Examining the causal mechanism behind rare hereditary diseases nature
For droplet formation experiments in Fig., proteins were diluted to desired concentrations in storage buffer, further diluted 1:1 in 20% PEG-8000 and mixed well with pipetting. Next, 10 µl of solution was immediately transferred on a chambered coverslip .
Droplets were imaged using a LSM880 confocal microscope with a ×63, 1.40 oil DIC objective. Images were acquired slightly above the solution interface; for FRAP experiments, images were acquired directly on the solution interface. Time series for FRAP experiments were acquired using 60 cycles of 2 s intervals, during which the eGFP signal was bleached using a 488 nm laser with 95% intensity after the second interval. FRAP was performed for at least ten droplets for both wild-type and mutant HMGB1 using 10 µM concentration. Recovery curves were fitted to a power-law model. For droplet assays using preassembled mCherry–HP1α, mCherry–MED1-IDR and mCherry–NPM1 condensates , mCherry-labelled proteins were diluted to 20 µM concentration in storage buffer, diluted 1:1 in 20% PEG-8000 and droplets were allowed to form for 1 h at room temperature, shielded from light. Next, eGFP–HMGB1 proteins or 5′ FAM-labelled synthetic IDR peptides were added to the desired concentration, thoroughly mixed and solutions were left to equilibrate for 45 min at room temperature, shielded from light. Droplets were imaged as described above. To test the contribution of RNA for the condensation propensity of HMGB1 IDR peptides, total RNA from V6.5 mouse embryonic stem cells was isolated using a Direct-zol RNA Miniprep kit and added in indicated concentrations into peptide dilutions. RNA–peptide dilutions were thoroughly mixed with pipetting, crowding agent was added and imaging was performed as described above.U2OS, HCT116 and HEK293T cells were cultured in DMEM with GlutaMAX supplemented with 10% FBS and 100 U mlpenicillin–streptomycin . MCF7 cells were cultured in RPMI-1640 supplemented with 20% FBS and 100 U ml), were grown in mTeSR Plus on plates coated with 1:100 diluted Matrigel in KnockOut DMEM and supplemented with 10 µM of the Rho kinase inhibitor Y-27632 once detached during passaging. Cells were cultured at 37 °C with 5% COin a humidified incubator. All cell lines were tested negative for mycoplasma contamination. For live-cell imaging and immunofluorescence, cells were seeded on chambered coverslips . On the next day, cells were transfected using FuGENE HD according to the manufacturer’s instructions. Human iPS cells were transfected using Lipofectamine 3000 according to the manufacturer’s instructions. For viability experiments, cells were cultured on 6-well plates. Transfection series were repeated at least twice for each experiment.Cells were grown on 6-well plates, transfected with FuGENE HD according to manufacturer’s instructions, and eGFPcells were sorted by FACS 48 h after transfections and lysed in TRIzol reagent . Experiments were performed in at least three biological replicates. RNA was extracted and cDNA synthesis was performed as described above, except that 125 ng of RNA was used. Primers are listed in Supplementary TableCells were imaged 24 h after transfections using a LSM880 confocal microscope equipped with an incubation chamber with 5% COand a heated stage at 37 °C. Images were acquired using a ×63, 1.40 oil DIC objective. To visualize cell nuclei, cells were incubated with 0.2 µg mlHoechst at least 10 min before imaging. To visualize nucleoli in living cells, we expressed RFP–fibrillarin fusion proteins by transfecting cells with pTagRFP-C1-fibrillarin plasmid together with plasmids for eGFP–HMGB1 and other transcription factor variants. FRAP experiments were performed for nucleolar regions in cells expressing wild-type or mutant eGFP–HMGB1, guided by the RFP–fibrillarin fluorescence channel. Time series for FRAP experiments were acquired using 20 cycles of 2 s intervals, during which the eGFP signal was bleached using a 488 nm laser with 85% intensity after the second interval. FRAP experiments with designed variants of HMGB1 and other frameshift variants were performed as described above, but using 85–100% laser intensities for bleaching with identical settings for each wild type–mutant comparison. Fluorescence intensities were acquired from around ten regions of interest from separate nuclei, quantified using ZEN Black 2.3 software and reported as relative values to the pre-bleaching time point. Time-lapse imaging of mutant HMGB1 expressing U2OS cells was performed on a Screenstar microplate with Zeiss Celldiscoverer 7. Images were acquired fully automated with a Plan-ApoChromat ×20 objective, NA = 0.7 and 1× tubelense using 15 min intervals and a camera binning of 1 × 1 pixel in 8-bit mode .For fixed-cell immunofluorescence, cells were fixed 24 h after transfections with 4% PFA in PBS for 10 min. After two washes with PBS, cells were permeabilized by incubating 30 min with 0.5% Triton X-100 at room temperature, washed three times with PBS and blocked for 1 h with blocking buffer at room temperature. Samples were incubated with primary antibodies diluted in blocking buffer overnight in 4 °C with gentle agitation. After four washes with blocking buffer, samples were incubated with secondary antibodies for 1 h at room temperature. Samples were washed two times with blocking buffer, incubated for 3 min with 0.25 µg mlU2OS cells were seeded on 24-well plates on sterilized 13 mm glass coverslips pretreated with 0.2% gelatin. The next day, cells were transfected with meGFP–HMGB1 full-length wild-type or mutant constructs using FuGENE HD according to the manufacturer’s instructions. After 24 h, pulse labelling of nascent peptide chains actively translated by the ribosome was performed by replacing the medium supplemented with 20 μM puromycin for 15 min at 37 °C, 5% CO. Cells were then washed three times with cold PBS, followed by fixation with 4% formaldehyde at room temperature, with shaking, for 20 min. Fixative was removed, and cells were washed two times with PBS, followed by incubation in blocking solution with shaking for 45 min at room temperature. Anti-puromycin and anti-GFP primary antibodies were applied in blocking solution supplemented with 0.4% Triton-X-100 and incubated overnight with shaking at 4 °C. Cells were then washed three times with PBS for 5 min at room temperature, followed by secondary antibodies incubated in blocking solution with 0.4% Triton-X-100 shaking for 2 h at room temperature. After three PBS washes, cells were incubated in DAPI in PBS for 30 min with shaking at room temperature, and washed with PBS an additional two times. Coverslips were removed from wells and sealed on poly--lysine slides with ProLong Gold Antifade Mountant . The experiment was performed in independent biological triplicates, with two to four technical replicate coverslips per conditions per experiment. Coverslips were imaged using a Zeiss Celldiscoverer 7 running Zen Blue v.3.2 . All images were acquired in a fully automated fashion with a Plan-ApoChromat ×20 objective, NA = 0.95 and a ×2 tube lens , and camera binning 2 × 2 pixels in 8-bit mode. The resulting lateral resolution is 0.227 µm pixel. All images were acquired in tile regions of typically 20 × 20 individual tiles, resulting in 400 individual images per coverslip. Focus stabilization was achieved with an automated combined hardware and software focusing strategy at each second position (Fig.
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