We report a concise procedure of fluorescence in situ hybridization (FISH) in the gonad and embryos of Caenorhabditis elegans for observing and quantifying repetitive sequences. We successfully observed and quantified two different repetitive sequences, telomere repeats and template of alternative lengthening of telomeres (TALT).
Telomere is a ribonucleoprotein structure that protects chromosomal ends from aberrant fusion and degradation. Telomere length is maintained by telomerase or an alternative pathway, known as alternative lengthening of telomeres (ALT)1. Recently, C. elegans has emerged as a multicellular model organism for the study of telomere and ALT2. Visualization of repetitive sequences in the genome is critical in understanding the biology of telomeres. While telomere length can be measured by telomere restriction fragment assay or quantitative PCR, these methods only provide the averaged telomere length. On the contrary, fluorescence in situ hybridization (FISH) can provide the information of the individual telomeres in cells. Here, we provide protocols and representative results of the method to determine telomere length of C. elegans by fluorescent in situ hybridization. This method provides a simple, but powerful, in situ procedure that does not cause noticeable damage to morphology. By using fluorescently labeled peptide nucleic acid (PNA) and digoxigenin-dUTP-labeled probe, we were able to visualize two different repetitive sequences: telomere repeats and template of ALT (TALT) in C. elegans embryos and gonads.
Telomere protects chromosomal ends from aberrant fusion and degradation. Mammalian telomere is composed of G-rich hexameric repeats, TTAGGG, and shelterin complexes. The telomere repeat sequence of the nematode is similar to those of mammals (TTAGGC). Most eukaryotes utilize telomerase to add telomere repeats to their chromosomal ends. However, 10 – 15% of cancer cells utilize telomerase independent mechanism, known as Alternative Lengthening of Telomeres (ALT)3. Previously, we reported that telomere repeats and its associated sequences, named as TALT, were amplified in the telomeres of telomerase mutant lines that survived critical sterility2.
Telomere length was measured by quantitative PCR or by Southern blot, which provides average length of total telomeres4,5,6,7. Read count of telomere repeat in whole genome sequencing data is also an indicator of total telomere contents8. Although Single TElomere Length Analysis (STELA) could provide the length of a single telomere, it cannot provide spatial information of telomeres9. While POT-1::mCherry reporter protein provides the spatial information of telomeres in vivo, it cannot represent lengths of double-stranded telomeres, as POT-1 is a single-strand telomere binding protein10.
While aforementioned methods provide the averaged information of repetitive sequences, fluorescence in situ hybridization (FISH) allows to observe the amount and spatial pattern of individual sequences of interest on a chromosomal scale. Instead of purification of DNA, tissues or cells are fixed to preserve the native spatial information in FISH. Thus, FISH is a both quantitative and qualitative tool for observation of individual repeat sequences, such as telomere repeats.
This protocol provides an efficient method for simultaneous detection of both telomere and other repeats based on improvements from previously described methods 11,12. C. elegans larvae or adults are multicellular organism with highly differentiated cells. The heterogeneity of cells impedes on the quantitative analysis of a large number of telomere spots. To maximize the number of cells analyzed, embryos are isolated and spread on the polylysine-coated slides for FISH. In addition, this protocol can also be combined with immunofluorescence.
As a proof that the protocol works, we show that it is possible to observe and quantify two different repetitive sequences. DNA probe against TALT1 was generated with simple PCR incorporating digoxigenin-dUTP. Then this TALT1 probe and fluorescence-labeled telomere PNA probe were hybridized simultaneously. Subsequently, digoxigenin was detected by canonical immunofluorescence methods. We present here the representative images where TALT1 colocalized with the telomere in trt-1 survivors.
The main advantage of our protocol is the simplicity of the procedure without noticeable damage to the morphology of cellular structure. Several steps were optimized for C. elegans FISH in this protocol. The critical steps for successful FISH include labeling of probes, fixation of embryos and penetration. Digoxigenin-dUTP labeling method provides an easy-to-use labeling method by PCR or nick-translation. To label long target sequence, nick-translation is preferred. In this case, the probes should be digested wi…
The authors have nothing to disclose.
Mutant worm strains were kindly provided by the Caenorhabditis Genetics Center. This research was supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (grant number: HI14C1277).
PNA probe | PANAGENE | custom order | |
Anti-Digoxigenin-Fluorescein, Fab fragments | Roche | 11207741910 | use 1:200 diluted in PBST |
Digoxigenin-dUTP | Roche | 11573152910 | |
Bovine serum albumin | SIGMA-ALDRICH | A-7906 | |
Paraformaldehyde | SIGMA-ALDRICH | P-6148 | prepare 4% paraformaldehyde by heating in DW with few drops of NaOH. add 0.1 volume of 10x PBS. |
Vectashield | Vector Laboratories | H-1200 | |
Hybridizaiton solution | 3X SSC, 50% formamide, 10% (w/v) dextran sulfate, 50 ug/ml heparin, 100 ug/ml yeast tRNA , 100ug/ml sonicated salmon sperm DNA | ||
Hybridizaiton wash solution | 2X SSC, 50% formamide | ||
Formamide | BIONEER | C-9012 | toxic |
Methanol | Carlo Erba | ||
Acetone | Carlo Erba | ||
Heparin | SIGMA-ALDRICH | H3393 | make 10 mg/ml for stock solution |
Dextran sulfate | SIGMA-ALDRICH | 67578 | |
10X PBS | For 1 Liter DW : 80 g NaCl, 2.0 g KCl, 27 g Na2HPO4:7H2O, 2.4 g KH2PO | ||
PBST | 1X PBS, 0.1% tween-20 | ||
Polysorbate 20 | SIGMA-ALDRICH | P-2287 | Commercial name is Tween-20 |
Poly-L-Lysine solution (0.1 % w/v) | SIGMA-ALDRICH | P-8920 | prepare fresh 0.01 % w/v solution before use |
M9 | 3 g KH2PO4, 6 g Na2HPO4, 5 g NaCl, 1 ml 1 M MgSO4, H2O to 1 L | ||
Bleaching solution | 20% sodium hypochlorite, 0.5 M KOH | ||
Antibody buffer | 1X PBST, 1mM EDTA, 0.1% BSA, 0.05% Sodium azide (toxic) | ||
Blocking solution | Antibody buffer with 5% bovine serum albumin (BSA) | ||
illustra Microspin G-50 | GE healthcare | 27-53310-01 | |
20X SSC | To make 1L, 175.3 g of NaCl, 88.2 g of sodium citrate, H2O to 1 L, adjust pH to 7.0 | ||
2X SSCT | 2X SSC, 0.1 % tween-20 | ||
10x digoxigenin-dUTP mix | 1 mM dATP, 1 mM dGTP, 1 mM dCTP, 0.65mM dTTP, 0.35mM DIG-11-dUTP | ||
PCR purification columns | Cosmo genetech | CMR0112 | |
Glass cleaner / ULTRA CLEAN | Dukssan pure chemicals | 8AV721 | |
Multi-well glass slide | MP biomedicals | 96041205 | |
Nematode growth media | to make 1 L, 3 g of NaCl, 17 g of agar, 2.5 g of peptone, H2O to 974 mL. Autoclave and cool the flask. Add 1 mL of 1M CaCl2, 1 ml of 4 mg/mL cholesterol in ethanol, 1 ml of 1 M MgSO4, 25 mL of 1 M KPO4. | ||
Levamisole | SIGMA-ALDRICH | 196142 | |
Razor | Feather | blade No. 11 | |
Rnase A | Enzynomics | ||
BSA | SIGMA-ALDRICH | A7906 | |
Equipments | |||
Confocal microsope | Zeiss | LSM 510 | EC Plan-Neofluar 100x was used as objective lens. |
Dry block / aluminum block | Labtech | LBH-T03 | Set temperature to 80℃ |
Humid chamber | Plastic box filled with paper towel soaked in DW | ||
Image Analysis Software | Dr. Peter Landsdorp | TFL-telo | http://www.flintbox.com/public/project/502 |