Pancreatic beta cells lack a low glucose and O2-inducible mitochondrial protein that augments cell survival

Structure and function of the S. pombe III-IV-cyt c supercomplex

The respiratory chain in aerobic organisms is composed of a number of membrane-bound protein complexes that link electron transfer to proton translocation across the membrane. In mitochondria, the final electron acceptor, complex IV (CIV), receives electrons from dimeric complex III (CIII2), via a mobile electron carrier, cytochrome c. In the present study, we isolated the CIII2CIV supercomplex from the fission yeast Schizosaccharomyces pombe and determined its structure with bound cyt. c using single-particle electron cryomicroscopy. A respiratory supercomplex factor 2 was found to be bound at CIV distally positioned in the supercomplex. In addition to the redox-active metal sites, we found a metal ion, presumably Zn2+, coordinated in the CIII subunit Cor1, which is encoded by the same gene (qcr1) as the mitochondrial-processing peptidase subunit β. Our data show that the isolated CIII2CIV supercomplex displays proteolytic activity suggesting a dual role of CIII2 in S. pombe. As in the supercomplex from S. cerevisiae, subunit Cox5 of CIV faces towards one CIII monomer, but in S. pombe, the two complexes are rotated relative to each other by ~45°. This orientation yields equal distances between the cyt. c binding sites at CIV and at each of the two CIII monomers. The structure shows cyt. c bound at four positions, but only along one of the two symmetrical branches. Overall, this combined structural and functional study reveals the integration of peptidase activity with the CIII2 respiratory system and indicates a two-dimensional cyt. c diffusion mechanism within the CIII2-CIV supercomplex.

Mitochondrial Factors in the Cell Nucleus

The origin of eukaryotic organisms involved the integration of mitochondria into the ancestor cell, with a massive gene transfer from the original proteobacterium to the host nucleus. Thus, mitochondrial performance relies on a mosaic of nuclear gene products from a variety of genomes. The concerted regulation of their synthesis is necessary for metabolic housekeeping and stress response. This governance involves crosstalk between mitochondrial, cytoplasmic, and nuclear factors. While anterograde and retrograde regulation preserve mitochondrial homeostasis, the mitochondria can modulate a wide set of nuclear genes in response to an extensive variety of conditions, whose response mechanisms often merge. In this review, we summarise how mitochondrial metabolites and proteins-encoded either in the nucleus or in the organelle-target the cell nucleus and exert different actions modulating gene expression and the chromatin state, or even causing DNA fragmentation in response to common stress conditions, such as hypoxia, oxidative stress, unfolded protein stress, and DNA damage.

HM-Chromanone Alleviates Hyperglycemia by Protecting Pancreatic Islet Cells in Streptozotocin-Induced Diabetic Mice

We examined the effects of HM-chromanone (HMC) on alleviating hyperglycemia and protecting pancreatic β-cells from streptozotocin (STZ)-induced damage in C57BL/6J mice. HMC was administered to STZ-induced diabetic mice at 10 or 30 mg/kg, for 14 days. Thereafter, changes in fasting blood glucose levels, insulin-secretion, histopathological examination of pancreas islet cell and apoptotic protein levels, and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay were determined. The results revealed that HMC dose-dependently improved blood glucose concentrations and alleviated pancreatic islet cells damage. In diabetic mice, degeneration of the islet cells was observed wherein they appeared shrunken, with hyaline deterioration, nuclear dissolution, and condensation. However, morphology of the islet cell was restored, and nuclei were visibly rounded in the HMC (30 mg/kg)-administered diabetic mice. In addition, β-cell numbers were markedly increased in HMC mice compared to STZ-induced diabetic mice, and the number of cells stained with glucagon was decreased. HMC markedly decreased the expression of proapoptotic proteins and increased antiapoptotic proteins, and the number of apoptotic cells detected by TUNEL was elevated. HMC decreased expression of interleukin (IL)-1β, IL-6, and tumor necrosis factor-α in diabetic mice. Moreover, HMC increased antioxidant-enzymes activity, and decreased reactive oxygen species generation. In conclusion, the results demonstrate the potential of HMC to alleviate hyperglycemia by protecting the pancreatic β-cells in diabetic mice.

HIG1 domain family member 1A is a crucial regulator of disorders associated with hypoxia

Mitochondria play a central role in cellular energy conversion, metabolism, and cell proliferation. The regulation of mitochondrial function by HIGD1A, which is located on the inner membrane of the mitochondria, is essential to maintain cell survival under hypoxic conditions. In recent years, there have been shown other cellular pathways and mechanisms involving HIGD1A diametrically or through its interaction. As a novel regulator, HIGD1A maintains mitochondrial integrity and enhances cell viability under hypoxic conditions, increasing cell resistance to hypoxia. HIGD1A mainly targets cytochrome c oxidase by regulating downstream signaling pathways, which affects the ATP generation system and subsequently alters mitochondrial respiratory function. In addition, HIGD1A plays a dual role in cell survival in distinct degree hypoxia regions of the tumor. Under mild and moderate anoxic areas, HIGD1A acts as a positive regulator to promote cell growth. However, HIGD1A plays a role in inhibiting cell growth but retaining cellular activity under severe anoxic areas. We speculate that HIGD1A engages in tumor recurrence and drug resistance mechanisms. This review will focus on data concerning how HIGD1A regulates cell viability under hypoxic conditions. Therefore, HIGD1A could be a potential therapeutic target for hypoxia-related diseases.

Cryo-EM structure and function of S. pombe complex IV with bound respiratory supercomplex factor

Fission yeast Schizosaccharomyces pombe serves as model organism for studying higher eukaryotes. We combined the use of cryo-EM and spectroscopy to investigate the structure and function of affinity purified respiratory complex IV (CIV) from S. pombe. The reaction sequence of the reduced enzyme with O₂ proceeds over a time scale of µs-ms, similar to that of the mammalian CIV. The cryo-EM structure of CIV revealed eleven subunits as well as a bound hypoxia-induced gene 1 (Hig1) domain of respiratory supercomplex factor 2 (Rcf2). These results suggest that binding of Rcf2 does not require the presence of a CIII-CIV supercomplex, i.e. Rcf2 is a component of CIV. An AlphaFold-Multimer model suggests that the Hig1 domains of both Rcf1 and Rcf2 bind at the same site of CIV suggesting that their binding is mutually exclusive. Furthermore, the differential functional effect of Rcf1 or Rcf2 is presumably caused by interactions of CIV with their different non-Hig1 domain parts.

Higd1a facilitates exercise-mediated alleviation of fatty liver in diet-induced obese mice

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) has emerged as the most common liver disease. Exercise is an effective strategy against NAFLD, but its underlying molecular mechanism is not completely understood.

METHODS

Higd1a, a mitochondrial inner membrane protein, was knocked down or overexpressed in mice livers by tail vein injection of adeno-associated virus (AAV) vectors. High fat diet-induced obese mice were subjected to treadmill training. Alpha mouse liver 12 (AML12) cells were used for in vitro studies.

RESULTS

Higd1a was upregulated in mice livers after treadmill exercise training. Knockdown of Higd1a in diet-induced obese mice livers impaired exercise-mediated alleviation of hepatic steatosis, liver injury and inflammation. On the contrary, hepatic overexpression of Higd1a ameliorated fatty liver, liver injury and inflammation in synergy with exercise. Mechanistically, deficiency of Higd1a in hepatocytes promoted free fatty acids (FFAs)-induced apoptosis and oxidative stress, and elevated the cytosolic level of oxidized mitochondrial DNA (ox-mtDNA) to activate NLRP3 inflammasome and JNK signaling, leading to decreased expression of critical genes involved in fatty acid oxidation (FAO), such as Ppara, Cpt1a and Acadm. Overexpression of Higd1a in hepatocytes blunted the above effects, which ultimately increased FAO genes expression and alleviated fat accumulation in hepatocytes.

CONCLUSION

These results identify a Higd1a-mediated inhibition of cytosolic ox-mtDNA/NLRP3 inflammasomes/JNK pathway that facilitates exercise-mediated alleviation of hepatosteatosis.

Identification of potential genes in endobronchial tuberculosis after bronchoscopic cryotherapy by transcriptome sequencing

BACKGROUND

Bronchoscopic cryotherapy facilitates the treatment of endobronchial tuberculosis (EBTB) and helps suppress progressive bronchial stenosis. However, the molecular mechanism of bronchoscopic cryotherapy in EBTB patients has not been reported.

METHODS

Transcriptome sequencing was performed to explore differentially expressed mRNAs (DEGs) in EBTB patients before and after bronchoscopic cryotherapy. Gene Ontology (GO) and KEGG analyses were carried out. Five genes (MKLN1, HIGD1A, PTGES, SKIL, and MCEMP1) were selected and validated using real-time qPCR (RT- qPCR).

RESULTS

In transcriptome analysis, 448 DEGs with p < 0.05 and|logFC| > 1 were identified; of these, 171 and 277 DEGs were significantly up- and down-regulated after bronchoscopic cryotherapy, respectively. Results displayed 337 biological process (GO-BP), 48 cellular component (GO-CC) and 62 molecular function (GO-MF) terms and 20 KEGG pathways. RT- qPCR results were consistent with the sequencing data.

CONCLUSION

These data provide informative evidence and recommendations for further scientific research on bronchoscopic cryotherapy for EBTB.

The functional role of Higd1a in mitochondrial homeostasis and in multiple disease processes

Higd1a is a conserved gene in evolution which is widely expressed in many tissues in mammals. Accumulating evidence has revealed multiple functions of Higd1a, as a mitochondrial inner membrane protein, in the regulation of metabolic homeostasis. It plays an important role in anti-apoptosis and promotes cellular survival in several cell types under hypoxic condition. And the survival of porcine Sertoli cells facilitated by Higd1a helps to support reproduction. In some cases, Higd1a can serve as a sign of metabolic stress. Over the past several years, a considerable amount of studies about how tumor fate is determined and how cancerous proliferation is regulated by Higd1a have been performed. In this review, we summarize the physiological functions of Higd1a in metabolic homeostasis and its pathophysiological roles in distinct diseases including cancer, nonalcoholic fatty liver disease (NAFLD), type II diabetes and mitochondrial diseases. The prospect of Higd1a with potential to preserve mammal health is also discussed. This review might pave the way for Higd1a-based research and application in clinical practice.

HIG1 domain family member 1A disrupts proliferation, migration and invasion of colon adenocarcinoma cells

HIG1 domain family member 1A (Higd-1a) interacts with dynamin-like 120 kDa protein to maintain the morphological and functional integrity of the mitochondria and thus plays an important role in the progression of malignant tumors. Higd-1a promotes the proliferation of pancreatic cancer cells and the growth of pancreatic cancer; however, no similar observations have been reported for colorectal cancer (CRC). This study, therefore, aimed to verify the role of Higd-1a in CRC. We downloaded data from the Genotype-Tissue Expression (GTEX) and The Cancer Genome Atlas (TCGA) databases and identified an association between Higd-1a levels in colon adenocarcinoma (COAD) tissues and poor survival using Kaplan-Meier curves. Subsequently, we overexpressed Higd-1a in the human COAD cell line HCT-8, knocked down Higd-1a expression in SW480 cells, and evaluated the effects via quantitative PCR (qPCR) and western blotting. MTT assays, colony formation assay, cell cycle analysis, annexin V-FITC/PI, wound-healing analysis, and transwell assay were used to test cell proliferation, formation of cell colonies, cell cycle progression, migration, invasiveness, and apoptosis. Higd-1a has low transcription levels in COAD tissue and suggests a poor prognosis. Higd-1a overexpression in HCT-8 cells weakened cell proliferation, formation of cell colonies, cell cycle progression, migration ability, and invasiveness, and increased apoptosis. Moreover, the decrease of Higd-1a in SW480 cells induced cell proliferation, formation of cell colonies, cell cycle progression, migration, and invasion, and inhibited apoptosis. Higd-1a is underexpressed in COAD cells and its overexpression impaired the proliferation, migration, and invasiveness of COAD cells.

Structure and Mechanism of Respiratory III-IV Supercomplexes in Bioenergetic Membranes

In the final steps of energy conservation in aerobic organisms, free energy from electron transfer through the respiratory chain is transduced into a proton electrochemical gradient across a membrane. In mitochondria and many bacteria, reduction of the dioxygen electron acceptor is catalyzed by cytochrome c oxidase (complex IV), which receives electrons from cytochrome bc1 (complex III), via membrane-bound or water-soluble cytochrome c. These complexes function independently, but in many organisms they associate to form supercomplexes. Here, we review the structural features and the functional significance of the nonobligate III2IV1/2 Saccharomyces cerevisiae mitochondrial supercomplex as well as the obligate III2IV2 supercomplex from actinobacteria. The analysis is centered around the Q-cycle of complex III, proton uptake by CytcO, as well as mechanistic and structural solutions to the electronic link between complexes III and IV.

HIGD‑1B inhibits hypoxia‑induced mitochondrial fragmentation by regulating OPA1 cleavage in cardiomyocytes

The dynamic regulation of mitochondrial morphology is key for eukaryotic cells to manage physiological challenges. Therefore, it is important to understand the molecular basis of mitochondrial dynamic regulation. The aim of the present study was to explore the role of HIG1 hypoxia inducible domain family member 1B (HIGD‑1B) in hypoxia‑induced mitochondrial fragmentation. Protein expression was determined via western blotting. Immunofluorescence assays were performed to detect the subcellular location of HIGD‑1B. Cell Counting Kit‑8 assays and flow cytometry were carried out to measure cell viability and apoptosis, respectively. Protein interactions were evaluated by co‑immunoprecipitation. In the present study, it was found that HIGD‑1B serves a role in cell survival by maintaining the integrity of the mitochondria under hypoxic conditions. Knockdown of HIGD‑1B promoted mitochondrial fragmentation, while overexpression of HIGD‑1B increased survival by preventing activation of caspase‑3 and ‑9. HIGD‑1B expression was associated with cell viability and apoptosis in cardiomyocytes. Furthermore, HIGD‑1B delayed the cleavage process of optic atrophy 1 (OPA1) and stabilized mitochondrial morphology by interacting with OPA1. Collectively, the results from the present study identified a role for HIGD‑1B as an inhibitor of the mitochondrial fission in cardiomyocytes.

Applying Immunomodulation to Promote Longevity of Immunoisolated Pancreatic Islet Grafts

Islet transplantation is a promising therapy for insulin-dependent diabetes, but large-scale application is hampered by the lack of a consistent source of insulin-producing cells and need for lifelong administration of immunosuppressive drugs, which are associated with severe side effects. To avoid chronic immunosuppression, islet grafts can be enveloped in immunoisolating polymeric membranes. These immunoisolating polymeric membranes protect islet grafts from cell-mediated rejection while allowing diffusion of oxygen, nutrients, and insulin. Although clinical trials have shown the safety and feasibility of encapsulated islets to control glucose homeostasis, the strategy does up till now not support long-term graft survival. This partly can be explained by a significant loss of insulin-producing cells in the immediate period after implantation. The loss can be prevented by combining immunoisolation with immunomodulation, such as combined administration of immunomodulating cytokines or coencapsulation of immunomodulating cell types such as regulatory T cells, mesenchymal stem cells, or Sertoli cells. Also, administration of specific antibodies or apoptotic donor leucocytes is considered to create a tolerant microenvironment around immunoisolated grafts. In this review, we describe the outcomes and limitations of these approaches, as well as the recent progress in immunoisolating devices. Impact statement Immunoisolation by enveloping islets in semipermeable membranes allows for successful transplantation of islet grafts in the absence of chronic immunosuppression, but the duration of graft survival is still not permanent. The reasons for long-term final graft failure is not fully understood, but combining immunoisolation with immunomodulation of tissues or host immune system has been proposed to enhance the longevity of grafts. This article reviews the recent progress and challenges of immunoisolation, as well as the benefits and feasibility of combining encapsulation approaches with immunomodulation to promote longevity of encapsulated grafts.

HIGD-Driven Regulation of Cytochrome c Oxidase Biogenesis and Function

The biogenesis and function of eukaryotic cytochrome c oxidase or mitochondrial respiratory chain complex IV (CIV) undergo several levels of regulation to adapt to changing environmental conditions. Adaptation to hypoxia and oxidative stress involves CIV subunit isoform switch, changes in phosphorylation status, and modulation of CIV assembly and enzymatic activity by interacting factors. The latter include the Hypoxia Inducible Gene Domain (HIGD) family yeast respiratory supercomplex factors 1 and 2 (Rcf1 and Rcf2) and two mammalian homologs of Rcf1, the proteins HIGD1A and HIGD2A. Whereas Rcf1 and Rcf2 are expressed constitutively, expression of HIGD1A and HIGD2A is induced under stress conditions, such as hypoxia and/or low glucose levels. In both systems, the HIGD proteins localize in the mitochondrial inner membrane and play a role in the biogenesis of CIV as a free unit or as part as respiratory supercomplexes. Notably, they remain bound to assembled CIV and, by modulating its activity, regulate cellular respiration. Here, we will describe the current knowledge regarding the specific and overlapping roles of the several HIGD proteins in physiological and stress conditions.

Rcf1 Modulates Cytochrome c Oxidase Activity Especially Under Energy-Demanding Conditions

The mitochondrial respiratory chain is assembled into supercomplexes. Previously, two respiratory supercomplex-associated proteins, Rcf1 and Rcf2, were identified in Saccharomyces cerevisiae, which were initially suggested to mediate supercomplex formation. Recent evidence suggests that these factors instead are involved in cytochrome c oxidase biogenesis. We demonstrate here that Rcf1 mediates proper function of cytochrome c oxidase, while binding of Rcf2 results in a decrease of cytochrome c oxidase activity. Chemical crosslink experiments demonstrate that the conserved Hig-domain as well as the fungi specific C-terminus of Rcf1 are involved in molecular interactions with the cytochrome c oxidase subunit Cox3. We propose that Rcf1 modulates cytochrome c oxidase activity by direct binding to the oxidase to trigger changes in subunit Cox1, which harbors the catalytic site. Additionally, Rcf1 interaction with cytochrome c oxidase in the supercomplexes increases under respiratory conditions. These observations indicate that Rcf1 could enable the tuning of the respiratory chain depending on metabolic needs or repair damages at the catalytic site.

Higd1a improves respiratory function in the models of mitochondrial disorder

The respiratory chain (RC) transports electrons to form a proton motive force that is required for ATP synthesis in the mitochondria. RC disorders cause mitochondrial diseases that have few effective treatments; therefore, novel therapeutic strategies are critically needed. We previously identified Higd1a as a positive regulator of cytochrome c oxidase (CcO) in the RC. Here, we test that Higd1a has a beneficial effect by increasing CcO activity in the models of mitochondrial dysfunction. We first demonstrated the tissue-protective effects of Higd1a via in situ measurement of mitochondrial ATP concentrations ([ATP]_mito) in a zebrafish hypoxia model. Heart-specific Higd1a overexpression mitigated the decline in [ATP]_mito under hypoxia and preserved cardiac function in zebrafish. Based on the in vivo results, we examined the effects of exogenous HIGD1A on three cellular models of mitochondrial disease; notably, HIGD1A improved respiratory function that was coupled with increased ATP synthesis and demonstrated cellular protection in all three models. Finally, enzyme kinetic analysis revealed that Higd1a significantly increased the maximal velocity of the reaction between CcO and cytochrome c without changing the affinity between them, indicating that Higd1a is a positive modulator of CcO. These results corroborate that Higd1a, or its mimic, provides therapeutic options for the treatment of mitochondrial diseases.

Higd-1a regulates the proliferation of pancreatic cancer cells through a pERK/p27KIP1/pRB pathway

Higd-1a/HIMP1-a/HIG1, a mitochondrial inner membrane protein, promotes cell survival under low glucose and hypoxic conditions. We previously reported that it interacts with Opa1, a factor involved in mitochondrial fusion, to regulate mitochondrial homeostasis. In the present study, we found that depletion of Higd-1a inhibited the proliferation of pancreatic cancer cells in vitro and in mice xenografts. Higd-1a knockdown did not itself lead to cell death but it caused cell cycle arrest through induction of p27^KIP1 and hypo-phosphorylation of RB protein. Knockdown of Higd-1a also induced cellular senescence as shown by increased granularity and SA-β-galactosidase activity. We further showed that the mitochondrial stress induced by Higd-1a led to reduced ERK phosphorylation. Inhibition of the ERK pathway with U0126 induced p27^KIP1 expression in the pancreatic cancer cells, confirming that the cell cycle retardation was the result of inhibition of the ERK pathway. Array analysis of human pancreatic cancers revealed that expression of Higd-1a was significantly elevated in pancreatic cancer tissues compared to normal tissue. Collectively, our results demonstrate that Higd-1a plays an important role in the proliferation of pancreatic cancer cells by regulating the pERK/p27^KIP1/pRB signaling pathway.

Higd1a Protects Cells from Lipotoxicity under High-Fat Exposure

Hypoxia-inducible gene domain family member 1A (Higd1a) has recently been reported to protect cells from hypoxia by helping to maintain normal mitochondrial function. The potential induction of Higd1a under high-fat exposure and whether it could protect cells from oxidative stress attracted our attention. Initially, 0.4 mM oleic acid and 0.2 mM palmitate were added to the growth media of HepG2 and LO2 cells for 72 hours. We discovered increased Higd1a expression, and knocking down Higd1a impaired mitochondrial transmembrane potential and induced cell apoptosis. We then identified that elevated reactive oxygen species (ROS) is responsible for increased Higd1a expression. Furthermore, we found that ROS promoted Higd1a expression by upregulating HIF-1a and PGC-1a expressions, and these two proteins could exert synergistic effects in inducing Higd1a expression. Taken together, these data suggest that Higd1a plays positive roles in protecting cells from oxidative stress, and ROS could induce Higd1a expression by upregulating PGC-1a and HIF-1a expressions.

The yeast mitochondrial proteins Rcf1 and Rcf2 support the enzymology of the cytochrome c oxidase complex and generation of the proton motive force

The yeast mitochondrial proteins Rcf1 and Rcf2 are associated with a subpopulation of the cytochrome bc ₁-cytochrome c oxidase supercomplex and have been proposed to play a role in the assembly and/or modulation of the activity of the cytochrome c oxidase (complex IV, CIV). Yeast mutants deficient in either Rcf1 or Rcf2 proteins can use aerobic respiration-based metabolism for growth, but the absence of both proteins results in a strong growth defect. In this study, using assorted biochemical and biophysical analyses of Rcf1/Rcf2 single and double null-mutant yeast cells and mitochondria, we further explored how Rcf1 and Rcf2 support aerobic respiration and growth. We show that the absence of Rcf1 physically reduces the levels of CIV and diminishes the ability of the CIV that is present to maintain a normal mitochondrial proton motive force (PMF). Although the absence of Rcf2 did not noticeably affect the physical content of CIV, the PMF generated by CIV was also lower than normal. Our results indicate that the detrimental effects of the absence of Rcf1 and Rcf2 proteins on the CIV complex are distinct in terms of CIV assembly/accumulation and additive in terms of the ability of CIV to generate PMF. Thus, the combined absence of Rcf1 and Rcf2 alters both CIV physiology and assembly. We conclude that the slow aerobic growth of the Rcf1/Rcf2 double null mutant results from diminished generation of mitochondrial PMF by CIV and limits the level of CIV activity required for maintenance of the PMF and growth under aerobic conditions.

The Expression of Hypoxia-Induced Gene 1 (Higd1a) in the Central Nervous System of Male and Female Rats Differs According to Age

HIGD1A (hypoxia-induced gene domain protein-1a), a mitochondrial inner membrane protein present in various cell types, has been mainly associated with anti-apoptotic processes in response to stressors. Our previous findings have shown that Higd1a mRNA is widely expressed across the central nervous system (CNS), exhibiting an increasing expression in the spinal cord from postnatal day 1 (P1) to 15 (P15) and changes in the distribution pattern from P1 to P90. During the first weeks of postnatal life, the great plasticity of the CNS is accompanied by cell death/survival decisions. So we first describe HIGD1A expression throughout the brain during early postnatal life in female and male pups. Secondly, based on the fact that in some areas this process is influenced by the sex of individuals, we explore HIGD1A expression in the sexual dimorphic nucleus (SDN) of the medial preoptic area, a region that is several folds larger in male than in female rats, partly due to sex differences in the process of apoptosis during this period. Immunohistochemical analysis revealed that HIGD1A is widely but unevenly expressed throughout the brain. Quantitative Western blot analysis of the parietal cortex, diencephalon, and spinal cord from both sexes at P1, P5, P8, and P15 showed that the expression of this protein is predominantly high and changes with age but not sex. Similarly, in the sexual dimorphic nucleus, the expression of HIGD1A varied according to age, but we were not able to detect significant differences in its expression according to sex. Altogether, these results suggest that HIGD1A protein is expressed in several areas of the central nervous system following a pattern that quantitatively changes with age but does not seem to change according to sex.

miR-375 down-regulation of the rearranged L-myc fusion and hypoxia-induced gene domain protein 1A genes and effects on Sertoli cell proliferation

Objective

This study aimed to screen and identify the target genes of miR-375 in pig Sertoli (ST) cells and to elucidate the effect of miR-375 on the proliferation of ST cells.

Methods

In this study, bioinformatics software was used to predict and verify miR-375 target genes. Quantitative polymerase chain reaction (PCR) was used to detect the relationship between miR-375 and its target genes in ST cells. Enzyme-linked immunosorbent assay (ELISA) of rearranged L-myc fusion (RLF) and hypoxia-induced gene domain protein 1A (HIGD1A) was performed on porcine ST cells, which were transfected with a miR-375 mimics and inhibitor to verify the results. Dual luciferase reporter gene assays were performed to assess the interactions among miR-375, RLF, and HIGD1A. The effect of miR-375 on the proliferation of ST cells was analyzed by CellTiter 96 AQueous One Solution Cell Proliferation Assay (MTS).

Results

Five possible target genes of miR-375, including RLF, HIGD1A, colorectal cancer associated 2, POU class 3 homeobox 1, and WW domain binding protein 1 like, were found. The results of quantitative PCR suggested that mRNA expression of RLF and HIGD1A had a negative correlation with miR-375, indicating that RLF and HIGD1A are likely the target genes of miR-375. The ELISA results revealed that RLF and HIGD1A were negatively correlated with the miR-375 protein level. The luminescence results for the miR-375 group co-transfected with wild-type RLF and HIGD1A vector were significantly lower than those of the miR-375 group co-transfected with the blank vector or mutant RLF and HIGD1A vectors. The present findings suggest that RLF and HIGD1A are target genes of miR-375 and that miR-375 inhibits ST cell proliferation according to MTS analysis.

Conclusion

It was speculated that miR-375 affects cell proliferation through its target genes, which play an important role in the development of testicular tissue.

Naringin Protects Pancreatic β-Cells Against Oxidative Stress-Induced Apoptosis by Inhibiting Both Intrinsic and Extrinsic Pathways in Insulin-Deficient Diabetic Mice

SCOPE

Oxidative stress has been suggested to play a central role in the pathogenesis of diabetes, as well as other metabolic disorders. Naringin, a major flavanone glycoside in citrus species, has been shown to display strong antioxidant potential in in vitro and in vivo models of oxidative stress; however, the underlying protective mechanisms in diabetes are unclear.

METHODS AND RESULTS

To study the protective effects and molecular mechanisms of naringin in preventing islet dysfunction and diabetes, we examined glucose homeostasis, β-cell apoptosis, and inflammatory response in insulin-deficient diabetic mice exposed to acute oxidative stress with streptozotocin (STZ). Naringin dose-dependently ameliorated hyperglycemia and islet dysfunction in insulin-deficient diabetic mice. Naringin counteracted STZ-induced β-cell apoptosis by inhibiting both the intrinsic (mitochondria-mediated) and extrinsic (death receptor-mediated) pathways. Furthermore, these protective effects were associated with suppression of DNA damage response and nuclear factor-kappa B- and mitogen-activated protein kinase-mediated signaling pathways, as well as reduction of reactive oxygen species accumulation and pro-inflammatory cytokine production in the pancreas.

CONCLUSION

Taken together, our study provides insights into the underlying mechanisms through which naringin protects the pancreatic β-cells against oxidative stress-induced apoptosis.

Subunit composition of respiratory chain complex 1 and its responses to oxygen in mitochondria from human donor livers

OBJECTIVE

Donor liver function in transplantation is defined by mitochondrial function and the ability of mitochondria to recover from the sequence of warm and/or cold ischemia. Mitochondrial resilience maybe related to assembly and- subunit composition of Complex 1. The aim of this study was to determine if Complex 1 subunit composition was different in donor livers of varying quality and whether oxygen exposure had any effect.

RESULTS

Five human livers not suitable for transplant were split. One half placed in cold static storage and the other half exposed to 40% oxygen for 2 h. Protein was extracted for western blot. Membranes were probed with antibodies against β-actin and the following subunits of Complex 1: MTND1, NDUFA10, NDUFB6 and NDUFV2. No difference in steady state Complex 1 subunit composition was demonstrated between donor livers of varying quality, in terms of steatosis or mode of donation. Neither did exposure to oxygen influence Complex 1 subunit composition. This small observational study on subunit levels suggest that Complex 1 is fully assembled as no degradation of subunits associated with the different parts of the enzyme was seen.

Mutational Analysis of the QRRQ Motif in the Yeast Hig1 Type 2 Protein Rcf1 Reveals a Regulatory Role for the Cytochrome c Oxidase Complex

The yeast Rcf1 protein is a member of the conserved family of proteins termed the hypoxia-induced gene (domain) 1 (Hig1 or HIGD1) family. Rcf1 interacts with components of the mitochondrial oxidative phosphorylation system, in particular the cytochrome bc₁ (complex III)-cytochrome c oxidase (complex IV) supercomplex (termed III-IV) and the ADP/ATP carrier proteins. Rcf1 plays a role in the assembly and modulation of the activity of complex IV; however, the molecular basis for how Rcf1 influences the activity of complex IV is currently unknown. Hig1 type 2 isoforms, which include the Rcf1 protein, are characterized in part by the presence of a conserved motif, (Q/I)X₃(R/H)XRX₃Q, termed here the QRRQ motif. We show that mutation of conserved residues within the Rcf1 QRRQ motif alters the interactions between Rcf1 and partner proteins and results in the destabilization of complex IV and alteration of its enzymatic properties. Our findings indicate that Rcf1 does not serve as a stoichiometric component, i.e. as a subunit of complex IV, to support its activity. Rather, we propose that Rcf1 serves to dynamically interact with complex IV during its assembly process and, in doing so, regulates a late maturation step of complex IV. We speculate that the Rcf1/Hig1 proteins play a role in the incorporation and/or remodeling of lipids, in particular cardiolipin, into complex IV and. possibly, other mitochondrial proteins such as ADP/ATP carrier proteins.

A novel rice protein family of OsHIGDs may be involved in early signalling of hypoxia-promoted stem growth in deepwater rice

OsHIGDs was identified as a novel hypoxia-responsive protein family. Among them, OsHIGD2 is characterized as a mitochondrial protein and is related to hypoxia signalling through interacting with mitochondrial proteins of critical functions in reducing cell damages caused by hypoxia. Recent evidence supports ethylene as a key factor in modulating plant responses to submergence stress. Meanwhile, there has been general consent that ethylene is not the only signal for the submergence-induced stem growth. In this study, we confirmed that hypoxia also promotes stem elongation in deepwater rice even in the absence of ethylene. As components of ethylene-independent hypoxia signalling, five HIGD (hypoxia-induced gene domain) protein genes were identified. Among the genes, OsHIGD2 showed the fastest and strongest induction by hypoxia as well as submergence. Co-expression analysis indicated that OsHIGD2 had a simultaneous expression pattern with fermentation-related genes, such as ADH1 (alcohol dehydrogenase 1) and PDC2 (pyruvate decarboxylase 2). Transient expression of OsHIGD2 in leaf epidermal cells of Nicotiana benthamiana provided evidence that the protein is localized to mitochondria. We further identified OsHIGD2-interacting proteins through the yeast two-hybrid assay using OsHIGD2 as bait. As a result, three mitochondrial proteins were discovered that function in the regulation of redox potential or reduction of protein damages caused by reactive oxygen species. In this report, we propose that OsHIGD2 is a mitochondrial protein which takes part in the early stage of hypoxia signalling by interacting with proteins that are related to oxygen utilization.

HIGD1A Regulates Oxygen Consumption, ROS Production, and AMPK Activity during Glucose Deprivation to Modulate Cell Survival and Tumor Growth

Hypoxia-inducible gene domain family member 1A (HIGD1A) is a survival factor induced by hypoxia-inducible factor 1 (HIF-1). HIF-1 regulates many responses to oxygen deprivation, but viable cells within hypoxic perinecrotic solid tumor regions frequently lack HIF-1α. HIGD1A is induced in these HIF-deficient extreme environments and interacts with the mitochondrial electron transport chain to repress oxygen consumption, enhance AMPK activity, and lower cellular ROS levels. Importantly, HIGD1A decreases tumor growth but promotes tumor cell survival in vivo. The human Higd1a gene is located on chromosome 3p22.1, where many tumor suppressor genes reside. Consistent with this, the Higd1a gene promoter is differentially methylated in human cancers, preventing its hypoxic induction. However, when hypoxic tumor cells are confronted with glucose deprivation, DNA methyltransferase activity is inhibited, enabling HIGD1A expression, metabolic adaptation, and possible dormancy induction. Our findings therefore reveal important new roles for this family of mitochondrial proteins in cancer biology.

Higd1a is a positive regulator of cytochrome c oxidase

Cytochrome c oxidase (CcO) is the only enzyme that uses oxygen to produce a proton gradient for ATP production during mitochondrial oxidative phosphorylation. Although CcO activity increases in response to hypoxia, the underlying regulatory mechanism remains elusive. By screening for hypoxia-inducible genes in cardiomyocytes, we identified hypoxia inducible domain family, member 1A (Higd1a) as a positive regulator of CcO. Recombinant Higd1a directly integrated into highly purified CcO and increased its activity. Resonance Raman analysis revealed that Higd1a caused structural changes around heme a, the active center that drives the proton pump. Using a mitochondria-targeted ATP biosensor, we showed that knockdown of endogenous Higd1a reduced oxygen consumption and subsequent mitochondrial ATP synthesis, leading to increased cell death in response to hypoxia; all of these phenotypes were rescued by exogenous Higd1a. These results suggest that Higd1a is a previously unidentified regulatory component of CcO, and represents a therapeutic target for diseases associated with reduced CcO activity.

Respiratory supercomplexes: plasticity and implications

The plasticity model of the electron transport chain has slowly begun to replace both the liquid model of free complexes and the solid model of supercomplexes. The plasticity model predicts that respiratory complexes exist and function both as single complexes and as supercomplexes. The advantages of this system is an electron transport train which is able to adapt to changes in its environment. This review will investigate the current body of work on supercomplexes including their assembly, regulation, and plasticity, and particularly their role in the generation of reactive oxygen species and aging.

Sumoylation modulates oxidative stress relevant to the viability and functionality of pancreatic beta cells

Sumoylation is an evolutionarily conserved regulatory mechanism to play an important role in various cellular processes through modulation of protein localization, stability and functionality. Recent studies including ours have consistently demonstrated that sumoylation provides protection for cells against oxidative stress. Given that pancreatic beta cells are a vulnerable target of oxidative stress, we thus in this minireview, updated the advancement of sumoylation in the regulation of ROS generation, and discussed its impact on several critical signaling pathways relevant to beta cells against oxidative stress and maintenance of functionality. Specifically, we bring together how sumoylation represses intracellular ROS formation, and protects beta cells against oxidative stress through regulating IκB/NFκB, JNK/c-Jun, and Maf/Nrf2 pathways. The tight implication of sumoylation in oxidative stress reflects that it could be an essential mechanism for beta cells to adapt to the detrimental cellular microenvironment.

Higd-1a interacts with Opa1 and is required for the morphological and functional integrity of mitochondria

The activity and morphology of mitochondria are maintained by dynamic fusion and fission processes regulated by a group of proteins residing in, or attached to, their inner and outer membranes. Hypoxia-induced gene domain protein-1a (Higd-1a)/HIMP1-a/HIG1, a mitochondrial inner membrane protein, plays a role in cell survival under hypoxic conditions. In the present study, we showed that Higd-1a depletion resulted in mitochondrial fission, depletion of mtDNA, disorganization of cristae, and growth retardation. We demonstrated that Higd-1a functions by specifically binding to Optic atrophy 1 (Opa1), a key element in fusion of the inner membrane. In the absence of Higd-1a, Opa1 was cleaved, resulting in the loss of its long isoforms and accumulation of small soluble forms. The small forms of Opa1 do not interact with Higd-1a, suggesting that a part of Opa1 in or proximal to the membrane is required for that interaction. Opa1 cleavage, mitochondrial fission, and cell death induced by dissipation of the mitochondrial membrane potential were significantly inhibited by ectopic expression of Higd-1a. Furthermore, growth inhibition due to Higd-1a depletion could be overcome by overexpression of a noncleavable form of Opa1. Collectively, our observations demonstrate that Higd-1a inhibits Opa1 cleavage and is required for mitochondrial fusion by virtue of its interaction with Opa1.

Evolution of three parent genes and their retrogene copies in Drosophila species

Retrogenes form a class of gene duplicate lacking the regulatory sequences found outside of the mRNA-coding regions of the parent gene. It is not clear how a retrogene's lack of parental regulatory sequences affects the evolution of the gene pair. To explore the evolution of parent genes and retrogenes, we investigated three such gene pairs in the family Drosophilidae; in Drosophila melanogaster, these gene pairs are CG8331 and CG4960, CG17734 and CG11825, and Sep2 and Sep5. We investigated the embryonic expression patterns of these gene pairs across multiple Drosophila species. Expression patterns of the parent genes and their single copy orthologs are relatively conserved across species, whether or not a species has a retrogene copy, although there is some variation in CG8331 and CG17734. In contrast, expression patterns of the retrogene orthologs have diversified. We used the genome sequences of 20 Drosophila species to investigate coding sequence evolution. The coding sequences of the three gene pairs appear to be evolving predominantly under negative selection; however, the parent genes and retrogenes show some distinct differences in amino acid sequence. Therefore, in general, retrogene expression patterns and coding sequences are distinct compared to their parents and, in some cases, retrogene expression patterns diversify.

Respiratory supercomplexes: structure, function and assembly

The mitochondrial respiratory chain consists of 5 enzyme complexes that are responsible for ATP generation. The paradigm of the electron transport chain as discrete enzymes diffused in the inner mitochondrial membrane has been replaced by the solid state supercomplex model wherein the respiratory complexes associate with each other to form supramolecular complexes. Defects in these supercomplexes, which have been shown to be functionally active and required for forming stable respiratory complexes, have been associated with many genetic and neurodegenerative disorders demonstrating their biomedical significance. In this review, we will summarize the functional and structural significance of supercomplexes and provide a comprehensive review of their assembly and the assembly factors currently known to play a role in this process.

Nuclear localization of the mitochondrial factor HIGD1A during metabolic stress

Cellular stress responses are frequently governed by the subcellular localization of critical effector proteins. Apoptosis-inducing Factor (AIF) or Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH), for example, can translocate from mitochondria to the nucleus, where they modulate apoptotic death pathways. Hypoxia-inducible gene domain 1A (HIGD1A) is a mitochondrial protein regulated by Hypoxia-inducible Factor-1α (HIF1α). Here we show that while HIGD1A resides in mitochondria during physiological hypoxia, severe metabolic stress, such as glucose starvation coupled with hypoxia, in addition to DNA damage induced by etoposide, triggers its nuclear accumulation. We show that nuclear localization of HIGD1A overlaps with that of AIF, and is dependent on the presence of BAX and BAK. Furthermore, we show that AIF and HIGD1A physically interact. Additionally, we demonstrate that nuclear HIGD1A is a potential marker of metabolic stress in vivo, frequently observed in diverse pathological states such as myocardial infarction, hypoxic-ischemic encephalopathy (HIE), and different types of cancer. In summary, we demonstrate a novel nuclear localization of HIGD1A that is commonly observed in human disease processes in vivo.

Hypoxia-inducible factor-1 alpha in the heart: a double agent?

Hypoxia-inducible factor (HIF) is a set of transcription factors that regulate the cellular response to hypoxia. There is a great body of evidence supporting the protective role of HIF-1α in cardiovascular pathophysiology, however, newer studies are hinting at a maladaptive and deleterious role of this transcription factor that merits further investigation. There is a general agreement, however, that HIF-mediated responses appear to differ under conditions of acute and chronic oxygen deprivation. The intensity and sustainability of HIF-1α activation are major determinants of whether the responses are pathological or beneficial. HIF activation is seen to be beneficial in the setting of acute myocardial ischemia and deleterious in chronic conditions. In this review, we will focus on recent insights into the role of HIF-1α in the heart and especially in the setting of ischemic heart disease.

The archipelago ubiquitin ligase subunit acts in target tissue to restrict tracheal terminal cell branching and hypoxic-induced gene expression

The Drosophila melanogaster gene archipelago (ago) encodes the F-box/WD-repeat protein substrate specificity factor for an SCF (Skp/Cullin/F-box)-type polyubiquitin ligase that inhibits tumor-like growth by targeting proteins for degradation by the proteasome. The Ago protein is expressed widely in the fly embryo and larva and promotes degradation of pro-proliferative proteins in mitotically active cells. However the requirement for Ago in post-mitotic developmental processes remains largely unexplored. Here we show that Ago is an antagonist of the physiologic response to low oxygen (hypoxia). Reducing Ago activity in larval muscle cells elicits enhanced branching of nearby tracheal terminal cells in normoxia. This tracheogenic phenotype shows a genetic dependence on sima, which encodes the HIF-1α subunit of the hypoxia-inducible transcription factor dHIF and its target the FGF ligand branchless (bnl), and is enhanced by depletion of the Drosophila Von Hippel Lindau (dVHL) factor, which is a subunit of an oxygen-dependent ubiquitin ligase that degrades Sima/HIF-1α protein in metazoan cells. Genetic reduction of ago results in constitutive expression of some hypoxia-inducible genes in normoxia, increases the sensitivity of others to mild hypoxic stimulus, and enhances the ability of adult flies to recover from hypoxic stupor. As a molecular correlate to these genetic data, we find that Ago physically associates with Sima and restricts Sima levels in vivo. Collectively, these findings identify Ago as a required element of a circuit that suppresses the tracheogenic activity of larval muscle cells by antagonizing the Sima-mediated transcriptional response to hypoxia.

Proinsulin atypical maturation and disposal induces extensive defects in mouse Ins2+/Akita β-cells

Because of its low relative folding rate and plentiful manufacture in β-cells, proinsulin maintains a homeostatic balance of natively and plentiful non-natively folded states (i.e., proinsulin homeostasis, PIHO) through the integration of maturation and disposal processes. PIHO is susceptible to genetic and environmental influences, and its disorder has been critically linked to defects in β-cells in diabetes. To explore this hypothesis, we performed polymerase chain reaction (PCR), metabolic-labeling, immunoblotting, and histological studies to clarify what defects result from primary disorder of PIHO in model Ins2^+/Akita β-cells. We used T antigen-transformed Ins2^+/Akita and control Ins2^+/+ β-cells established from Akita and wild-type littermate mice. In Ins2^+/Akita β-cells, we found no apparent defect at the transcriptional and translational levels to contribute to reduced cellular content of insulin and its precursor and secreted insulin. Glucose response remained normal in proinsulin biosynthesis but was impaired for insulin secretion. The size and number of mature insulin granules were reduced, but the size/number of endoplasmic reticulum, Golgi, mitochondrion, and lysosome organelles and vacuoles were expanded/increased. Moreover, cell death increased, and severe oxidative stress, which manifested as increased reactive oxygen species, thioredoxin-interacting protein, and protein tyrosine nitration, occurred in Ins2^+/Akita β-cells and/or islets. These data show the first clear evidence that primary PIHO imbalance induces severe oxidative stress and impairs glucose-stimulated insulin release and β-cell survival as well as producing other toxic consequences. The defects disclosed/clarified in model Ins2^+/Akita β-cells further support a role of the genetic and stress-susceptible PIHO disorder in β-cell failure and diabetes.

β-Cells with relative low HIMP1 overexpression levels in a transgenic mouse line enhance basal insulin production and hypoxia/hypoglycemia tolerance

Rodent pancreatic β-cells that naturally lack hypoglycemia/hypoxia inducible mitochondrial protein 1 (HIMP1) are susceptible to hypoglycemia and hypoxia influences. A linkage between the hypoglycemia/hypoxia susceptibility and the lack of HIMP1 is suggested in a recent study using transformed β-cells lines. To further illuminate this linkage, we applied mouse insulin 1 gene promoter (MIP) to control HIMP1-a isoform cDNA and have generated three lines (L1 to L3) of heterozygous HIMP1 transgenic (Tg) mice by breeding of three founders with C57BL/6J mice. In HIMP1-Tg mice/islets, we performed quantitative polymerase chain reaction (PCR), immunoblot, histology, and physiology studies to investigate HIMP1 overexpression and its link to β-cell function/survival and body glucose homeostasis. We found that the HIMP1 level increased steadily in β-cells of L1 to L3 heterozygous HIMP1-Tg mice. HIMP1 overexpression at relatively lower levels in L1 heterozygotes results in a negligible decline in blood glucose concentrations and an insignificant elevation in blood insulin levels, while HIMP1 overexpression at higher levels are toxic, causing hyperglycemia in L2/3 heterozygotes. Follow-up studies in 5-30-week-old L1 heterozygous mice/islets found that HIMP1 overexpression at relatively lower levels in β-cells has enhanced basal insulin biosynthesis, basal insulin secretion, and tolerances to low oxygen/glucose influences. The findings enforced the linkage between the hypoglycemia/hypoxia susceptibility and the lack of HIMP1 in β-cells, and show a potential value of HIMP1 overexpression at relatively lower levels in modulating β-cell function and survival.

Identification of a protein mediating respiratory supercomplex stability

The complexes of the electron transport chain associate into large macromolecular assemblies, which are believed to facilitate efficient electron flow. We have identified a conserved mitochondrial protein, named respiratory supercomplex factor 1 (Rcf1-Yml030w), that is required for the normal assembly of respiratory supercomplexes. We demonstrate that Rcf1 stably and independently associates with both Complex III and Complex IV of the electron transport chain. Deletion of the RCF1 gene caused impaired respiration, probably as a result of destabilization of respiratory supercomplexes. Consistent with the hypothetical function of these respiratory assemblies, loss of RCF1 caused elevated mitochondrial oxidative stress and damage. Finally, we show that knockdown of HIG2A, a mammalian homolog of RCF1, causes impaired supercomplex formation. We suggest that Rcf1 is a member of an evolutionarily conserved protein family that acts to promote respiratory supercomplex assembly and activity.

Temporal distribution of Hig-1 (hypoxia-induced gene 1) mRNA and protein in rat spinal cord: changes during postnatal life

Several cellular and molecular events responsible for the development of the central nervous system (CNS), particularly those related to the development of ordered neural connections, occur during the first days of postnatal life, being days 1 through 10 a critical period to reach maturity and establish innervations. We have previously characterized hypoxia-induced gene 1 (Hig-1) and described an increase in its expression from day 1 to 15 of postnatal life in the spinal cord. Hig-1 mRNA has an open reading frame for a 93 amino acid protein, but its function has not been completely elucidated. Recently, several analyses in many cell types have related Hig-1 expression with differentiation or cell death/survival balance. With the aim of further characterizing the presence of Hig-1 in the CNS, we analyzed the cellular distribution of HIG-1 protein in rat's spinal cord at postnatal days 1, 8, 15, and 90 (P1-P90). We found an interesting change in the protein expression pattern, shifting from neurons at P1 to glial cells at P90, which points towards a functional role for this protein in the spinal cord throughout development. We also compared the protein distribution with the cellular distribution of the mRNA and of an antisense RNA.

HIG1, a novel regulator of mitochondrial γ-secretase, maintains normal mitochondrial function

The γ-secretase complex (which contains presenilins, nicastrin, anterior pharynx defective-1, and presenilin enhancer-2) cleaves type I transmembrane proteins, including Notch and amyloid precursor protein. Dysregulated γ-secretase activity has been implicated in the pathogenesis of Alzheimer's disease, stroke, atherosclerosis, and cancer. Tight regulation of γ-secretase activity is required for normal physiology. Here, we isolated HIG1 (hypoxia inducible gene 1, domain member 1A) from a functional screen of γ-secretase inhibitory genes. HIG1 was highly expressed in the brain. Interestingly, HIG1 was localized to the mitochondria and was directly bound to γ-secretase components on the mitochondrial membrane in SK-N-SH neuroblastoma cells. Overexpresssion of HIG1 attenuated hypoxia-induced γ-secretase activation on the mitochondrial membrane and the accumulation of intracellular amyloid β. This accumulation was accompanied by hypoxia-induced mitochondrial dysfunction. The latter half domain of HIG1 was required for binding to the γ-secretase complex and suppression of γ-secretase activity. Moreover, depletion of HIG1 increased γ-secretase activation and enhanced hypoxia-induced mitochondrial dysfunction. In summary, HIG1 is a novel modulator of the mitochondrial γ-secretase complex, and may play a role in the maintenance of normal mitochondrial function.

Rcf1 and Rcf2, members of the hypoxia-induced gene 1 protein family, are critical components of the mitochondrial cytochrome bc1-cytochrome c oxidase supercomplex

We report that Rcf1 (formerly Aim31), a member of the conserved hypoxia-induced gene 1 (Hig1) protein family, represents a novel component of the yeast cytochrome bc(1)-cytochrome c oxidase (COX) supercomplex. Rcf1 (respiratory supercomplex factor 1) partitions with the COX complex, and evidence that it may act as a bridge to the cytochrome bc(1) complex is presented. Rcf1 interacts with the Cox3 subunit and can do so prior to their assembly into the COX complex. A close proximity of Rcf1 and members of the ADP/ATP carrier (AAC) family was also established. Rcf1 displays overlapping function with another Hig1-related protein, Rcf2 (formerly Aim38), and their joint presence is required for optimal COX enzyme activity and the correct assembly of the cytochrome bc(1)-COX supercomplex. Rcf1 and Rcf2 can independently associate with the cytochrome bc(1)-COX supercomplex, indicating that at least two forms of this supercomplex exist within mitochondria. We provide evidence that the association with the cytochrome bc(1)-COX supercomplex and regulation of the COX complex are a conserved feature of Hig1 family members. Based on our findings, we propose a model where the Hig1 proteins regulate the COX enzyme activity through Cox3 and associated Cox12 protein, in a manner that may be influenced by the neighboring AAC proteins.

Supramolecular organization in prokaryotic respiratory systems

Prokaryotes are characterized by an extreme flexibility of their respiratory systems allowing them to cope with various extreme environments. To date, supramolecular organization of respiratory systems appears as a conserved evolutionary feature as supercomplexes have been isolated in bacteria, archaea, and eukaryotes. Most of the yet identified supercomplexes in prokaryotes are involved in aerobic respiration and share similarities with those reported in mitochondria. Supercomplexes likely reflect a snapshot of the cellular respiration in a given cell population. While the exact nature of the determinants for supramolecular organization in prokaryotes is not understood, lipids, proteins, and subcellular localization can be seen as key players. Owing to the well-reported supramolecular organization of the mitochondrial respiratory chain in eukaryotes, several hypotheses have been formulated to explain the consequences of such arrangement and can be tested in the context of prokaryotes. Considering the inherent metabolic flexibility of a number of prokaryotes, cellular distribution and composition of the supramolecular assemblies should be studied in regards to environmental signals. This would pave the way to new concepts in cellular respiration.

The survival effect of mitochondrial Higd-1a is associated with suppression of cytochrome C release and prevention of caspase activation

Higd-1a (hypoxia induced gene domain family-1a) is a mitochondrial inner membrane protein with a conformation of N-terminal outside-C-terminal outside and loop inside. There are four Higd genes, Higd-1a, -1b, -1c and -2a, in the mouse. Higd-1a and -2a are expressed primarily in the brain, heart, kidney and leukocytes. HIF (hypoxia-inducible factor) overexpression induced the endogenous expression and promoter activity of Higd-1a. Mutation of the HRE (hypoxia-response element) site at -32bp in the Higd-1a promoter reduced the promoter activity, suggesting that transcription of Higd-1a is regulated by binding of the transcription factor HIF to the HRE. Higd-1a promoted cell survival under hypoxia. RAW264.7 cells stably transfected with Higd-1a underwent less apoptosis than control cells in a hypoxic condition, and hypoxia-induced apoptosis was strongly enhanced when endogenous Higd-1a was silenced by siRNA. The survival effect of Higd-1a was completely abolished by deletion of the 26 N-terminal amino acids, and we showed that Higd-1a increased survival by inhibiting cytochrome C release and reducing the activities of caspases. However, expression of Bcl-2, Bax, Bad, and BNIP3 and translocation of AIF were unaffected under the same conditions. Higd-2a also enhanced cell survival under hypoxia. Cells transfected with Higd-2a underwent less apoptosis than control cells in hypoxic conditions, and hypoxia-induced apoptosis increased when endogenous Higd-2a was depleted. Together these observations indicate that Higd-1a is induced by hypoxia in a HIF-dependent manner and its anti-apoptotic effect results from inhibiting cytochrome C release and reducing caspase activities.

Reg2 protects mouse insulinoma cells from streptozotocin-induced mitochondrial disruption and apoptosis

We reported previously that pancreas-specific ablation of IGF-I in mice induced an increased expression of regenerating family proteins Reg2 and Reg3&#x03B2; in the pancreas and protected them from streptozotocin (Stz)-induced &#x03B2;-cell damage. We, therefore, assessed the effect of ectopically introduced Reg2 on Stz-induced apoptosis in MIN6 mouse insulinoma cells and report here that Reg2 protects MIN6 cells from Stz-induced apoptosis by attenuating its ability to disrupt mitochondrial membrane integrity, activate caspase-3 and promote poly-ADP ribose polymerase cleavage, and induce apoptosis. These changes correlated with suppression of c-jun N-terminal kinase (JNK) phosphorylation by Stz. Reg2 inhibited Stz-induced proapoptotic events as well as the inactivation of JNK. Inclusion of chemical inhibitor of JNK to Reg2 expressing cells rendered them sensitive to Stz. These data demonstrate that Reg2 protects insulin-producing cells against Stz-induced apoptosis by interfering with its cytotoxic signaling upstream of the intrinsic proapoptotic events by preventing its ability to inactivate JNK.

The isolated pancreatic islet as a micro-organ and its transplantation to cure diabetes: celebrating the legacy of Paul Lacy

Over the past three decades the pancreatic islet of Langerhans has taken center stage as an endocrine micro-organ whose glucoregulatory function is highly explicable on the basis of the increasingly well understood activities of three highly interactive secretory cells. Islet dysfunction underlies both type 1 and type 2 diabetes mellitus (DM); its protection from immune attack and gluco-and lipo-toxicity may prevent the development of DM; and its replacement by non-surgical transplantation may be curative of DM. During a career marked by vision, focus and tenacity, Paul Lacy contributed substantially to the development of each of these concepts. In this review we focus on Lacy's contribution to the development of the concept of the islet as a micro-organ, how this foreshadowed our current detailed understanding of single cell function and cell-cell interactions and how this led to a reduced model of islet function encouraging islet transplantation. Next, we examine how clinical allotransplantation, first undertaken by Lacy, has contributed to a more complex view of the interaction of islet endocrine cells with its circulation and neighboring tissues, both "in situ" and after transplantation. Lastly, we consider recent developments in some alternative approaches to treatment of DM that Lacy could glimpse on the horizon but did not have the chance to participate in.

Gene expression in a Drosophila model of mitochondrial disease

Background

A point mutation in the Drosophila gene technical knockout (tko), encoding mitoribosomal protein S12, was previously shown to cause a phenotype of respiratory chain deficiency, developmental delay, and neurological abnormalities similar to those presented in many human mitochondrial disorders, as well as defective courtship behavior.

Methodology/Principal Findings

Here, we describe a transcriptome-wide analysis of gene expression in tko^25t mutant flies that revealed systematic and compensatory changes in the expression of genes connected with metabolism, including up-regulation of lactate dehydrogenase and of many genes involved in the catabolism of fats and proteins, and various anaplerotic pathways. Gut-specific enzymes involved in the primary mobilization of dietary fats and proteins, as well as a number of transport functions, were also strongly up-regulated, consistent with the idea that oxidative phosphorylation OXPHOS dysfunction is perceived physiologically as a starvation for particular biomolecules. In addition, many stress-response genes were induced. Other changes may reflect a signature of developmental delay, notably a down-regulation of genes connected with reproduction, including gametogenesis, as well as courtship behavior in males; logically this represents a programmed response to a mitochondrially generated starvation signal. The underlying signalling pathway, if conserved, could influence many physiological processes in response to nutritional stress, although any such pathway involved remains unidentified.

Conclusions/Significance

These studies indicate that general and organ-specific metabolism is transformed in response to mitochondrial dysfunction, including digestive and absorptive functions, and give important clues as to how novel therapeutic strategies for mitochondrial disorders might be developed.

Reactive oxygen species-mediated pancreatic beta-cell death is regulated by interactions between stress-activated protein kinases, p38 and c-Jun N-terminal kinase, and mitogen-activated protein kinase phosphatases

Pancreatic beta-cells are susceptible to reactive oxygen species (ROS), which are known to be generated by high or low glucose (LG), hypoxic, or cytokine-producing conditions. When we cultured mouse beta-cell-derived MIN6 cells in a LG condition, we detected a significant generation of ROS, including hydrogen peroxide, which was comparable to the ROS production in hypoxic or cytokine-treated conditions. ROS accumulation induced by the LG culture led to cell death, which was prevented by the ROS scavengers N-acetylcysteine and manganese(III)tetrakis(4-benzoic acid) porphyrin. We next investigated the mechanism of stress-activated protein kinases (SAPKs), c-jun N-terminal kinase (JNK) and p38, in ROS-induced MIN6 cell death. Activation of p38 occurred immediately after the LG culture, whereas JNK activation increased slowly 8 h later. Adenoviral p38 expression decreased MIN6 cell death, whereas the JNK expression increased it. Consistently, blocking p38 activation by inhibitors increased beta-cell death, whereas JNK inhibitors decreased it. We then examined the role of MAPK phosphatases (MKPs) specific for stress-activated protein kinases in beta-cell death. We found that MKP-1 presented an increase in its oxidized product after the LG culture. ROS scavengers prevented the appearance of this oxidized product and JNK activation. Thus, ROS-induced MKP inactivation causes sustained activation of JNK, which contributes to beta-cell death. Adenoviral overexpression of MKP-1 and MKP-7 prevented the phosphorylation of JNK at 36 h after the LG culture, and decreased MIN6 beta-cell death. We suggest that beta-cell death is regulated by interactions between JNK and its specific MKPs.

Intermittent hypoxia reverses the diurnal glucose rhythm and causes pancreatic beta-cell replication in mice

Obstructive sleep apnoea (OSA) and type 2 diabetes frequently co-exist and potentially interact haemodynamically and metabolically. However, the confounding effects of obesity have obscured the examination of any independent or interactive effects of the hypoxic stress of OSA and the hyperglycaemia of type 2 diabetes on haemodynamic and metabolic outcomes. We have developed a chronically catheterized, unhandled, lean murine model to examine the effects of intermittent hypoxic (IH) exposure and exogenous glucose infusion on the diurnal pattern of arterial blood pressure and blood glucose, as well as pancreatic beta-cell growth and function. Four experimental groups of adult male C57BL/J mice were exposed to 80 h of (1) either IH (nadir of inspired oxygen 5-6% at 60 cycles h(-1) for 12 h during light period) or intermittent air (IA; control) and (2) continuous infusion of either 50% dextrose or saline (control). IH exposure during saline infusion caused a sustained increase in arterial blood pressure of 10 mmHg (P < 0.0001), reversed the normal diurnal rhythm of blood glucose (P < 0.03), doubled corticosterone levels (P < 0.0001), and increased replication of pancreatic beta-cells from 1.5 +/- 0.3 to 4.0 +/- 0.8% bromodeoxyuridine (BrdU)-positive) beta-cells. The combined stimulus of IH exposure and glucose infusion attenuated the hypertension, exacerbated the reversed diurnal glucose rhythm, and produced the highest rates of apoptosis in beta-cells, without any additive effects on beta-cell replication. We conclude that, in contrast to the development of sustained hypertension, IH impaired glucose homeostasis only during periods of hypoxic exposure. IH acted as a stimulus to pancreatic beta-cell replication, but the presence of hyperglycaemia may increase the hypoxic susceptibility of beta-cells. This model will provide a basis for future mechanistic studies as well as assessing the metabolic impact of common comorbities in OSA, including obesity, insulin resistance and type 2 diabetes.

Gene expression profile of bioreactor-cultured cardiac cells: activation of morphogenetic pathways for tissue engineering

Cells grown in three-dimensional (3D) culture take on in vivo phenotypes and organize into tissue-like structures. Understanding the pathways and mechanisms contributing to this in vitro tissuegenesis is a critical goal of tissue engineering. To identify pathways relevant to cardiac tissue engineering, we compared mRNA expression profiles from bioreactor-cultured 3D aggregates of primary neonatal rat heart cells (NRHCs), which form layered structures similar to cardiac tissue, and standard plate-cultured NRHCs, which do not. In a series of two experiments, NRHCs were grown on solid microcarrier surfaces within clinostatically rotated polytetrafluoroethylene (PTFE) vessels and compared to parallel cultures grown on standard tissue culture plates without rotation. After 1, 4, and 6 days, gene expression profiles were analyzed using Affymetrix Rat Genome U34A (RG-U34A) arrays. The results were validated using real-time PCR, and the data set was filtered to generate a list of 93 probe sets that were substantially the same in replicate samples but substantially different between the bioreactor and plate groups. Cluster analysis indicated that the bioreactor and plate samples had similar expression patterns on day 1 but that these patterns diverged thereafter. Database for Annotation, Visualization, and Integrated Discovery (DAVID) analysis revealed a number of statistically significant gene groupings, including groups associated with muscle development and morphogenesis. Further analysis of the annotated gene list indicated that 13 of the 93 filtered genes were associated with endothelial cells, blood vessels, or angiogenesis. These results suggest that 3D aggregate culture of NRHCs in bioreactors is associated with the differential expression of morphogenic and angiogenic pathways similar to those seen during cardiac development.