Med13p Prevents Stress-Independent Mitochondrial Hyperfragmentation and Aberrant Apoptosis Activation in Saccharomyces cerevisiae by Controlling Cyclin C Nuclear Localization

During aging, and as a result of environmental changes, cells are exposed to elevated levels of reactive oxygen species (ROS). High ROS levels induce lipid oxidation, protein aggregation, mitochondrial hyperfragmentation, DNA damage and programmed cell death (PCD), also called apoptosis. PCD is a highly regulated process and its misregulation has been linked to neurodegenerative diseases and cancer development. Our hypothesis is that cyclin C plays a role in the initiation of apoptosis. During normal conditions, cyclin C represses the transcription of stress response genes (SRG). In response to stress, cyclin C translocates to the cytoplasm where it facilitates mitochondrial hyperfragmentation and PCD. Our laboratory demonstrated that cyclin C loss increases viability of hydrogen peroxide exposed cells. Furthermore, cyclin C overexpression allows stress-independent relocalization into the cytoplasm which triggers mitochondria hyperfragmentation and increases cell sensitivity to oxidative stress. These results indicate that cyclin C localization is critical for proper regulation of PCD. Stress-induced cyclin C cytoplasmic translocalization and mitochondrial hyperfragmentation insures timely activation of apoptosis in stressed cells. Therefore, understanding the molecular switch controlling cyclin C stress induced relocalization may provide ways to protect cells from dangerous ROS or promote the death of transformed cells. In this work, I describe the role of Med13p as the gatekeeper that maintains cyclin C nuclear localization in unstressed cells. Med13p, together with cyclin C, cyclin dependent kinase (Cdk8p) and Med12 form a complex termed the Cdk8 module. This module associates with the RNA polymerase II holoenzyme complex and mediates gene repression. Using a combination of genetic and biochemical approaches, I demonstrated that loss of Med13p (med13Δ) permitted cyclin C relocalization into the cytoplasm in the absence of stress. Aberrant cyclin C localization to the cytoplasm has three consequences. First, more than 90% of cells exhibited hyperfragmented mitochondria. Second, the lack of mitochondrial fusion in unstressed cells induces mtDNA instability resulting in respiration deficiency. Third, cells become hyper-sensitive to oxidative stress. These data support a model that Med13p plays an important role in protecting the cell from aberrant ROS-induced apoptosis by retaining cyclin C in the nucleus

Med13p Prevents Mitochondrial Fission and Programmed Cell Death in Yeast Through Nuclear Retention of Cyclin C

The yeast cyclin C-Cdk8 kinase forms a complex with Med13p to repress the transcription of genes involved in the stress response and meiosis. In response to oxidative stress, cyclin C displays nuclear to cytoplasmic relocalization that triggers mitochondrial fission and promotes programmed cell death. In this report, we demonstrate that Med13p mediates cyclin C nuclear retention in unstressed cells. Deleting MED13 allows aberrant cytoplasmic cyclin C localization and extensive mitochondrial fragmentation. Loss of Med13p function resulted in mitochondrial dysfunction and hypersensitivity to oxidative stress-induced programmed cell death that were dependent on cyclin C. The regulatory system controlling cyclin C-Med13p interaction is complex. First, a previous study found that cyclin C phosphorylation by the stress-activated MAP kinase Slt2p is required for nuclear to cytoplasmic translocation. This study found that cyclin C-Med13p association is impaired when the Slt2p target residue is substituted with a phosphomimetic amino acid. The second step involves Med13p destruction mediated by the 26S proteasome and cyclin C-Cdk8p kinase activity. In conclusion, Med13p maintains mitochondrial structure, function, and normal oxidative stress sensitivity through cyclin C nuclear retention. Releasing cyclin C from the nucleus involves both its phosphorylation by Slt2p coupled with Med13p destruction

Stress-Induced Nuclear-to-Cytoplasmic Translocation of Cyclin C Promotes Mitochondrial Fission in Yeast

Mitochondrial morphology is maintained by the opposing activities of dynamin-based fission and fusion machines. In response to stress, this balance is dramatically shifted toward fission. This study reveals that the yeast transcriptional repressor cyclin C is both necessary and sufficient for stress-induced hyperfission. In response to oxidative stress, cyclin C translocates from the nucleus to the cytoplasm, where it is destroyed. Prior to its destruction, cyclin C both genetically and physically interacts with Mdv1p, an adaptor that links the GTPase Dnm1p to the mitochondrial receptor Fis1p. Cyclin C is required for stress-induced Mdv1p mitochondrial recruitment and the efficient formation of functional Dnm1p filaments. Finally, coimmunoprecipitation studies and fluorescence microscopy revealed an elevated association between Mdv1p and Dnm1p in stressed cells that is dependent on cyclin C. This study provides a mechanism by which stress-induced gene induction and mitochondrial fission are coordinated through translocation of cyclin C

The probable, possible, and novel functions of ERp29

The luminal endoplasmic reticulum (ER) protein of 29 kDa (ERp29) is a ubiquitously expressed cellular agent with multiple critical roles. ERp29 regulates the biosynthesis and trafficking of several transmembrane and secretory proteins, including the cystic fibrosis transmembrane conductance regulator (CFTR), the epithelial sodium channel (ENaC), thyroglobulin, connexin 43 hemichannels, and proinsulin. ERp29 is hypothesized to promote ER t

Zika Fetal Neuropathogenesis: Etiology of a Viral Syndrome

The ongoing Zika virus epidemic in the Americas and the observed association with both fetal abnormalities (primary microcephaly) and adult autoimmune pathology (Guillain–Barré syndrome) has brought attention to this neglected pathogen. While initial case studies generated significant interest in the Zika virus outbreak, larger prospective epidemiology and basic virology studies examining the mechanisms of Zika viral infection and associated pathophysiology are only now starting to be published. In this review, we analyze Zika fetal neuropathogenesis from a comparative pathology perspective, using the historic metaphor of “TORCH” viral pathogenesis to provide context. By drawing parallels to other viral infections of the fetus, we identify common themes and mechanisms that may illuminate the observed pathology. The existing data on the susceptibility of various cells to both Zika and other flavivirus infections are summarized. Finally, we highlight relevant aspects of the known molecular mechanisms of flavivirus replication

Expression of ZIKV receptors in human brain and placental tissue.

<p>NA = data not available.</p

Alignment of first 130 nucleotides of 3’UTR of ZIKV, illustrating Musashi binding element (MBE) location and associated mutations over time and geographic spread.

<p>Sequences shown are the only ones that are unique for country and/or sequence; duplicates of the same country were discarded. Alignment was performed using the MAFFT multiple sequence alignment program for unix-like operating systems. Visualization was performed using Geneious. There is presence of stem-loop I (SL I) and stem-loop II (SL II) on those sequences, with SL II being partially shown. There is also presence of MBE on SL II, with two SNPs on African sequences, which could potentially change the RNA structure and availability of the element. SL I and SL II were annotated from Zhu Z. et al. MBE was annotated using the UTRscan tool of UTRSite (<a href="http://utrsite.ba.itb.cnr.it/" target="_blank">http://utrsite.ba.itb.cnr.it/</a>).</p

Control of Plasma Membrane Permeability by ABC Transporters

International audienceATP-binding cassette transporters Pdr5 and Yor1 from Saccharomyces cerevisiae control the asymmetric distribution of phospholipids across the plasma membrane as well as serving as ATP-dependent drug efflux pumps. Mutant strains lacking these transporter proteins were found to exhibit very different resistance phenotypes to two inhibitors of sphingolipid biosynthesis that act either late (aureobasidin A [AbA]) or early (myriocin [Myr]) in the pathway leading to production of these important plasma membrane lipids. These pdr5 Delta yor1 strains were highly AbA resistant but extremely sensitive to Myr. We provide evidence that these phenotypic changes are likely due to modulation of the plasma membrane flippase complexes, Dnf1/Lem3 and Dnf2/Lem3. Flippases act to move phospholipids from the outer to the inner leaflet of the plasma membrane. Genetic analyses indicate that lem3 Delta mutant strains are highly AbA sensitive and Myr resistant. These phenotypes are fully epistatic to those seen in pdr5 Delta yor1 strains. Direct analysis of AbA-induced signaling demonstrated that loss of Pdr5 and Yor1 inhibited the AbA-triggered phosphorylation of the AGC kinase Ypk1 and its substrate Orm1. Microarray experiments found that a pdr5 Delta yor1 strain induced a Pdr1-dependent induction of the entire Pdr regulon. Our data support the view that Pdr5/Yor1 negatively regulate flippase function and activity of the nuclear Pdr1 transcription factor. Together, these data argue that the interaction of the ABC transporters Pdr5 and Yor1 with the Lem3-dependent flippases regulates permeability of AbA via control of plasma membrane protein function as seen for the high-affinity tryptophan permease Tat2

Complete Genome Sequences of Seven EA Cluster Microbacteriophages, Bustleton, MillyPhilly, Riyhil, Phriends, Pherbot, PrincePhergus, and TinSulphur

Seven EA cluster microbacteriophages were isolated from soil collected around Philadelphia, PA, using the bacterial host Microbacterium foliorum All of these phages have a highly conserved genome with regions of diversity localized to the 3' end. In phage Phriends (EA1 cluster), this region contains an orpham gene with no known function.</p