The fission yeast DNA structure checkpoint protein Rad26ATRIP/LCD1/UVSD accumulates in the cytoplasm following microtubule destabilization

A strategy for the investigation of toxic mechanisms and protection by efflux pumps using Schizosaccharomyces pombe strains: Application to rotenone

Effects not related with the inhibition of complex I of the mitochondrial electron transport chain are studied in S. pombe, which lacks it. This study aims: First, the use of a strategy with S. pombe strains to investigate the toxicity, mechanisms of action, interactions and detoxication by efflux pumps. Second, to investigate the mechanisms of toxic action of rotenone. In the dose-response assessment, the yeast presented a good correlation with the toxicity in Daphnia magna for 15 chemicals. In the mechanistic study, the mph1Δ strain presented marked specificity to the interaction with microtubules by carbendazim. DNA damage caused by hydroxyurea, an inhibitor of deoxynucleotide synthesis, was identified with marked specificity with the rad3Δ strain. The sty1Δ strain was very sensitive to the oxidative and osmotic stress induced by hydrogen peroxide and potassium chloride, respectively, being more sensitive to oxidative stress than the pap1Δ strain. The protection by exclusion pumps was also evaluated. Rotenone presented low toxicity in S. pombe due to the lack of its main target, and the marked protection by the exclusion transporters Bfr1, Pmd1, Caf5 and Mfs1. Marked cellular stress was detected. Finally, the toxicity of rotenone could be potentiated by the fungicide carbendazim and the antimetabolite hydroxyurea. In conclusion, the use of S. pombe strains is a valid strategy to: a) assess global toxicity; b) investigate the main mechanisms of toxic action, particularly spindle and DNA interferences, and osmotic and oxidative stress not related to complex I inhibition; c) explore the detoxication by efflux pumps; and d) evaluate possible chemical interactions. Therefore, it should be useful for the investigation of adverse outcome pathways.

Fission yeast Ase1PRC1 is required for the G2-microtubule damage response

Schizosaccharomyces pombe delays entry into mitosis following G₂ microtubule damage. This pathway is dependent on Rad26^ATRIP, the regulatory subunit of the Rad26^ATRIP/Rad3^ATR DNA damage response (DDR) complex. However, this G₂ microtubule damage response pathway acts independently of the G₂ DNA damage checkpoint pathway. To identify other proteins in this G₂ microtubule damage pathway, we previously screened a cDNA overexpression library for genes that rescued the sensitivity of rad26Δ cells to the microtubule poison thiabendazole. A partial cDNA fragment encoding only the C-terminal regulatory region of the microtubule bundling protein Ase1 ^PRC1 was isolated. This fragment lacks the Ase1^PRC1 dimerization and microtubule binding domains and retains the conserved C-terminal unstructured regulatory region. Here, we report that ase1Δ cells fail to delay entry into mitosis following G₂ microtubule damage. Microscopy revealed that Rad26^ATRIP foci localized alongside Ase1^PRC1 filaments, although we suggest that this is related to microtubule-dependent double strand break mobility that facilitates homologous recombination events. Indeed, we report that the DNA repair protein Rad52 co-localizes with Rad26^ATRIP at these foci, and that localization of Rad26^ATRIP to these foci depends on a Rad26^ATRIP N-terminal region containing a checkpoint recruitment domain. To our knowledge, this is the first report implicating Ase1^PRC1 in regulation of the G₂/M transition.

Effect of Surface Coating of Gold Nanoparticles on Cytotoxicity and Cell Cycle Progression

Gold nanoparticles (GNPs) are usually wrapped with biocompatible polymers in biomedical field, however, the effect of biocompatible polymers of gold nanoparticles on cellular responses are still not fully understood. In this study, GNPs with/without polymer wrapping were used as model probes for the investigation of cytotoxicity and cell cycle progression. Our results show that the bovine serum albumin (BSA) coated GNPs (BSA-GNPs) had been transported into lysosomes after endocytosis. The lysosomal accumulation had then led to increased binding between kinesin 5 and microtubules, enhanced microtubule stabilization, and eventually induced G₂/M arrest through the regulation of cadherin 1. In contrast, the bare GNPs experienced lysosomal escape, resulting in microtubule damage and G₀/G₁ arrest through the regulation of proliferating cell nuclear antigen. Overall, our findings showed that both naked and BSA wrapped gold nanoparticles had cytotoxicity, however, they affected cell proliferation via different pathways. This will greatly help us to regulate cell responses for different biomedical applications.

Pap1 + confers microtubule damage resistance to mut2a, an extragenic suppressor of the rad26:4A allele in S. pombe

The DNA structure checkpoint protein Rad26^ATRIP is also required for an interphase microtubule damage response. This checkpoint delays spindle pole body separation and entry into mitosis following treatment of cells with microtubule poisons. This checkpoint requires cytoplasmic Rad26^ATRIP, which is compromised by the rad26:4A allele that inhibits cytoplasmic accumulation of Rad26^ATRIP following microtubule damage. The rad26::4a allele also disrupts minichromosome stability and cellular morphology, suggesting that the interphase microtubule damage checkpoint pathway operates in an effort to maintain chromosome stability and proper cell shape. To identify other proteins of the Rad26-dependent interphase microtubule damage response, we used ultra violet (UV) radiation to identify extragenic interaction suppressors of the rad26::4A growth defect on microtubule poisons. One suppressor mutation, which we named mut2a, permitted growth of rad26:4A cells on MBC media and conferred sensitivity to a microtubulin poison upon genetic outcross. In an attempt to clone this interaction suppressor using a genomic library complementation strategy, we instead isolated pap1 ⁺ as an extracopy suppressor of the mut2a growth defect. We discuss the mechanism by which pap1 ⁺ overexpression may allow growth of mut2a cells in conditions that destabilize microtubules.

A genomic Multiprocess survey of machineries that control and link cell shape, microtubule organization, and cell-cycle progression

Understanding cells as integrated systems requires that we systematically decipher how single genes affect multiple biological processes and how processes are functionally linked. Here, we used multiprocess phenotypic profiling, combining high-resolution 3D confocal microscopy and multiparametric image analysis, to simultaneously survey the fission yeast genome with respect to three key cellular processes: cell shape, microtubule organization, and cell-cycle progression. We identify, validate, and functionally annotate 262 genes controlling specific aspects of those processes. Of these, 62% had not been linked to these processes before and 35% are implicated in multiple processes. Importantly, we identify a conserved role for DNA-damage responses in controlling microtubule stability. In addition, we investigate how the processes are functionally linked. We show unexpectedly that disruption of cell-cycle progression does not necessarily affect cell size control and that distinct aspects of cell shape regulate microtubules and vice versa, identifying important systems-level links across these processes.

Characterization of hNek6 interactome reveals an important role for its short N-terminal domain and colocalization with proteins at the centrosome

Physical protein-protein interactions are fundamental to all biological processes and are organized in complex networks. One branch of the kinome network is the evolutionarily conserved NIMA-related serine/threonine kinases (Neks). Most of the 11 mammalian Neks studied so far are related to cell cycle regulation, and due to association with diverse human pathologies, Neks are promising chemotherapeutic targets. Human Nek6 was associated to carcinogenesis, but its interacting partners and signaling pathways remain elusive. Here we introduce hNek6 as a highly connected member in the human kinase interactome. In a more global context, we performed a broad data bank comparison based on degree distribution analysis and found that the human kinome is enriched in hubs. Our networks include a broad set of novel hNek6 interactors as identified by our yeast two-hybrid screens classified into 18 functional categories. All of the tested interactions were confirmed, and the majority of tested substrates were phosphorylated in vitro by hNek6. Notably, we found that hNek6 N-terminal is important to mediate the interactions with its partners. Some novel interactors also colocalized with hNek6 and γ-tubulin in human cells, pointing to a possible centrosomal interaction. The interacting proteins link hNek6 to novel pathways, for example, Notch signaling and actin cytoskeleton regulation, or give new insights on how hNek6 may regulate previously proposed pathways such as cell cycle regulation, DNA repair response, and NF-κB signaling. Our findings open new perspectives in the study of hNek6 role in cancer by analyzing its novel interactions in specific pathways in tumor cells, which may provide important implications for drug design and cancer therapy.

Antioxidants from the Leaves of Cinnamomum kotoense

Two novel antioxidants, obtusilactone A (1) and (-)-sesamin (2) have been identified in Cinnamomum kotoense Kanehira. Both showed effective 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity compared with vitamin C, and reducing power compared with BHA. These results suggest that these constituents of C. kotoense act as natural antioxidants and play a potential role in cancer prevention.

Fission yeast Rad26ATRIP delays spindle-pole-body separation following interphase microtubule damage

The conserved fission yeast protein Rad26(ATRIP) preserves genomic stability by occupying central positions within DNA-structure checkpoint pathways. It is also required for proper cellular morphology, chromosome stability and following treatment with microtubule poisons. Here, we report that mutation of a putative nuclear export sequence in Rad26(ATRIP) disrupted its cytoplasmic localization in untreated cells and conferred abnormal cellular morphology, minichromosome instability and sensitivity to microtubule poisons without affecting DNA-structure checkpoint signaling. This mutation also disrupted a delay to spindle-pole-body separation that occurred following microtubule damage in G(2). Together, these results demonstrate that Rad26(ATRIP) participates in two genetically defined checkpoint pathways--one that responds to genomic damage and the other to microtubule damage. This response to microtubule damage delays spindle-pole-body separation and, in doing so, might preserve both cellular morphology and chromosome stability.