Cell Biology

A fundamental understanding of gene regulation requires a quantitative characterization of the spatial organization and dynamics of chromatin. The advent of fluorescence super-resolution microscopy techniques such as photoactivated localization microscopy (PALM) presented a breakthrough to visualize structural features with a resolution of ~20 nm in fixed cells. However, until recently the long acquisition time of super-resolution images prevented high-resolution measurements in living cells due to spreading of localizations caused by chromatin motion. Here, we present a step-by step protocol for our recently developed approach for correlatively imaging telomeres with conventional fluorescence and PALM, in order to obtain time-averaged super-resolution images and dynamic parameters in living cells. First, individual single molecule localizations are assigned to a locus as it moves, allowing to discriminate between bound and unbound dCas9 molecules, whose mobilities overlap. By subtracting the telomere trajectory from the localization of bound molecules, the motion blurring is then corrected, and high-resolution structural characterizations can be made. These structural parameters can also be related to local chromatin motion or larger scale domain movement. This protocol therefore improves the ability to analyze the mobility and time-averaged nanoscopic structure of locus-specific chromatin with single-molecule sensitivity.

Genome sizes of Zygnema spp. vary greatly, being unknown whether polyploidization occurred. The exact number of chromosomes in this genus is unknown since counting methods established for higher plants cannot be applied to green algae. The massive presence of pectins and arabinogalactan proteins in the cell wall interferes with the uptake of staining solutions; moreover, cell divisions in green algae are not restricted to meristems as in higher plants, which is another limiting factor. Cell divisions occur randomly in the thallus, due to the intercalary growth of algal filaments. Therefore, we increased the number of cell divisions via synchronization by changing the light cycle (10:14 h light/dark). The number of observed mitotic stages peaked at the beginning of the dark cycle. This protocol describes two methods for the visualization of chromosomes in the filamentous green alga Zygnema. Existing protocols were modified, leading to improved acetocarmine and haematoxylin staining methods as investigated by light microscopy. A freeze-shattering approach with liquid nitrogen was applied to increase the accessibility of the haematoxylin dye. These modified protocols allowed reliable chromosome counting in the genus Zygnema.

Key features

• Improved method for chromosome staining in filamentous green algae.

• Optimized for the Zygnema strains SAG 698-1a (Z. cylindricum), SAG 698-1b (Z. circumcarinatum), and SAG 2419 (Zygnema ‘Saalach’).

• This protocol builds upon the methods of chromosomal staining in green algae developed by Wittmann (1965), Staker (1971), and Fujii and Guerra (1998).

• Cultivation and synchronization: 14 days; fixation and permeabilization: 24 h; staining: 1 h; image analysis and chromosome number quantification: up to 20 h.

The stable HBV-transfected cell lines, which based on stable integration of replication-competent HBV genome into hepatic cells, are widely used in basic research and antiviral drug evaluation against HBV. However, previous reported strategies to generate HBV-replicating cell lines, which primarily rely on random integration of exogenous DNA by plasmid transfection, are inefficient and time-consuming. We newly developed an all-in-one Sleeping Beauty transposon system (denoted pTSMP-HBV vector) for robust generation of stable HBV-replicating cell lines of different genotype. The pTSMP-HBV vector contains HBV 1.3-copy genome and dual selection markers (mCherry and puromycin resistance gene), allowing rapid enrichment of stably-transfected cells via red fluorescence-activated cell sorting and puromycin antibiotic selection. In this protocol, we described the detailed procedure for constructing the HBV-replicating stable cells and systematically evaluating HBV replication and viral protein expression profiles of these cells.

Somatic chromosomes are usually studied from the root tip cells of the plants for cytological investigations. In dioecious plant species like Coccinia grandis, it is very difficult to get meristematic root tip cells from the mature plants of the respective sex forms. In this report, young leaves of the respective sexual phenotypes were used as tissue samples for mitotic chromosome analysis. For meiotic preparation, flower buds of appropriate size were selected for chromosomal studies. Following protocols could be effectively used for routine chromosome preparations in other plant species as well.

ROS-induced DNA damage is repaired in living cells within a temporal and spatial context, and chromatin structure is critical to a consideration of DNA repair processes in situ. It’s well known that chromatin remodeling factors participate in many DNA damage repair pathways, indicating the importance of chromatin remodeling in facilitating DNA damage repair. To date, there has been no method to induce site-specific oxidative DNA damage in living cells. Therefore, it is not known whether the DNA repair mechanisms differ within active or condensed chromatin. We recently established a novel method, DTG (Damage Targeted at one Genome-site), to study DNA damage response of reactive oxygen species (ROS)-induced DNA damage in living cell at one genome loci with active or inactive transcription. For this, we integrated a tetracycline responsive elements (TRE) cassette (~90 kb) at X-chromosome in U2OS cells (Lan et al., 2010), then fused KillerRed (KR), a light-stimulated ROS-inducer which can specifically produce ROS-induced DNA damage, to a tet-repressor (tetR-KR, OFF) or a transcription activator (TA-KR, ON) (Lan et al., 2014) (Figure 1). TetR-KR or TA-KR binds to the TRE cassette and induces ROS damage under hetero- or euchromatin states, respectively. How chromatin states regulate the DNA damage response processes can be examined by using this powerful method.

A chromosome is the structure that organizes DNA and protein in cells. It is a single piece of coiled DNA containing coding and non-coding sequences. Human cells have 23 pairs of chromosomes including 22 pairs of autosomes and one pair of sex chromosome, giving a total of 46 per cell. In tumor cells, chromosomal instability has been considered to be one of the hallmarks of tumor formation. Here we use the karyotype analysis to separate the chromosomes and observe the chromosomes in tumor cells with a microscope.

This protocol is a more detailed version of previous protocols (Yang et al., 2011; Bolaños-Villegas et al., 2013) developed for the examination of meiotic chromosome spreads. Meiotic chromosome spreads are useful to determine the presence of defects in chromosome pairing and segregation. The protocol also describes how to perform fluorescent in situ hybridization experiments with a centromere probe used to label chromosomes.

This protocol is optimized for immunoFISH staining of adherent cultured mammalian cells. It combines immunofluorescence for DNA damage response factors (e.g. 53BP1) and FISH against telomeric DNA.

Fluorescence in situ hybridization (FISH) is a method that uses a fluorescently labeled DNA probe for mapping the position of a genetic element on chromosomes. A DNA probe is prepared by incorporating Cy-3 or Cy-5 labeled nucleotides into DNA by nick-translation or a random primed labeling method. This protocol was used to map genes (Sharakhova et al., 2010) and microsatellite markers (Kamali et al., 2011; Peery et al., 2011) on polytene chromosomes from ovarian nurse cells and salivary glands of malaria mosquitoes. Detailed physical genome mapping performed on polytene chromosomes has the potential to link DNA sequences to specific chromosomal structures such as heterochromatin (Sharakhova et al., 2010). This method also allows comparative cytogenetic studies (Sharakhova et al., 2011; Xia et al., 2010), and reconstruction of species phylogenies (Kamali et al., 2012).