FISH Going Meso-Scale: A Microscopic Search for Chromatin Domains

The intimate relationships between genome structure and function direct efforts toward deciphering three-dimensional chromatin organization within the interphase nuclei at different genomic length scales. For decades, major insights into chromatin structure at the level of large-scale euchromatin and heterochromatin compartments, chromosome territories, and subchromosomal regions resulted from the evolution of light microscopy and fluorescence in situ hybridization. Studies of nanoscale nucleosomal chromatin organization benefited from a variety of electron microscopy techniques. Recent breakthroughs in the investigation of mesoscale chromatin structures have emerged from chromatin conformation capture methods (C-methods). Chromatin has been found to form hierarchical domains with high frequency of local interactions from loop domains to topologically associating domains and compartments. During the last decade, advances in super-resolution light microscopy made these levels of chromatin folding amenable for microscopic examination. Here we are reviewing recent developments in FISH-based approaches for detection, quantitative measurements, and validation of contact chromatin domains deduced from C-based data. We specifically focus on the design and application of Oligopaint probes, which marked the latest progress in the imaging of chromatin domains. Vivid examples of chromatin domain FISH-visualization by means of conventional, super-resolution light and electron microscopy in different model organisms are provided.

CRISPR/dCas9 platforms in plants: strategies and applications beyond genome editing

Clustered regularly interspaced short palindromic repeat (CRISPR) and Cas9-associated protein systems provide a powerful genetic manipulation tool that can drive plant research forward. Nuclease-dead Cas9 (dCas9) is an enzymatically inactive mutant of Cas9 in which its endonuclease activity is non-functional. The applications of CRISPR/dCas9 have expanded and diversified in recent years. Originally, dCas9 was used as a CRISPR/Cas9 re-engineering tool that enables targeted expression of any gene or multiple genes through recruitment of transcriptional effector domains without introducing irreversible DNA-damaging mutations. Subsequent applications have made use of its ability to recruit modifying enzymes and reporter proteins to DNA target sites. In this paper, the most recent progress in the applications of CRISPR/dCas9 in plants, which include gene activation and repression, epigenome editing, modulation of chromatin topology, live-cell chromatin imaging and DNA-free genetic modification, will be reviewed. The associated strategies for exploiting the CRISPR/dCas9 system for crop improvement with a dimer of the future of the CRISPR/dCas9 system in the functional genomics of crops and the development of traits will be briefly discussed.

Chromatin imaging and new technologies for imaging the nucleome

Synergistic developments in advanced fluorescent imaging and labeling techniques enable direct visualization of the chromatin structure and dynamics at the nanoscale level and in live cells. Super-resolution imaging encompasses a class of constantly evolving techniques that break the diffraction limit of fluorescence microscopy. Structured illumination microscopy provides a twofold resolution improvement and can readily achieve live multicolor imaging using conventional fluorophores. Single-molecule localization microscopy increases the spatial resolution by approximately 10-fold at the expense of slower acquisition speed. Stimulated emission-depletion microscopy generates a roughly fivefold resolution improvement with an imaging speed proportional to the scanning area. In parallel, advanced labeling strategies have been developed to "light up" global and sequence-specific DNA regions. DNA binding dyes have been exploited to achieve high labeling densities in single-molecule localization microscopy and enhance contrast in correlated light and electron microscopy. New-generation Oligopaint utilizes bioinformatics analyses to optimize the design of fluorescence in situ hybridization probes. Through sequential and combinatorial labeling, direct characterization of the DNA domain volume and length as well as the spatial organization of distinct topologically associated domains has been reported. In live cells, locus-specific labeling has been achieved by either inserting artificial loci next to the gene of interest, such as the repressor-operator array systems, or utilizing genome editing tools, including zinc finer proteins, transcription activator-like effectors, and the clustered regularly interspaced short palindromic repeats systems. Combined with single-molecule tracking, these labeling techniques enable direct visualization of intra- and inter-chromatin interactions. This article is categorized under: Laboratory Methods and Technologies > Imaging.