SUMMARY The spatiotemporal organization and dynamics of chromatin play critical roles in regulating genome function. However, visualizing specific, endogenous genomic loci remains challenging in living cells. Here, we demonstrate such an imaging technique by repurposing the bacterial CRISPR/Cas system. Using an EGFP-tagged endonuclease-deficient Cas9 protein and a structurally optimized small guide (sg) RNA, we show robust imaging of repetitive elements in telomeres and coding genes in living cells. Furthermore, an array of sgRNAs tiling along the target locus enables the visualization of non-repetitive genomic sequences. Using this method, we have studied telomere dynamics during elongation or disruption, the subnuclear localization of the MUC4 loci, the cohesion of replicated MUC4 loci on sister chromatids, and their dynamic behaviors during mitosis. This CRISPR imaging tool has potential to significantly improve the capacity to study the conformation and dynamics of native chromosomes in living human cells.

No abstract

In addition to the popular method of fluorescent protein fusion, live cell protein imaging has now seen more and more application of epitope tags. The small size of these tags may reduce functional perturbation and enable signal amplification. To address their background issue, we adapt self-complementing split fluorescent proteins as epitope tags for live cell protein labelling. The two tags, GFP11 and sfCherry11 are derived from the eleventh β-strand of super-folder GFP and sfCherry, respectively. The small size of FP11-tags enables a cost-effective and scalable way to insert them into endogenous genomic loci via CRISPR-mediated homology-directed repair. Tandem arrangement FP11-tags allows proportional enhancement of fluorescence signal in tracking intraflagellar transport particles, or reduction of photobleaching for live microtubule imaging. Finally, we show the utility of tandem GFP11-tag in scaffolding protein oligomerization. These experiments illustrate the versatility of FP11-tag as a labelling tool as well as a multimerization-control tool for both imaging and non-imaging applications.

In order to elucidate the functional organization of the genome, it is vital to directly visualize the interactions between genomic elements in living cells. For this purpose, we engineered the Cas9 protein from Staphylococcus aureus (SaCas9) for the imaging of endogenous genomic loci, which showed a similar robustness and efficiency as previously reported for Streptococcus pyogenes Cas9 (SpCas9). Imaging readouts allowed us to characterize the DNA-binding activity of SaCas9 and to optimize its sgRNA scaffold. Combining SaCas9 and SpCas9, we demonstrated two-color CRISPR imaging with the capability to resolve genomic loci spaced by <300 kb. Combinatorial color-mixing further enabled us to code multiple genomic elements in the same cell. Our results highlight the potential of combining SpCas9 and SaCas9 for multiplexed CRISPR-Cas9 applications, such as imaging and genome engineering.

During apoptosis, dying cells are swiftly removed by phagocytes. How apoptotic cells are recognized by phagocytes is not fully understood. Here we report the identification and characterization of the C. elegans ttr-52 gene, which is required for efficient cell corpse engulfment and encodes a transthyretin-like protein. The TTR-52 protein is expressed in and secreted from C. elegans endoderm and clusters around apoptotic cells. Genetic analysis indicates that TTR-52 acts in the cell corpse engulfment pathway mediated by CED-1, CED-6, and CED-7 and affects clustering of the phagocyte receptor CED-1 around apoptotic cells. Interestingly, TTR-52 recognizes surface exposed phosphatidylserine (PS) in vivo and binds to both PS and the extracellular domain of CED-1 in vitro. Therefore, TTR-52 is the first bridging molecule identified in C. elegans that mediates recognition of apoptotic cells by cross-linking the PS “eat me” signal with the phagocyte receptor CED-1.

Endocytic sorting is achieved through the formation of morphologically and functionally distinct sub-domains within early endosomes. Cargoes destined for recycling are sorted to and transported through newly-formed tubular membranes, but the processes that regulate membrane tubulation are poorly understood. Here, we identified a novel Caenorhabditis elegans Cdc50 family protein, CHAT-1, which acts as the chaperone of the TAT-1 P4-ATPase to regulate membrane phosphatidylserine (PS) asymmetry and endocytic transport. In chat-1 and tat-1 mutants, the endocytic sorting process is disrupted, leading to defects in both cargo recycling and degradation. TAT-1 and CHAT-1 colocalize to the tubular domain of the early endosome, the tubular endocytic recycling compartment (ERC), and the recycling endosome where PS is enriched on the cytosolic surface. Loss of tat-1 and chat-1 function disrupts membrane PS asymmetry and abrogates the tubular membrane structure. Our data suggest that CHAT-1 and TAT-1 maintain membrane phosphatidylserine asymmetry, thus promoting membrane tubulation and regulating endocytic sorting and recycling.

The lack of efficient tools to image non-repetitive genes in living cells has limited our ability to explore the functional impact of the spatiotemporal dynamics of such genes. Here, we addressed this issue by developing a CRISPR-Tag system using one to four highly active sgRNAs to specifically label protein-coding genes with a high signal-to-noise ratio for visualization by wide-field fluorescence microscopy. Our approach involves assembling a CRISPR-Tag within the intron region of a fluorescent protein and then integrating this cassette to N- or C-terminus of a specific gene, which enables simultaneous real-time imaging of protein and DNA of human protein-coding genes, such as HIST2H2BE, LMNA and HSPA8 in living cells. This CRISPR-Tag system, with a minimal size of ~250 bp DNA tag, represents an easily and broadly applicable technique to study the spatiotemporal organization of genomic elements in living cells.

The three-dimensional organization of the genome plays important roles in regulating the functional output of the genome, and even the maintenance of epigenetic inheritance and genome stability. Here, we review and compare a number of newly developed methods that enables the direct visualization of specific, endogenous DNA sequences in living cells, especially that utilizing the CRISPR-Cas9 system. We further discuss the practical considerations in implementing the CRIPSR imaging technique in order to achieve sufficient signal-to-background level, high specificity and high labeling efficiency. With these DNA labeling methods enabling the tracking of the copy number, localization and movement of genomic elements, we discuss their potential applications in understanding the searching and targeting mechanism of Cas9-sgRNA complex, investigating chromosome organization, as well as visualizing genome instability and rearrangement.