Interaction with DNA polymerase η is required for nuclear accumulation of REV1 and suppression of spontaneous mutations in human cells

Defects in the gene encoding human Polη result in xeroderma pigmentosum variant (XP-V), an inherited cancer-prone syndrome. Polη catalyzes efficient and accurate translesion DNA synthesis (TLS) past UV-induced lesions. In addition to Polη, human cells have multiple TLS polymerases such as Polι, Polκ, Polζ and REV1. REV1 physically interacts with other TLS polymerases, but the physiological relevance of the interaction remains unclear. Here we developed an antibody that detects the endogenous REV1 protein and found that human cells contain about 60,000 of REV1 molecules per cell as well as Polη. In un-irradiated cells, formation of nuclear foci by ectopically expressed REV1 was enhanced by the co-expression of Polη. Importantly, the endogenous REV1 protein accumulated at the UV-irradiated areas of nuclei in Polη-expressing cells but not in Polη-deficient XP-V cells. UV-irradiation induced nuclear foci of REV1 and Polη proteins in both S-phase and G1 cells, suggesting that these proteins may function both during and outside S phase. We reconstituted XP-V cells with wild-type Polη or with Polη mutants harboring substitutions in phenylalanine residues critical for interaction with REV1. The REV1-interaction-deficient Polη mutant failed to promote REV1 accumulation at sites of UV-irradiation, yet (similar to wild-type Polη) corrected the UV sensitivity of XP-V cells and suppressed UV-induced mutations. Interestingly however, spontaneous mutations of XP-V cells were only partially suppressed by the REV1-interaction deficient mutant of Polη. Thus, Polη–REV1 interactions prevent spontaneous mutations, probably by promoting accurate TLS past endogenous DNA lesions, while the interaction is dispensable for accurate Polη-mediated TLS of UV-induced lesions.     

Involvement of intracellular transport in TREK-1c current run-up in 293T cells

The TREK-1 channel, the TWIK-1-related potassium (K⁺) channel, is a member of a family of 2-pore-domain K⁺ (K2P) channels, through which background or leak K⁺ currents occur. An interesting feature of the TREK-1 channel is the run-up of current: i.e. the current through TREK-1 channels spontaneously increases within several minutes of the formation of the whole-cell configuration. To investigate whether intracellular transport is involved in the run-up, we established 293T cell lines stably expressing the TREK-1c channel (K2P2.1) and examined the effects of inhibitors of membrane protein transport, N-methylmaleimide (NEM), brefeldin-A, and an endocytosis inhibitor, pitstop2, on the run-up. The results showing that NEM and brefeldin-A inhibited and pitstop2 facilitated the run-up suggest the involvement of intracellular protein transport. Correspondingly, in cells stably expressing the mCherry-TREK-1 fusion protein, NEM decreased and pitstop2 increased the cell surface localization of the fusion protein. Furthermore, the run-up was inhibited by the intracellular application of a peptide of the C-terminal fragment TREK335–360, corresponding to the interaction site with microtubule-associated protein 2 (Mtap2). This peptide also inhibited the co-immunoprecipitation of Mtap2 with anti-mCherry antibody. The extracellular application of an ezrin inhibitor (NSC668394) also suppressed the run-up and surface localization of the fusion protein. The co-application of these inhibitors abolished the TREK-1c current, suggesting that the additive effects of ezrin and Mtap2 enhance the surface expression of TREK-1c channels and the run-up. These findings clearly showed the involvement of intracellular transport in TREK-1c current run-up and its mechanism.     

A mouse model reveals that Mfsd2a is critical for unfolded protein response upon exposure to tunicamycin

Major facilitator superfamily domain containing 2a (Mfsd2a) is a member of the major facilitator superfamily. Mfsd2a functions as a transporter for docosahexaenoic acid and also plays a role in the unfolded protein response (UPR) upon tunicamycin (TM) exposure. UPR is involved in the pathogenesis of various human diseases. TM and thapsigargin are representative experimental reagents that induce UPR. To elucidate the detailed function of Mfsd2a in UPR in vivo, we generated Mfsd2a-deficient mice and investigated the role of Mfsd2a during UPR induced by TM or thapsigargin. Phenotypically, Mfsd2a-deficient mice were small and short-lived. No gross anatomical abnormalities in Mfsd2a-deficient mice compared with the wild-type mice were exhibited. Embryonic fibroblasts derived from Mfsd2a-null mice failed to show induction of GRP78 and DDIT3 expressions upon TM exposure but not upon Tg exposure. This phenomenon could not be overcome despite the exposure under high TM concentration. Reconstitution of Mfsd2a in Mfsd2a-null MEF showed hypersensitivity to TM. Furthermore, we examined the physiological role of Mfsd2a against TM using an in vivo mouse model. DDIT3 induction by TM was drastically attenuated in both the liver and brain of Mfsd2a-deficient mice. These results reveal that Mfsd2a plays a critical role in UPR upon TM exposure.     

Quantitative determination of cyclic polylactic acid oligomers in serum by direct injection liquid chromatography tandem mass spectrometry

Polylactic acid (PLA) is a biodegradable polymer, currently used in pharmaceutical and surgical devices. There is a concern that cyclic polylactic acid (CPLA), which is a by-product of PLA synthesis, may be introduced into the human body as an undesirable contaminant. We carried out a quantitation investigation of the CPLA heptamer (CPLA-7) by liquid chromatography mass spectrometry (LC-MS). We found that CPLA-7 binds strongly with serum proteins and that only 62% of CPLA-7 was recovered after routine deproteination; therefore, we directly injected serum into the LC-MS/MS system after passage through a bovine serum albumin (BSA)-coated chromatographic column and found the recovery of CPLA-7 was improved to 84%, and that the detection (S/N=3) and quantitation limit (S/N=10 and below 15% relative standard deviation) were 1.5 and 2.5 ng/mL, respectively. We conclude that direct injection LC-MS/MS, using a BSA column, is a simple and effective quantitative analysis method for CPLA in serum.     

Active gamma-secretase is localized to detergent-resistant membranes in human brain

Several lines of evidence suggest that polymerization of the amyloid beta-peptide (Abeta) into amyloid plaques is a pathogenic event in Alzheimer's disease (AD). Abeta is produced from the amyloid precursor protein as the result of sequential proteolytic cleavages by beta-secretase and gamma-secretase, and it has been suggested that these enzymes could be targets for treatment of AD. gamma-Secretase is an aspartyl protease complex, containing at least four transmembrane proteins. Studies in cell lines have shown that gamma-secretase is partially localized to lipid rafts, which are detergent-resistant membrane microdomains enriched in cholesterol and sphingolipids. Here, we studied gamma-secretase in detergent-resistant membranes (DRMs) prepared from human brain. DRMs prepared in the mild detergent CHAPSO and isolated by sucrose gradient centrifugation were enriched in gamma-secretase components and activity. The DRM fraction was subjected to size-exclusion chromatography in CHAPSO, and all of the gamma-secretase components and a lipid raft marker were found in the void volume (> 2000 kDa). Co-immunoprecipitation studies further supported the notion that the gamma-secretase components are associated even at high concentrations of CHAPSO. Preparations from rat brain gave similar results and showed a postmortem time-dependent decline in gamma-secretase activity, suggesting that DRMs from fresh rat brain may be useful for gamma-secretase activity studies. Finally, confocal microscopy showed co-localization of gamma-secretase components and a lipid raft marker in thin sections of human brain. We conclude that the active gamma-secretase complex is localized to lipid rafts in human brain.     

Nucleotide-dependent conformational changes and assembly of the AAA ATPase SKD1/VPS4B

SKD1/VPS4B belongs to the adenosine triphosphatases associated with diverse cellular activities (AAA) family and regulates multivesicular body (MVB) biogenesis. SKD1 changes its oligomeric state during the ATPase cycle and subsequently releases endosomal sorting complex required for transport (ESCRT) complexes from endosomes during the formation of MVBs. In this study, we describe domain motions in monomeric SKD1 on ATP and ADP binding. Nucleotides bind between the alpha/beta and the alpha-helical domains of SKD1, inducing a approximately 20 degrees domain rotation and closure of the binding site, which are similar to the changes observed in the AAA+ ATPase, HslU. Gel filtration and small-angle X-ray scattering experiments showed that the ATP-bound form of SKD1 oligomerizes in solution, whereas ADP-bound and apo forms of SKD1 exist as monomers, even though the conformations of the ADP- and ATP-bound forms are nearly identical. Nucleotide-bound SKD1 structures are compatible with a hexameric ring arrangement reminiscent of the AAA ATPase p97 D1 ring. In the hexameric ring model of SKD1, Arg290 from a neighboring molecule binds to the gamma-phosphate of ATP, which promotes oligomerization of the ATP-bound form. ATP hydrolysis would eliminate this interaction and subsequent nucleotide release causes the domains to rotate, which together lead to the disassembly of the SKD1 oligomer.     

Expression analysis of UDP-glucose:flavonoid 3-O-glucosyltransferase (UFGT) gene in an interspecific hybrid grape between Vitis ficifolia var. ganebu and Vitis vinifera cv. Muscat of Alexandria

Kadainou R-1, an interspecific hybrid grape derived from red (Vitis ficifolia var. ganebu) and white (V. vinifera cv. Muscat of Alexandria) grapes, accumulates high concentrations of anthocyanin in the berry skin. Hence, the expression of uridine 5′-diphosphate (UDP)-glucose:flavonoid 3-O-glucosyltransferase (UFGT), the key enzyme of the anthocyanin pathway, was examined in the berry skin of Kadainou R-1. As information on gene sequences of V. ficifolia var. ganebu and other wild grape species was unavailable, we performed GeneChip hybridization using biotin-labeled genomic deoxyribonucleic acid (DNA) to investigate how the genomic sequences of V. vinifera varieties and that of V. ficifolia var. ganebu differ. The study showed a lower correlation coefficient between V. vinifera cultivars and V. ficifolia var. ganebu than that among V. vinifera cultivars. The sequences of the UFGT gene derived from both parents of the red and white cultivars were sequenced in Kadainou R1 and revealed that both were expressed irrespective of the fact that it was not expressed in the white grape (male parent).