementing efforts to address the well-acknowledged issues of credibility and reproducibility.

Keywords:  Complex II; coenzyme Q; electron transfer system; fatty acid oxidation; flavin adenine dinucleotide; succinate dehydrogenase; tricarboxylic acid cycle

DOI:  https://doi.org/10.1016/j.jbc.2023.105470
Biochim Biophys Acta Bioenerg. 2023 Dec 16. pii: S0005-2728(23)00073-7. [Epub ahead of print] 149027

Performance of TMRM and Mitotrackers in mitochondrial morphofunctional analysis of primary human skin fibroblasts.

Shruti Desai, Sander Grefte, Els van de Westerlo, Susette Lauwen, Angela Paters, Jochen H M Prehn, Zhuohui Gan, Jaap Keijer, Merel J W Adjobo-Hermans, Werner J H Koopman.

  Mitochondrial membrane potential (Δψ) and morphology are considered key readouts of mitochondrial functional state. This morphofunction can be studied using fluorescent dyes ("probes") like tetramethylrhodamine methyl ester (TMRM) and Mitotracker (MT) dyes. Although these dyes are broadly used, information comparing their performance in mitochondrial morphology quantification and Δψ-sensitivity in the same cell model is still scarce. Here we applied epifluorescence microscopy of primary human skin fibroblasts to evaluate TMRM, Mitotracker Red CMXros (CMXros), Mitotracker Red CMH2Xros (CMH2Xros), Mitotracker Green FM (MG) and Mitotracker Deep Red FM (MDR). All probes were suited for automated quantification of mitochondrial morphology parameters when Δψ was normal, although they did not deliver quantitatively identical results. The mitochondrial localization of TMRM and MTs was differentially sensitive to carbonyl cyanide-4-phenylhydrazone (FCCP)-induced Δψ depolarization, decreasing in the order: TMRM ≫ CHM2Xros = CMXros = MDR > MG. To study the effect of reversible Δψ changes, the impact of photo-induced Δψ "flickering" was studied in cells co-stained with TMRM and MG. During a flickering event, individual mitochondria displayed subsequent TMRM release and uptake, whereas this phenomenon was not observed for MG. Spatiotemporal and computational analysis of the flickering event provided evidence that TMRM redistributes between adjacent mitochondria by a mechanism dependent on Δψ and TMRM concentration. In summary, this study demonstrates that: (1) TMRM and MTs are suited for automated mitochondrial morphology quantification, (2) numerical data obtained with different probes is not identical, and (3) all probes are sensitive to FCCP-induced Δψ depolarization, with TMRM and MG displaying the highest and lowest sensitivity, respectively. We conclude that TMRM is better suited for integrated analysis of Δψ and mitochondrial morphology than the tested MTs under conditions that Δψ is not substantially depolarized.

Keywords:  FCCP; Flickering; Mitochondrial morphology; Mitotracker; TMRM

DOI:  https://doi.org/10.1016/j.bbabio.2023.149027
Nat Commun. 2023 Dec 20. 14(1): 8474

Mtfp1 ablation enhances mitochondrial respiration and protects against hepatic steatosis.

Cecilia Patitucci, Juan Diego Hernández-Camacho, Elodie Vimont, Sonny Yde, Thomas Cokelaer, Thibault Chaze, Quentin Giai Gianetto, Mariette Matondo, Anastasia Gazi, Ivan Nemazanyy, David A Stroud, Daniella H Hock, Erminia Donnarumma, Timothy Wai.

Hepatic steatosis is the result of imbalanced nutrient delivery and metabolism in the liver and is the first hallmark of Metabolic dysfunction-associated steatotic liver disease (MASLD). MASLD is the most common chronic liver disease and involves the accumulation of excess lipids in hepatocytes, inflammation, and cancer. Mitochondria play central roles in liver metabolism yet the specific mitochondrial functions causally linked to MASLD remain unclear. Here, we identify Mitochondrial Fission Process 1 protein (MTFP1) as a key regulator of mitochondrial and metabolic activity in the liver. Deletion of Mtfp1 in hepatocytes is physiologically benign in mice yet leads to the upregulation of oxidative phosphorylation (OXPHOS) activity and mitochondrial respiration, independently of mitochondrial biogenesis. Consequently, liver-specific knockout mice are protected against high fat diet-induced steatosis and metabolic dysregulation. Additionally, Mtfp1 deletion inhibits mitochondrial permeability transition pore opening in hepatocytes, conferring protection against apoptotic liver damage in vivo and ex vivo. Our work uncovers additional functions of MTFP1 in the liver, positioning this gene as an unexpected regulator of OXPHOS and a therapeutic candidate for MASLD.

DOI: https://doi.org/10.1038/s41467-023-44143-9
J Lipid Res. 2023 Dec 20. pii: S0022-2275(23)00164-5. [Epub ahead of print] 100491

Unmasking Crucial Residues in Adipose Triglyceride Lipase (ATGL) for Co-Activation with Comparative Gene Identification-58 (CGI-58).

Natalia Kulminskaya, Carlos Francisco Rodriguez Gamez, Peter Hofer, Ines Kathrin Cerk, Noopur Dubey, Roland Viertlmayr, Theo Sagmeister, Tea Pavkov-Keller, Rudolf Zechner, Monika Oberer.

  Lipolysis is an essential metabolic process that releases unesterified fatty acids from neutral lipid stores to maintain energy homeostasis in living organisms. Adipose triglyceride lipase (ATGL) plays a key role in intracellular lipolysis and can be co-activated upon interaction with the protein comparative gene identification-58 (CGI-58). The underlying molecular mechanism of ATGL stimulation by CGI-58 is incompletely understood. Based on analysis of evolutionary conservation, we used site directed mutagenesis to study a C-terminally truncated variant and full-length mouse ATGL providing insights in the protein co-activation on a per-residue level. We identified the region from residues N209-N215 in ATGL as essential for co-activation by CGI-58. ATGL variants with amino-acids exchanges in this region were still able to hydrolyze triacylglycerol at the basal level and to interact with CGI-58, yet could not be activated by CGI-58. Our studies also demonstrate that full-length mouse ATGL showed higher tolerance to specific single amino acid exchanges in the N209-N215 region upon CGI-58 co-activation compared to C-terminally truncated ATGL variants. The region is either directly involved in protein-protein interaction or essential for conformational changes required in the co-activation process. Three-dimensional models of the ATGL/CGI-58 complex with the artificial intelligence software AlphaFold demonstrated that a large surface area is involved in the protein-protein interaction. Mapping important amino acids for co-activation of both proteins, ATGL and CGI-58, onto the 3D model of the complex locates these essential amino acids at the predicted ATGL/CGI-58 interface thus strongly corroborating the significance of these residues in CGI-58 mediated co-activation of ATGL.

Keywords:  ABHD5; ATGL; AlphaFold; CGI-58; PNPLA2; adipose triglyceride lipase; co-activation; comparative gene identification-58; lipolysis; protein structure; protein-protein interaction; triacylglycerol hydrolase activity

DOI:  https://doi.org/10.1016/j.jlr.2023.100491
Cell Rep. 2023 Dec 19. pii: S2211-1247(23)01596-6. [Epub ahead of print]43(1): 113584

Subcutaneous white adipose tissue independently regulates burn-induced hypermetabolism via immune-adipose crosstalk.

Carly M Knuth, Dalia Barayan, Ju Hee Lee, Christopher Auger, Lauar de Brito Monteiro, Zachary Ricciuti, Dea Metko, Lisa Wells, Hoon-Ki Sung, Robert A Screaton, Marc G Jeschke.

  Severe burns induce a chronic hypermetabolic state that persists well past wound closure, indicating that additional internal mechanisms must be involved. Adipose tissue is suggested to be a central regulator in perpetuating hypermetabolism, although this has not been directly tested. Here, we show that thermogenic adipose tissues are activated in parallel to increases in hypermetabolism independent of cold stress. Using an adipose tissue transplantation model, we discover that burn-derived subcutaneous white adipose tissue alone is sufficient to invoke a hypermetabolic response in a healthy recipient mouse. Concomitantly, transplantation of healthy adipose tissue alleviates metabolic dysfunction in a burn recipient. We further show that the nicotinic acetylcholine receptor signaling pathway may mediate an immune-adipose crosstalk to regulate adipose tissue remodeling post-injury. Targeting this pathway could lead to innovative therapeutic interventions to counteract hypermetabolic pathologies.

Keywords:  CP: Metabolism; adipose tissue; browning; burn injury; hypermetabolism; inflammation

DOI:  https://doi.org/10.1016/j.celrep.2023.113584
Proc Natl Acad Sci U S A. 2023 Dec 26. 120(52): e2312666120

Lysophosphatidic acid triggers inflammation in the liver and white adipose tissue in rat models of 1-acyl-sn-glycerol-3-phosphate acyltransferase 2 deficiency and overnutrition.

Ikki Sakuma, Rafael C Gaspar, Panu K Luukkonen, Mario Kahn, Dongyan Zhang, Xuchen Zhang, Sue Murray, Jaya Prakash Golla, Daniel F Vatner, Varman T Samuel, Kitt Falk Petersen, Gerald I Shulman.

  AGPAT2 (1-acyl-sn-glycerol-3-phosphate-acyltransferase-2) converts lysophosphatidic acid (LPA) into phosphatidic acid (PA), and mutations of the AGPAT2 gene cause the most common form of congenital generalized lipodystrophy which leads to steatohepatitis. The underlying mechanism by which AGPAT2 deficiency leads to lipodystrophy and steatohepatitis has not been elucidated. We addressed this question using an antisense oligonucleotide (ASO) to knockdown expression of Agpat2 in the liver and white adipose tissue (WAT) of adult male Sprague-Dawley rats. Agpat2 ASO treatment induced lipodystrophy and inflammation in WAT and the liver, which was associated with increased LPA content in both tissues, whereas PA content was unchanged. We found that a controlled-release mitochondrial protonophore (CRMP) prevented LPA accumulation and inflammation in WAT whereas an ASO against glycerol-3-phosphate acyltransferase, mitochondrial (Gpam) prevented LPA content and inflammation in the liver in Agpat2 ASO-treated rats. In addition, we show that overnutrition, due to high sucrose feeding, resulted in increased hepatic LPA content and increased activated macrophage content which were both abrogated with Gpam ASO treatment. Taken together, these data identify LPA as a key mediator of liver and WAT inflammation and lipodystrophy due to AGPAT2 deficiency as well as liver inflammation due to overnutrition and identify LPA as a potential therapeutic target to ameliorate these conditions.

Keywords:  AGPAT2; congenital generalized lipodystrophy; inflammation; lysophosphatidic acid; phosphatidic acid

DOI:  https://doi.org/10.1073/pnas.2312666120
Biochemistry (Mosc). 2023 Oct;88(10): 1513-1527

Investigation of the Mechanism of Membrane Potential Generation by Heme-Copper Respiratory Oxidases in a Real Time Mode.

Sergei A Siletsky.

  Heme-copper respiratory oxidases are highly efficient molecular machines. These membrane enzymes catalyze the final step of cellular respiration in eukaryotes and many prokaryotes: the transfer of electrons from cytochromes or quinols to molecular oxygen and oxygen reduction to water. The free energy released in this redox reaction is converted by heme-copper respiratory oxidases into the transmembrane gradient of the electrochemical potential of hydrogen ions H+). Heme-copper respiratory oxidases have a unique mechanism for generating H+, namely, a redox-coupled proton pump. A combination of direct electrometric method for measuring the kinetics of membrane potential generation with the methods of prestationary kinetics and site-directed mutagenesis in the studies of heme-copper oxidases allows to obtain a unique information on the translocation of protons inside the proteins in real time. The review summarizes the data of studies employing time-resolved electrometry to decipher the mechanisms of functioning of these important bioenergetic enzymes.

Keywords:  bioenergetics; capacitive potentiometry; cytochrome aa3; cytochrome oxidase; direct electrometric method; electrogenic proton transfer; generation; kinetics; photoreduction; proteoliposomes; proton pump; time resolution; zinc ions

DOI:  https://doi.org/10.1134/S0006297923100085
PNAS Nexus. 2023 Dec;2(12): pgad420

Regulation of lipolysis by 14-3-3 proteins on human adipocyte lipid droplets.

Qin Yang, Zinger Yang Loureiro, Anand Desai, Tiffany DeSouza, Kaida Li, Hui Wang, Sarah M Nicoloro, Javier Solivan-Rivera, Silvia Corvera.

  Adipocyte lipid droplets (LDs) play a crucial role in systemic lipid metabolism by storing and releasing lipids to meet the organism's energy needs. Hormonal signals such as catecholamines and insulin act on adipocyte LDs, and impaired responsiveness to these signals can lead to uncontrolled lipolysis, lipotoxicity, and metabolic disease. To investigate the mechanisms that control LD function in human adipocytes, we applied proximity labeling mediated by enhanced ascorbate peroxidase (APEX2) to identify the interactome of PLIN1 in adipocytes differentiated from human mesenchymal progenitor cells. We identified 70 proteins that interact specifically with PLIN1, including PNPLA2 and LIPE, which are the primary effectors of regulated triglyceride hydrolysis, and 4 members of the 14-3-3 protein family (YWHAB, YWHAE, YWHAZ, and YWHAG), which are known to regulate diverse signaling pathways. Functional studies showed that YWHAB is required for maximum cyclic adenosine monophosphate (cAMP)-stimulated lipolysis, as its CRISPR-Cas9-mediated knockout mitigates lipolysis through a mechanism independent of insulin signaling. These findings reveal a new regulatory mechanism operating in human adipocytes that can impact lipolysis and potentially systemic metabolism.

Keywords:  14-3-3 proteins; insulin; lipid droplet; lipolysis; proximity labeling

DOI:  https://doi.org/10.1093/pnasnexus/pgad420
Nature. 2023 Dec 20.

Mitochondrial dysfunction abrogates dietary lipid processing in enterocytes.

Chrysanthi Moschandrea, Vangelis Kondylis, Ioannis Evangelakos, Marija Herholz, Farina Schneider, Christina Schmidt, Ming Yang, Sandra Ehret, Markus Heine, Michelle Y Jaeckstein, Karolina Szczepanowska, Robin Schwarzer, Linda Baumann, Theresa Bock, Efterpi Nikitopoulou, Susanne Brodesser, Marcus Krüger, Christian Frezza, Joerg Heeren, Aleksandra Trifunovic, Manolis Pasparakis.

Digested dietary fats are taken up by enterocytes where they are assembled into pre-chylomicrons in the endoplasmic reticulum followed by transport to the Golgi for maturation and subsequent secretion to the circulation1. The role of mitochondria in dietary lipid processing is unclear. Here we show that mitochondrial dysfunction in enterocytes inhibits chylomicron production and the transport of dietary lipids to peripheral organs. Mice with specific ablation of the mitochondrial aspartyl-tRNA synthetase DARS2 (ref. 2), the respiratory chain subunit SDHA3 or the assembly factor COX10 (ref. 4) in intestinal epithelial cells showed accumulation of large lipid droplets (LDs) in enterocytes of the proximal small intestine and failed to thrive. Feeding a fat-free diet suppressed the build-up of LDs in DARS2-deficient enterocytes, which shows that the accumulating lipids derive mostly from digested fat. Furthermore, metabolic tracing studies revealed an impaired transport of dietary lipids to peripheral organs in mice lacking DARS2 in intestinal epithelial cells. DARS2 deficiency caused a distinct lack of mature chylomicrons concomitant with a progressive dispersal of the Golgi apparatus in proximal enterocytes. This finding suggests that mitochondrial dysfunction results in impaired trafficking of chylomicrons from the endoplasmic reticulum to the Golgi, which in turn leads to storage of dietary lipids in large cytoplasmic LDs. Taken together, these results reveal a role for mitochondria in dietary lipid transport in enterocytes, which might be relevant for understanding the intestinal defects observed in patients with mitochondrial disorders5.

DOI: https://doi.org/10.1038/s41586-023-06857-0
Curr Biol. 2023 Dec 18. pii: S0960-9822(23)01518-X. [Epub ahead of print]33(24): R1298-R1300

Temperature sensing: A potassium channel as cold sensor in the rain tree Samanea saman.

Ingo Dreyer, Fernando Vergara-Valladares.

The rain tree Samanea saman folds its leaves upon rainfall. New results now indicate that rain perception is in fact a temperature-sensing process, and that Samanea possess an ion channel with a strong temperature sensitivity that is involved in leaf movement.

DOI: https://doi.org/10.1016/j.cub.2023.11.004
bioRxiv. 2023 Dec 04. pii: 2023.12.03.569786. [Epub ahead of print]

Uncoupled substrate binding underlies the evolutionary switch between Na + and H + -coupled prokaryotic aspartate transporters.

Krishna D Reddy, Burha Rasool, Farideh Badichi Akher, Nemanja Kutlešić, Swati Pant, Olga Boudker.

Secondary active membrane transporters use the electrochemical energy of ion gradients to concentrate their substrates. Transporters within the same family often evolve to use different ions, driven by physiological needs or bioavailability. How such functional differences arise despite similar threedimensional protein structures is mostly unknown. We used phylogenetics and ancestral sequence reconstruction on prokaryotic glutamate transporters to recapitulate the evolutionary transition from Na + -coupled (GT-Na) to H + -coupled (GT-H) transport and discovered that it occurred via an intermediate clade (GT-Int). The reconstructed ancestral transporter AncGT-Int contains all residues required for Na + binding but switched from Na + -coupled substrate binding, characteristic of GT-Na transporters, to uncoupled binding. The high-resolution cryo-EM structures of AncGT-Int show that it binds substrate without Na + ions in the same manner as GT-Na transporters, which instead require Na + ions. Unlike GT-Na transporters, remodeled by ions into a high-affinity substrate-binding configuration, apo AncGT-Int is already in this configuration. Our results show how allosteric changes eliminated Na + dependence of Na + -coupled transporters before H + dependence arose, shedding light on ion coupling mechanisms and evolution in this family, and highlighting the power of phylogenetics and ancestral sequence reconstruction in the structure-function studies of membrane transporters.

DOI: https://doi.org/10.1101/2023.12.03.569786
Trends Endocrinol Metab. 2023 Dec 15. pii: S1043-2760(23)00243-6. [Epub ahead of print]

Mitochondrial quality control pathways sense mitochondrial protein import.

Laurie P Lee-Glover, Timothy E Shutt.

  Mitochondrial quality control (MQC) mechanisms are required to maintain a functional proteome, which enables mitochondria to perform a myriad of important cellular functions from oxidative phosphorylation to numerous other metabolic pathways. Mitochondrial protein homeostasis begins with the import of over 1000 nuclear-encoded mitochondrial proteins and the synthesis of 13 mitochondrial DNA-encoded proteins. A network of chaperones and proteases helps to fold new proteins and degrade unnecessary, damaged, or misfolded proteins, whereas more extensive damage can be removed by mitochondrial-derived vesicles (MDVs) or mitochondrial autophagy (mitophagy). Here, focusing on mechanisms in mammalian cells, we review the importance of mitochondrial protein import as a sentinel of mitochondrial function that activates multiple MQC mechanisms when impaired.

Keywords:  mitochondria; mitochondrial protein import; mitochondrial quality control; mitochondrial unfolded protein response; mitochondrial-derived vesicles; mitophagy

DOI:  https://doi.org/10.1016/j.tem.2023.11.004