Old dog, new tricks: Arf1 required for mitochondria homeostasis

ARF1 prevents aberrant type I interferon induction by regulating STING activation and recycling

Type I interferon (IFN) signalling is tightly controlled. Upon recognition of DNA by cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING) translocates along the endoplasmic reticulum (ER)-Golgi axis to induce IFN signalling. Termination is achieved through autophagic degradation or recycling of STING by retrograde Golgi-to-ER transport. Here, we identify the GTPase ADP-ribosylation factor 1 (ARF1) as a crucial negative regulator of cGAS-STING signalling. Heterozygous ARF1 missense mutations cause a previously unrecognized type I interferonopathy associated with enhanced IFN-stimulated gene expression. Disease-associated, GTPase-defective ARF1 increases cGAS-STING dependent type I IFN signalling in cell lines and primary patient cells. Mechanistically, mutated ARF1 perturbs mitochondrial morphology, causing cGAS activation by aberrant mitochondrial DNA release, and leads to accumulation of active STING at the Golgi/ERGIC due to defective retrograde transport. Our data show an unexpected dual role of ARF1 in maintaining cGAS-STING homeostasis, through promotion of mitochondrial integrity and STING recycling.

The Endoplasmic Reticulum-Mitochondria Encounter Structure and its Regulatory Proteins

In fungi, the endoplasmic reticulum-mitochondria encounter structure (ERMES) is present between the endoplasmic reticulon (ER) and mitochondria to promote the formation of the ER-mitochondria contact sites. Four constitutive components (Mmm1, Mdm12, Mdm34, and Mdm10) assemble to form the ERMES complex while regulator proteins are required for regulating the organization and function of the ERMES complex. Multiple regulator proteins, including Gem1, Lam6, Tom7, and Emr1, of the ERMES complex, have been identified recently. In this review, we discuss the organization of the ERMES complex and the roles of the regulator proteins of the ERMES complex.

The SNAP25 Interactome in Ventromedial Caudate in Schizophrenia Includes the Mitochondrial Protein ARF1

Abnormalities of SNAP25 (synaptosome-associated protein 25) amount and protein-protein interactions occur in schizophrenia, and may contribute to abnormalities of neurotransmitter release in patients. However, presynaptic terminal function depends on multiple subcellular mechanisms, including energy provided by mitochondria. To explore the SNAP25 interactome in schizophrenia, we immunoprecipitated SNAP25 along with interacting proteins from the ventromedial caudate of 15 cases of schizophrenia and 13 controls. Proteins were identified with mass spectrometry-based proteomics. As well as 15 SNARE- (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) associated proteins, we identified 17 mitochondria-associated and four other proteins. The mitochondrial small GTPase ARF1 (ADP-ribosylation factor 1) was identified in eight schizophrenia SNAP25 immunoprecipitates and none from controls (P = 0.004). Although the ARF1-SNAP25 interaction may be increased, immunoblotting demonstrated 21% lower ARF1-21 (21 kiloDaltons) in schizophrenia samples (P = 0.04). In contrast, the mitochondrial protein UQCRC1 (ubiquinol-cytochrome c reductase core protein 1) did not differ. Lower ARF1-21 levels were associated with the previously reported increased SNAP25-syntaxin interaction in schizophrenia (r = -0.39, P = 0.04). Additional immunoprecipitation studies confirmed the ARF1-21-SNAP25 interaction, independent of UQCRC1. Both ARF1 and SNAP25 were localized to synaptosomes. Confocal microscopy demonstrated co-localization of ARF1 and SNAP25, and further suggested fivefold enrichment of ARF1 in synaptosomes containing an excitatory marker (vesicular glutamate transporter) compared with synaptosomes containing an inhibitory marker (vesicular GABA transporter). The present findings suggest an association between abnormalities of SNARE proteins involved with vesicular neurotransmission and the mitochondrial protein ARF1 that may contribute to the pathophysiology of schizophrenia.

Arf1 regulates the ER-mitochondria encounter structure (ERMES) in a reactive oxygen species-dependent manner

The Arf family of small GTP-binding and -hydrolyzing proteins are some of the most important intracellular regulators of membrane dynamics. In this study, we identified the Golgi-localized Arf family G protein Arf1 in Candida albicans and confirmed its conserved function in regulating the secretory pathway. Interestingly, deletion of ARF1 resulted in intracellular reactive oxygen species (ROS) accumulation, and induced formation of the endoplasmic reticulum (ER)-mitochondria encounter structure (ERMES). Moreover, N-acetylcysteine-mediated ROS scavenging in the arf1Δ/Δ strain attenuated ERMES formation, indicating that intracellular ROS accumulation resulting from ARF1 deletion facilitated ERMES formation. In addition, Arf1 regulated many key physiological processes in C. albicans, including cell cycle progression, morphogenesis and virulence. This study uncovers a role for Arf family G proteins in regulating ERMES formation and sheds new light on the crucial contribution of ROS to membrane dynamics.

ER fatalities-The role of ER-mitochondrial contact sites in yeast life and death decisions

Following extracellular stress signals, all eukaryotic cells choose whether to elicit a pro-survival or pro-death response. The decision over which path to take is governed by the severity and duration of the damage. In response to mild stress, pro-survival programs are initiated (unfolded protein response, autophagy, mitophagy) whereas severe or chronic stress forces the cell to abandon these adaptive programs and shift towards regulated cell death to remove irreversibly damaged cells. Both pro-survival and pro-death programs involve regulated communication between the endoplasmic reticulum (ER) and mitochondria. In yeast, recent data suggest this inter-organelle contact is facilitated by the endoplasmic reticulum mitochondria encounter structure (ERMES). These membrane contacts are not only important for the exchange of cellular signals, but also play a role in mitochondrial tethering during mitophagy, mitochondrial fission and mitochondrial inheritance. This review focuses on recent findings in yeast that shed light on how ER-mitochondrial communication mediates critical cell fate decisions.

Proteomics of Breast Muscle Tissue Associated with the Phenotypic Expression of Feed Efficiency within a Pedigree Male Broiler Line: I. Highlight on Mitochondria

As feed represents 60 to 70% of the cost of raising an animal to market weight, feed efficiency (the amount of dry weight intake to amount of wet weight gain) remains an important genetic trait in animal agriculture. To gain greater understanding of cellular mechanisms of feed efficiency (FE), shotgun proteomics was conducted using in-gel trypsin digestion and tandem mass spectrometry on breast muscle samples obtained from pedigree male (PedM) broilers exhibiting high feed efficiency (FE) or low FE phenotypes (n = 4 per group). The high FE group had greater body weight gain (P = 0.004) but consumed the same amount of feed (P = 0.30) from 6 to 7 wk resulting in higher FE (P < 0.001). Over 1800 proteins were identified, of which 152 were different (P < 0.05) by at least 1.3 fold and ≤ 15 fold between the high and low FE phenotypes. Data were analyzed for a modified differential expression (DE) metric (Phenotypic Impact Factors or PIF) and interpretation of protein expression data facilitated using the Ingenuity Pathway Analysis (IPA) program. In the entire data set, 228 mitochondrial proteins were identified whose collective expression indicates a higher mitochondrial expression in the high FE phenotype (binomial probability P < 0.00001). Within the top up and down 5% PIF molecules in the dataset, there were 15 mitoproteome proteins up-regulated and only 5 down-regulated in the high FE phenotype. Pathway enrichment analysis also identified mitochondrial dysfunction and oxidative phosphorylation as the number 1 and 5 differentially expressed canonical pathways (up-regulated in high FE) in the proteomic dataset. Upstream analysis (based on DE of downstream molecules) predicted that insulin receptor, insulin like growth receptor 1, nuclear factor, erythroid 2-like 2, AMP activated protein kinase (α subunit), progesterone and triiodothyronine would be activated in the high FE phenotype whereas rapamycin independent companion of target of rapamycin, mitogen activated protein kinase 4, and serum response factor would be inhibited in the high FE phenotype. The results provide additional insight into the fundamental molecular landscape of feed efficiency in breast muscle of broilers as well as further support for a role of mitochondria in the phenotypic expression of FE.

Funding provided by USDA-NIFA (#2013–01953), Arkansas Biosciences Institute (Little Rock, AR), McMaster Fellowship (AUS to WB) and the Agricultural Experiment Station (Univ. of Arkansas, Fayetteville).

Actin, Membrane Trafficking and the Control of Prion Induction, Propagation and Transmission in Yeast

The model eukaryotic yeast Saccharomyces cerevisiae has proven a useful model system in which prion biogenesis and elimination are studied. Several yeast prions exist in budding yeast and a number of studies now suggest that these alternate protein conformations may play important roles in the cell. During the last few years cellular factors affecting prion induction, propagation and elimination have been identified. Amongst these, proteins involved in the regulation of the actin cytoskeleton and dynamic membrane processes such as endocytosis have been found to play a critical role not only in facilitating de novo prion formation but also in prion propagation. Here we briefly review prion formation and maintenance with special attention given to the cellular processes that require the functionality of the actin cytoskeleton.

Altered Gene Expression in the Schistosome-Transmitting Snail Biomphalaria glabrata following Exposure to Niclosamide, the Active Ingredient in the Widely Used Molluscicide Bayluscide

In view of the call by the World Health Organization (WHO) for elimination of schistosomiasis as a public health problem by 2025, use of molluscicides in snail control to supplement chemotherapy–based control efforts is likely to increase in the coming years. The mechanisms of action of niclosamide, the active ingredient in the most widely used molluscicides, remain largely unknown. A better understanding of its toxicology at the molecular level will both improve our knowledge of snail biology and may offer valuable insights into the development of better chemical control methods for snails. We used a recently developed Biomphalaria glabrata oligonucleotide microarray (31K features) to investigate the effect of sublethal exposure to niclosamide on the transcriptional responses of the snail B. glabrata relative to untreated snails. Most of the genes highly upregulated following exposure of snails to niclosamide are involved in biotransformation of xenobiotics, including genes encoding cytochrome P450s (CYP), glutathione S-transferases (GST), and drug transporters, notably multi-drug resistance protein (efflux transporter) and solute linked carrier (influx transporter). Niclosamide also induced stress responses. Specifically, six heat shock protein (HSP) genes from three super-families (HSP20, HSP40 and HSP70) were upregulated. Genes encoding ADP-ribosylation factor (ARF), cAMP response element-binding protein (CREB) and coatomer, all of which are involved in vesicle trafficking in the Golgi of mammalian cells, were also upregulated. Lastly, a hemoglobin gene was downregulated, suggesting niclosamide may affect oxygen transport. Our results show that snails mount substantial responses to sublethal concentrations of niclosamide, at least some of which appear to be protective. The topic of how niclosamide’s lethality at higher concentrations is determined requires further study. Given that niclosamide has also been used as an anthelmintic drug for decades and has been found to have activity against several types of cancer, our findings may be of relevance in understanding how both parasites and neoplastic cells respond to this compound.

Schistosomes are snail-transmitted parasites that continue to infect over 230 million people worldwide and cause the disease schistosomiasis. Currently there is no effective vaccine against the disease. Control programs have relied primarily on use of chemotherapy with praziquantel to eliminate adult worms from infected people. An increasing body of evidence, however, suggests that praziquantel-based control programs are not likely to be sufficient to achieve sustainable transmission control. Snail control achieved by focal use of molluscicides, especially in combination with other methods like chemotherapy, sanitation and health education, offers considerable promise for reduction of disease transmission. Consequently, use of molluscicides in snail control is likely to increase in the coming years. We undertook a microarray study to assess transcriptional responses to niclosamide, the active ingredient in commonly-used molluscicides, in the schistosome-transmitting snail Biomphalaria glabrata. We show that niclosamide activates components in snails’ pathways known to be involved in biotransformation of xenobiotics and stress responses. We suggest that major alterations in vesicle trafficking and interference with oxygen transport also follow niclosamide exposure. The results contribute to our understanding of molecular impacts of niclosamide exposure on snails, and provide a basis for further studies to define the mode of action of niclosamide and other molluscicides in the future.