Correction: Surveillance-Activated Defenses Block the ROS-Induced Mitochondrial Unfolded Protein Response.

[This corrects the article DOI: 10.1371/journal.pgen.1003346.].

Surveillance-activated defenses block the ROS-induced mitochondrial unfolded protein response.

Disturbance of cellular functions results in the activation of stress-signaling pathways that aim at restoring homeostasis. We performed a genome-wide screen to identify components of the signal transduction of the mitochondrial unfolded protein response (UPR(mt)) to a nuclear chaperone promoter. We used the ROS generating complex I inhibitor paraquat to induce the UPR(mt), and we employed RNAi exposure post-embryonically to allow testing genes whose knockdown results in embryonic lethality. We identified 54 novel regulators of the ROS-induced UPR(mt). Activation of the UPR(mt), but not of other stress-signaling pathways, failed when homeostasis of basic cellular mechanisms such as translation and protein transport were impaired. These mechanisms are monitored by a recently discovered surveillance system that interprets interruption of these processes as pathogen attack and depends on signaling through the JNK-like MAP-kinase KGB-1. Mutation of kgb-1 abrogated the inhibition of ROS-induced UPR(mt), suggesting that surveillance-activated defenses specifically inhibit the UPR(mt) but do not compromise activation of the heat shock response, the UPR of the endoplasmic reticulum, or the SKN-1/Nrf2 mediated response to cytosolic stress. In addition, we identified PIFK-1, the orthologue of the Drosophila PI 4-kinase four wheel drive (FWD), and found that it is the only known factor so far that is essential for the unfolded protein responses of both mitochondria and endoplasmic reticulum. This suggests that both UPRs may share a common membrane associated mechanism.

Cell-nonautonomous signaling of FOXO/DAF-16 to the stem cells of Caenorhabditis elegans.

In Caenorhabditis elegans (C. elegans), the promotion of longevity by the transcription factor DAF-16 requires reduced insulin/IGF receptor (IIR) signaling or the ablation of the germline, although the reason for the negative impact of germ cells is unknown. FOXO/DAF-16 activity inhibits germline proliferation in both daf-2 mutants and gld-1 tumors. In contrast to its function as a germline tumor suppressor, we now provide evidence that somatic DAF-16 in the presence of IIR signaling can also result in tumorigenic activity, which counteracts robust lifespan extension. In contrast to the cell-autonomous IIR signaling, which is required for larval germline proliferation, activation of DAF-16 in the hypodermis results in hyperplasia of the germline and disruption of the surrounding basement membrane. SHC-1 adaptor protein and AKT-1 kinase antagonize, whereas AKT-2 and SGK-1 kinases promote, this cell-nonautonomous DAF-16 function. Our data suggest that a functional balance of DAF-16 activities in different tissues determines longevity and reveals a novel, cell-nonautonomous role of FOXO/DAF-16 to affect stem cells.

SnAvi--a new tandem tag for high-affinity protein-complex purification.

Systematic tandem-affinity-purification (TAP) of protein complexes was tremendously successful in yeast and has changed the general concept of how we understand protein function in eukaryotic cells. The transfer of this method to other model organisms has been difficult and may require specific adaptations. We were especially interested to establish a cell-type-specific TAP system for Caenorhabditis elegans, a model animal well suited to high-throughput analysis, proteomics and systems biology. By combining the high-affinity interaction between in vivo biotinylated target-proteins and streptavidin with the usage of a newly identified epitope of the publicly shared SB1 monoclonal antibody we created a novel in vivo fluorescent tag, the SnAvi-Tag. We show the versatile application of the SnAvi-Tag in Escherichia coli, vertebrate cells and in C. elegans for tandem affinity purification of protein complexes, western blotting and also for the in vivo sub-cellular localization of labelled proteins.

Linker histone HIS-24 (H1.1) cytoplasmic retention promotes germ line development and influences histone H3 methylation in Caenorhabditis elegans.

RNA interference with one of the eight Caenorhabditis elegans linker histone genes triggers desilencing of a repetitive transgene and developmental defects in the hermaphrodite germ line. These characteristics are similar to the phenotype of the C. elegans Polycomb group genes mes-2, mes-3, mes-4, and mes-6 (M. A. Jedrusik and E. Schulze, Development 128:1069-1080, 2001; I. Korf, Y. Fan, and S. Strome, Development 125:2469-2478, 1998). These Polycomb group proteins contribute to germ line-specific chromatin modifications. Using a his-24 deletion mutant and an isoform-specific antibody, we characterized the role of his-24 in C. elegans germ line development. We describe an unexpected cytoplasmic retention of HIS-24 in peculiar granular structures. This phenomenon is confined to the developing germ lines of both sexes. It is strictly dependent on the activities of the chromatin-modifying genes mes-2, mes-3, mes-4, and mes-6, as well as on the C. elegans sirtuin gene sir-2.1. A temperature shift experiment with a mes-3(ts) mutant revealed that mes gene activity is required in a time window ranging from L3 to the early L4 stage before the onset of meiosis. We find that the his-24(ok1024) mutant germ line is characterized by an increased level of the activating H3K4 methylation mark concomitant with a decrease of the repressive H3K9 methylation. In the germ line of his-24(ok1024) mes-3(bn35) double mutant animals, the repressive H3K27 methylation is more reduced than in the respective mes single mutant. These observations distinguish his-24 as an unusual element in the developmental regulation of germ line chromatin structure in C. elegans.

Analysis of the novel excretory cell expressed ECP-1 protein and its proposed ECP-1/IFC-2 fusion protein EXC-2 in the nematode Caenorhabditis elegans.

The multigene family of cytoplasmic intermediate filament (IF) proteins in C. elegans covers eleven members, of which four (IFA-1 to IFA-3, IFB-1), which form an obligate heteropolymeric IF system, are essential for development. The six other C. elegans IF proteins IFB-2, IFC-1, IFC-2, IFD-1, IFD-2 and IFP-1 are co-expressed in the intestinal terminal web during different developmental stages, reveal various differently penetrant RNAi phenotypes and form another heteropolymeric IFB-2/IFCDP-1 IF system in C. elegans. Interestingly, the alternatively spliced IFC-2 variant, called EXC-2, was recently found also to be needed for a normal excretory system maturation in C. elegans. In order to better understand the IFC-2 function in the nematode tissue, we retrieved from the WormBase its multiple predicted alternatively spliced transcripts and analysed them using the molecular, immunofluorescence and RNAi approaches. We found that the 21-exon long genomic fragment encodes, besides the two different intestinal IFC-2a and IFC-2b IF proteins, also the novel excretory cell/IF unrelated protein ECP-1 and probably also the large ECP-1/IFC-2 fusion protein EXC-2, which all seem to be tissue-specific regulated from different promoters. Our analyses provide a framework for investigating interactions between the novel ECP-1, EXC-2 and some other proteins, including IFs, which show a similar excretory canal phenotype and are essential for development of the C. elegans excretory cells.

A mutation in CHN-1/CHIP suppresses muscle degeneration in Caenorhabditis elegans.

Duchenne muscular dystrophy (DMD) is one of the most severe X-linked, inherited diseases of childhood, characterized by progressive muscle wasting and weakness as the consequence of mutations in the dystrophin gene. The protein encoded by dystrophin is a huge cytosolic protein that links the intracellular F-actin filaments to the members of the dystrophin-glycoprotein-complex (DGC). Dystrophin deficiency results in the absence or reduction of complex components that are degraded through an unknown pathway. We show here that muscle degeneration in a Caenorhabditis elegans DMD model is efficiently reduced by downregulation of chn-1, encoding the homologue of the human E3/E4 ubiquitylation enzyme CHIP. A deletion mutant of chn-1 delays the cell death of body-wall muscle cells and improves the motility of animals carrying mutations in dystrophin and MyoD. Elimination of chn-1 function in the musculature, but not in the nervous system, is sufficient for this effect, and can be phenocopied by proteasome inhibitor treatment. This suggests a critical role of CHIP/CHN-1-mediated ubiquitylation in the control of muscle wasting and degeneration and identifies a potential new drug target for the treatment of this disease.

G1 phase-dependent nucleolar accumulation of human histone H1x.

BACKGROUND INFORMATION: H1 histones are a protein family comprising several subtypes. Although specific functions of the individual subtypes could not be determined so far, differential roles are indicated by varied nuclear distributions as well as differential expression patterns of the H1 subtypes. Although the group of replication-dependent H1 subtypes is synthesized during S phase, the replacement H1 subtype, H1 degrees , is also expressed in a replication-independent manner in non-proliferating cells. Recently we showed, by protein biochemical analysis, that the ubiquitously expressed subtype H1x is enriched in the micrococcal nuclease-resistant part of chromatin and that, although it shares common features with H1 degrees , its expression is differentially regulated, since, in contrast to H1 degrees , growth arrest or induction of differentiation did not induce an accumulation of H1x. RESULTS: In the present study, we show that H1x exhibits a cell-cycle-dependent change of its nuclear distribution. This H1 subtype showed a nucleolar accumulation during the G(1) phase, and it was evenly distributed in the nucleus during S phase and G(2). Immunocytochemical analysis of the intranucleolar distribution of H1x indicated that it is located mainly in the condensed nucleolar chromatin. In addition, we demonstrate that the amount of H1x protein remained nearly unchanged during S phase progression, which is in contrast to the replication-dependent subtypes. CONCLUSION: These results suggest that the differential localization of H1x provides a mechanism for a control of H1x activity by means of shuttling between nuclear subcompartments instead of a controlled turnover of the protein.

Telomeric position effect variegation in Saccharomyces cerevisiae by Caenorhabditis elegans linker histones suggests a mechanistic connection between germ line and telomeric silencing.

Linker histones are nonessential for the life of single-celled eukaryotes. Linker histones, however, can be important components of specific developmental programs in multicellular animals and plants. For Caenorhabditis elegans a single linker histone variant (H1.1) is essential in a chromatin silencing process which is crucial for the proliferation and differentiation of the hermaphrodite germ line. In this study we analyzed the whole linker histone complement of C. elegans by telomeric position effect variegation in budding yeast. In this assay an indicator gene (URA3) placed close to the repressive telomeric chromatin structure is subject to epigenetically inherited gene inactivation. Just one out of seven C. elegans linker histones (H1.1) was able to enhance the telomeric position effect in budding yeast. Since these results reflect the biological function of H1.1 in C. elegans, we suggest that chromatin silencing in C. elegans is governed by molecular mechanisms related to the telomere-dependent silencing in budding yeast. We confirmed this hypothesis by testing C. elegans homologs of three yeast genes which are established modifiers of the yeast telomeric chromatin structure (SIR2, SET1, and RAD17) for their influence on repeat-dependent transgene silencing for C. elegans.

In vivo and in vitro evidence that the four essential intermediate filament (IF) proteins A1, A2, A3 and B1 of the nematode Caenorhabditis elegans form an obligate heteropolymeric IF system.

The in vitro polymerization and tissue-specific expression patterns of the four essential intermediate filament (IF) proteins (A1, A2, A3, B1) and the non-essential IF protein A4 were analyzed. Recombinant B1, used as a probe in blot overlay assays of the 11 Caenorhabditis elegans IF proteins, reacted strongly with proteins A1 to A4, indicating a heterotypic interaction. Obligate heteropolymeric filament assembly in vitro was confirmed by electron microscopy. Protein B1 formed long IF when mixed with an equimolar amount of A1, A2 or A3. Developmentally regulated coexpression of B1 and one or more members of the A family was found with GFP-promoter reporters. This coexpression pattern argues for a heteropolymer system in vivo. One or both splice variants of the B1 gene are always coexpressed in a tissue-specific manner with at least one member of the A family in hypodermis, pharynx, pharyngeal-intestinal valve, excretory cells, uterus, vulva and rectum. Interestingly, while the intestine normally lacks a B1/A pair, the dauer larva shows intestinal B1 and A4. These results are in line with similar postembryonic phenotypes of the hypodermis induced by RNA interference (RNAi) of genes B1, A2 and A3. Similarly, defects of the pharynx and its A1-GFP containing tonofilaments observed in the postembryonic B1 RNAi phenotype are consistent with the coexpression of B1 and A1 in the marginal cells. Thus RNAi analyses provide independent evidence for the existence of the B1/A obligate heteropolymer system in vivo. Proteins A1 and B1 have a similar and rather slow turnover rate in photobleaching experiments of the pharynx tonofilaments.

Expression profiles of the essential intermediate filament (IF) protein A2 and the IF protein C2 in the nematode Caenorhabditis elegans.

The multigene family of intermediate filament (IF) proteins in Caenorhabditis elegans covers 11 members of which four (A1-3, B1) are essential for development. Suppression of a fifth gene (C2) results in a dumpy phenotype. Expression patterns of three essential genes (A1, A3, B1) were already reported. To begin to analyze the two remaining RNAi phenotypes we followed the expression of the A2 and C2 proteins. Expression of A2 mRNA starts in larval stage L1 and continues in the adult. Transgenic A2 promoter/gfp larvae strongly display GFP in the main body hypodermis but not in seam cells. This pattern and the muscle displacement/paralysis induced by RNAi silencing are consistent with the role of this protein in keeping the correct hypodermis/muscle relationship during development. IF protein C2 occurs in the cytoplasm and desmosomes of intestinal cells and in pharynx desmosomes. Expression of C2 starts in the late embryo and persists in all further stages.

Functional analysis of the single calmodulin gene in the nematode Caenorhabditis elegans by RNA interference and 4-D microscopy.

Calmodulin (CaM), a small calcium-binding protein, is the key mediator of numerous calcium-induced changes in cellular activity. Its ligands include enzymes, cytoskeletal proteins and ion channels, identified in large part by biochemical and cell biological approaches. Thus far it has been difficult to assess the function of CaM genetically, because of the maternal supply in Drosophila and the presence of at least three nonallelic genes in vertebrates. Here we use the unique possibility offered by the C. elegans model system to inactivate the single CaM gene (cmd-1) through RNA interference (RNAi). We show that the RNAi microinjection approach results in a severe embryonic lethal phenotype. Embryos show disturbed morphogenesis, aberrant cell migration patterns, a striking hyperproliferation of cells and multiple defects in apoptosis. Finally, we show that RNAi delivery by the feeding protocol does not allow the efficient silencing of the CaM gene obtained by microinjection. General differences between the two delivery methods are discussed.

Targeted purification of SnAvi-tagged proteins.

Tandem affinity purification (TAP) is a powerful technique to identify protein complex members. The modular composition of TAP-tags allows two sequential protein enrichment steps and thereby drastically reduces the amount of contaminants. Here, we describe the application of the SnAvi-tag-a TAP-tag useful in different expression systems. Due to its modular composition, this tag is multifunctional and facilitates among others the in vivo visualization of tagged proteins and their cell type specific activation.

Mitochondrial stress: balancing friend and foe.

Mitochondria are vital organelles of the aerobic eukaryotic cell. Their dysfunction associates with aging and widespread age-related diseases. To sustain mitochondrial integrity, the cell executes a distinct set of stress-induced protective responses. The mitochondrial unfolded protein response (UPR(mt)) is a response of the cell to mitochondrial damage. The transcription factor ATFS-1 triggers UPR(mt) effector gene expression in the nucleus. The selective exclusion of ATFS-1 from mitochondrial import by stress-induced alterations of the mitochondrial membrane potential is currently discussed as key activation mechanism. Surprisingly, UPR(mt) activation often coincides with a lifespan extension in Caenorhabditis elegans and the same has recently been reported for mammalian cells. This review summarizes the current model of the UPR(mt), its inducers, and its crosstalk with other cellular stress responses. It focuses on the role of mitochondrial function as a regulator of aging and longevity.

Expression pattern and first functional characterization of riok-1 in Caenorhabditis elegans.

Rio kinases are atypical serine/threonine kinases that emerge as potential cooperation partners in Ras-driven tumors. In the current study, we performed an RNAi screen in Caenorhabditis elegans to identify suppressors of oncogenic Ras signaling. Aberrant Ras/Raf signaling in C. elegans leads to the formation of a multi-vulva (Muv) phenotype. We found that depletion of riok-1, the C. elegans orthologue of the mammalian RioK1, suppressed the Muv phenotype. By using a promoter GFP construct, we could show that riok-1 is expressed in neuronal cells, the somatic gonad, the vulva, the uterus and the spermatheca. Furthermore, we observed developmental defects in the gonad upon riok-1 knockdown in a wildtype background. Our data suggest that riok-1 is a modulator of the Ras signaling pathway, suggesting implications for novel interventions in the context of Ras-driven tumors.

Temperature- and touch-sensitive neurons couple CNG and TRPV channel activities to control heat avoidance in Caenorhabditis elegans.

BACKGROUND: Any organism depends on its ability to sense temperature and avoid noxious heat. The nematode Caenorhabditis elegans responds to noxious temperatures exceeding ∼35°C and also senses changes in its environmental temperature in the range between 15 and 25°C. The neural circuits and molecular mechanisms involved in thermotaxis have been successfully studied, whereas details of the thermal avoidance behavior remain elusive. In this work, we investigate neurological and molecular aspects of thermonociception using genetic, cell biological and physiological approaches. METHODOLOGY/PRINCIPAL FINDINGS: We show here that the thermosensory neurons AFD, in addition to sensing temperature within the range within which the animals can thrive, also contribute to the sensation of noxious temperatures resulting in a reflex-like escape reaction. Distinct sets of interneurons are involved in transmitting thermonociception and thermotaxis, respectively. Loss of AFD is partially compensated by the activity of a pair of multidendritic, polymodal neurons, FLP, whereas laser ablation of both types of neurons abrogated the heat response in the head of the animals almost completely. A third pair of heat sensory neurons, PHC, is situated in the tail. We find that the thermal avoidance response requires the cell autonomous function of cGMP dependent Cyclic Nucleotide-Gated (CNG) channels in AFD, and the heat- and capsaicin-sensitive Transient Receptor Potential Vanilloid (TRPV) channels in the FLP and PHC sensory neurons. CONCLUSIONS/SIGNIFICANCE: Our results identify distinct thermal responses mediated by a single neuron, but also show that parallel nociceptor circuits and molecules may be used as back-up strategies to guarantee fast and efficient responses to potentially detrimental stimuli.

Novel roles of Caenorhabditis elegans heterochromatin protein HP1 and linker histone in the regulation of innate immune gene expression.

Linker histone (H1) and heterochromatin protein 1 (HP1) are essential components of heterochromatin which contribute to the transcriptional repression of genes. It has been shown that the methylation mark of vertebrate histone H1 is specifically recognized by the chromodomain of HP1. However, the exact biological role of linker histone binding to HP1 has not been determined. Here, we investigate the function of the Caenorhabditis elegans H1 variant HIS-24 and the HP1-like proteins HPL-1 and HPL-2 in the cooperative transcriptional regulation of immune-relevant genes. We provide the first evidence that HPL-1 interacts with HIS-24 monomethylated at lysine 14 (HIS-24K14me1) and associates in vivo with promoters of genes involved in antimicrobial response. We also report an increase in overall cellular levels and alterations in the distribution of HIS-24K14me1 after infection with pathogenic bacteria. HIS-24K14me1 localization changes from being mostly nuclear to both nuclear and cytoplasmic in the intestinal cells of infected animals. Our results highlight an antimicrobial role of HIS-24K14me1 and suggest a functional link between epigenetic regulation by an HP1/H1 complex and the innate immune system in C. elegans.

Characterisation of human histone H1x.

The members of the H1 histone family can be classified into three groups, which are the main class subtypes expressed in somatic cells, the developmental- and tissue-specific subtypes, and the replacement subtype H1(o). Until now, the subtype H1x was not classified, since it has not yet been thoroughly examined. The results of this study show that H1x shares similarities but also exhibits slight differences in its biochemical behaviour in comparison to the main class H1 histones. In HeLa cells it is located in the nucleus and partially associated with nucleosomes. Nevertheless, it is, like H1(o), mainly located in chromatin regions that are not affected by micrococcal nuclease digestion. Further common features of H1x and the replacement histone H1(o) are that the genes of both subtypes are solitarily located and give rise to polyadenylated mRNA. However, comparison of the inducibility of their expression revealed that their genes are regulated differentially.

A novel linker histone-like protein is associated with cytoplasmic filaments in Caenorhabditis elegans.

The histone H1 complement of Caenorhabditis elegans contains a single unusual protein, H1.X. Although H1.X possesses the globular domain and the canonical three-domain structure of linker histones, the amino acid composition of H1.X is distinctly different from conventional linker histones in both terminal domains. We have characterized H1.X in C. elegans by antibody labeling, green fluorescent protein fusion protein expression and RNA interference. Unlike normal linker histones, H1.X is a cytoplasmic as well as a nuclear protein and is not associated with chromosomes. H1.X is most prominently expressed in the marginal cells of the pharynx and is associated with a peculiar cytoplasmic cytoskeletal structure therein, the tonofilaments. Additionally H1.X::GFP is expressed in the cytoplasm of body and vulva muscle cells, neurons, excretory cells and in the nucleoli of embryonic blastomeres and adult gut cells. RNA interference with H1.X results in uncoordinated and egg laying defective animals, as well as in a longitudinally enlarged pharynx. These phenotypes indicate a cytoplasmic role of H1.X in muscle growth and muscle function.