Mar 06, 2023

Public workspacesingle-cell Micro-C protocol

DOI
dx.doi.org/10.17504/protocols.io.kqdg39wbzg25/v1
  • 1Biomedical Pioneering Innovation Center, Peking University, Beijing 100871, China;
  • 2Department of Neurobiology, Stanford University, Stanford, CA 94305, USA
Honggui Wu
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DOI: dx.doi.org/10.17504/protocols.io.kqdg39wbzg25/v1
External link: https://www.biorxiv.org/content/10.1101/2023.02.18.529096v1.article-metrics
Protocol Citation: Honggui Wu, Longzhi Tan 2023. single-cell Micro-C protocol. protocols.io https://dx.doi.org/10.17504/protocols.io.kqdg39wbzg25/v1
Manuscript citation:
License: This is an open access protocol distributed under the terms of the Creative Commons Attribution License,  which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Protocol status: Working
We use this protocol and it's working
Created: March 04, 2023
Last Modified: March 06, 2023
Protocol Integer ID: 78092
Keywords: single-cell 3D genome structure, single-cell Hi-C, chromosome conformation, Micro-C
Abstract
Here we present a single-cell 3D genome mapping assay with improved spatial resolution with the help of micrococcal nuclease (MNase), termed single-cell Micro-C (scMicro-C). To achieve scMicro-C, we made three improvements. First, we titrated MNase digestion to reduce DNA loss and produce proper DNA fragments. Second, we solubilized chromatin with an ionic detergent, sodium dodecyl sulfate (SDS), which dramatically improved ligation efficiency. Third, we adopted our high-coverage transposon-based whole-genome amplification method, META (19), using Tn5 (20), to enhance the detection of chromatin “contacts.” scMicro-C retains the high signal-to-noise and high-resolution characteristics of bulk Micro-C, as indicated the accurate detection chromatin loops and other fine-scale chromatin structures. Furthermore, scMicro-C retains nucleosome and other chromatin-bound proteins (e.g., transcription factor) occupancy information. With our previously developed Dip-C algorithm, we confirmed that scMicro-C enables the reproducible reconstruction of 3D genome structures in single-cell at 5 kb resolution. With such kilobase 3D structures, we were able to explore the fine-scale chromatin folding in individual cells.
Crosslink Cells for Micro-C
Crosslink Cells for Micro-C
Prepare 1 x BSA in PBS for Washing.
  • Add Amount50 µL 10% BSA (MACS 130-091-376, final 1x) to Amount50 mL 1x PBS.

Prepare 1% Paraformaldehyde (Amount1 mL for 1 million cells), recover to room temperature.
  • Amount1 mL 32% PFA (EMS 15714, final 1%)
  • Amount31 mL 1x PBS


Prepare 3 mM DSG (Amount1 mL for 1 million cells).
  • Dissolve 50 mg DSG (Thermo Fisher 20593) with Amount511 µL DMSO to a final concentration of 300 mM.
  • Add Amount300 µL 300 mM DSG to Amount29.7 mL 1 x PBS to a final concentration of 3 mM, protected from light.

Harvest cells.
  • Collect cell growth with appropriate density and high viability.
  • Wash cells once with 1x PBS.
Fixation.
  • Resuspend cell pellets in 1% PFA, and incubate at room temperature with rotation for 10 min.
  • At the end of incubation, add 2 M Tris-HCl ph 7.5 to a final concentration of 0.75 M, and incubate at RT for 5 min.
  • Centrifuge at 3000 x g for 5 min in a swing bucket centrifuge.
Remove the supernatant, then wash twice with Amount5 mL ice-cold 1 x BSA in PBS.

Crosslink with DSG.
  • Resuspend in 3 mM DSG, and incubate at RT for 45 min with rotation.
  • At the end of incubation, add 2 M Tris-HCl ph 7.5 to a final concentration of 0.75 M, and incubate at RT for 5 min.
  • Centrifuge at 3000 x g for 5 min in a swing bucket centrifuge.


Count cells and aliquot.
  • Remove the supernatant, then wash twice with Amount5 mL ice-cold 1 x BSA in PBS.
  • Count cell number, then aliquot 1 million cells each to 1.5 mL tube.
  • Store at -80°C (up to a year).

Prepare Micro-C Buffers
Prepare Micro-C Buffers
Prepare Micro-C Buffers
Amount10 mL Prepare Micro-C Buffer 1 (Amount300 µL per sample )
  • Amount50 µL 5 M NaCl (Invitrogen AM9760G; final 50 mM)
  • Amount50 µL 1 M Tris pH 7.5 (Invitrogen 15567027; final 10 mM)
  • Amount25 µL 1 M MgCl2 (Invitrogen AM9530G; final 5 mM)
  • Amount5 µL 1 M CaCl2 (Sigma 21115; final 1 mM)
  • Amount50 µL 5% Digitonin (Sigma D141-100MG, final: 0.05%)
  • Amount50 µL 100 x protease inhibitor (Roche 4693159001) (final: 1 X)
  • Amount4.77 mL nuclease-free H2O








Prepare Micro-C Buffer 2 (Amount10 mL )
  • Amount100 µL 5 M NaCl (Invitrogen AM9760G; final 50 mM)
  • Amount100 µL 1 M Tris pH 7.5 (Invitrogen 15567027; final 10 mM)
  • Amount100 µL 1 M MgCl2 (Invitrogen AM9530G; final 10 mM)
  • Amount9.7 mL nuclease-free H2O






Prepare Micro-C Buffer 3 (Amount10 mL )
  • Amount500 µL 1 M Tris pH 7.5 (Invitrogen 15567027; final 50 mM)
  • Amount100 µL 1 M MgCl2 (Invitrogen AM9530G; final 10 mM)
  • Amount9.4 mL nuclease-free H2O



MNase Digestion
MNase Digestion
Dilute MNase.
  • Make MNase Dilution Buffer (10 mM Tris-HCl ph 7.5, 50 mM NaCl, 1 mM EDTA, 50% Glycerol).
  • Aliquot 10,000 Unit MNase (NEB M0247S, 2,000 U/µL) to Amount500 µL MNase Dilution Buffer to a final concentration of 20 U/µL.
  • Store at -80°C for several month.

Thaw 5 tubes of cell pellets on ice.
Resuspend in Amount100 µL Micro-C Buffer 1, incubate on ice for 20 min, and aspiration every 5 min.

Centrifuge at 800 x g at 4°C for 5 min.
Take 5 ul to check digestion efficiency, remaining centrifuge at 800 x g at 4°C for 5 min. Carefully discard the supernatant, and wash once with Amount500 µL Micro-C Buffer 2. Store at -80°C (stable for up to several weeks) or proceed to End Repair.

Check Digestion Efficiency
  • Take Amount5 µL above sample to check digestion efficiency.
  • Add 85 ul 1 x PBS, 5 ul 10% SDS (Sigma, 71736-100ML), and 5 ul 20 mg/mL ProteaseK (QIAGEN, 19131).
  • incubate at 65°C for 2 hr.
  • Purify with DNA clean column (ZYMO, D4014).
  • Run capillary electrophoresis (Fragment Analyzer/TepeStation/Bioanalyzer) to analyze digestion efficiency.
An exemplified MNase titration experiment: fragment size distribution with different MNase concentrations.

End Repair
End Repair
Thaw the cell pellet (with the most appropriate fragment size) on ice.
Prepare 0.5% SDS.
  • Add Amount25 µL 10% SDS to Amount475 µL Micro-C Buffer 2 to a final concentration of 0.5%.

Solubilize Chromatin.
  • Resuspend the cell pellets in Amount50 µL 0.5% SDS.
  • Incubate at 62°C for 10 min.
  • At the end of incubation, add Amount50 µL 5% Triton X-100 to quench SDS. Incubate at 37°C for 15 min.

Centrifuge at 800 x g for 5 min, carefully remove supernatant.
Prepare End Repair Mix 1 (Amount45 µL each sample).
  • Amount5 µL 10 X NEBuffer 2.1 (NEB B7202S)
  • Amount1 µL 100 mM ATP (Thermo R0441)
  • Amount2.5 µL 100 mM DTT (final: ~5 mM)
  • Amount2.5 µL 10 U/uL T4 PNK (NEB M0201S) (final: ~0.5 U/uL)
  • Amount34 µL nuclease-free H2O





  • Resuspend in Amount45 µL End Repair Mix1, and incubate at 37°C for 15 min.

  • At the end of incubation, add Amount5 µL 5 U/uL Klenow Fragment (NEB M0210S), incubate at 37°C for another 15 min.

Prepare End Repair Mix2 (Amount25 µL each sample)
  • Amount2.5 µL 10 X T4 DNA Ligase Buffer (NEB B0202S)
  • Amount0.5 µL 10 mM (each) dNTP (NEB N0447S)
  • Amount0.125 µL 20 mg/mL BSA (NEB B9000S)
  • Amount21.875 µL nuclease-free H2O




After Klenow incubation, add Amount25 µL End Repair Mix2 to each tube, incubate at 23°C for 45 min (shake 30 s and pause 2 min).

Add 5 ul 0.5 M EDTA (Invitrogen AM9260G).
Incubate at 65°C for 20 min.
Wash once with Amount1 mL Micro-C Buffer 3.

Proximity Ligation
Proximity Ligation
Prepare Ligation Mix (Amount250 µL each sample).
  • Amount25 µL 10 X T4 DNA Ligase Buffer (NEB B0202S)
  • Amount1.25 µL 20 mg/mL BSA (NEB B9000S)
  • Amount12.5 µL 400 U/uL T4 DNA Ligase (NEB M0202S)
  • Amount211.25 µL nuclease-free H2O



Resuspend the cell pellet in Amount250 µL Ligation Mix.

Take Amount25 µL as ligation efficiency control.

Check Ligation Efficiency
  • Take Amount25 µL ligation product above sample to check ligation efficiency.
  • Add 65 ul 1 x PBS, 5 ul 10% SDS (Sigma, 71736-100ML), and 5 ul 20 mg/mL ProteaseK (QIAGEN, 19131).
  • incubate at 65°C for 2 hr.
  • Purify with DNA clean column (ZYMO, D4014).
  • Run capillary electrophoresis (Fragment Analyzer/TepeStation/Bioanalyzer) to analyze ligation efficiency.
A typical fragment size distribution after digestion and ligation.

Single-cell Amplification
Single-cell Amplification
Primers
  • META 16 sequence

AB
META-16-1GGCACCG AAAA
META-16-2CTCGGCGA TAAA
META-16-3GGTGGAGC ATAA
META-16-4CGAGCGCA TTAA
META-16-5AGCCCGGT TATA
META-16-6TCGGCACC AATA
META-16-7GCCTGTGG ATTA
META-16-8GCGACCCT TTTA
META-16-9GCATGCGG TAAT
META-16-10GCGTTGCC ATAT
META-16-11GGCCGCAT TTAT
META-16-12ACCGCCTC TATT
META-16-13CCGTGCCA AAAT
META-16-14TCTCCGGG AATT
META-16-15CCGCGCTT ATTT
META-16-16CTGAGCTCG TTTT
  • META Transposon
AB
META Transposon 5'-[META sequence]-AGATGTGTATAAGAGACAG-3'
19 bp ME5'-/phos/-CTGTCTCTTATACACATCT-3'

  • First Step PCR Primers

AB
First PCR primers5'-[META sequence]-AGATGTGTATAAG-3'
Here we use 12 x 8 barcode combinations to distinguish individual cells.
  • META16-ADP1 cell barcode
AB
1-1GATATG
1-2ATACG
1-3CCGTCTG
1-4TGCG
1-5GAACTCG
1-6ATGTAG
1-7CCCG
1-8TATGT
1-9GAGTAAG
1-10ATCG
1-11CCTAG
1-12TGACCG
  • META16-ADP2 cell barcode

AB
2-1ACTCTA
2-2AGAGCAT
2-3GGTATG
2-4TCGATGC
2-5CTACTAG
2-6TATGCA
2-7CACACGA
2-8GTCGAT
Second Step Barcoded PCR Primers

AB
META16-ADP15'-CTTTCCCTACACGACGCTCTTCCGATCT-[cell barcode]-[META sequence]-AGATGTGTATAAG-3'
META16-ADP25'-GAGTTCAGACGTGTGCTCTTCCGATCT-[cell barcode]-[META sequence]-AGATGTGTATAAG-3'

Prepare Reagents.
Prepare 60 mg/mL Protease.
  • Dissolve 1 vial QIAGEN protease (QIAGEN 19155) in 2.78 mL 50% glycerol.
  • Aliquot and store at -80°C (stable for up half a year).
Cell Triton Lysis Buffer (Amount1 mL ).
  • Amount10 µL 1 M Tris pH 8.0 (Invitrogen AM9855G, final 10 mM)
  • Amount4 µL 5 M NaCl (Invitrogen AM9760G, final 20 mM)
  • Amount2 µL 0.5 M EDTA (Invitrogen AM9260G, final 1 mM)
  • Amount10 µL 10% Triton X-100 (Sigma 93443, final 0.1%)
  • Amount25 µL 1 M DTT (Sigma 646563, final 25 mM)
  • Amount5 µL 100 µM Carrier ssDNA (final 100 nM)
  • Add H2O to 1 mL
  • Amount25 µL 60 mg/mL QIAGEN protease (QIAGEN 19155, final 1.5 mg/mL) (freshly add when used)

AB
Carrier ssDNATCAGGTTTTCCTGAA



2 X Transposition Buffer (Amount1 mL )
  • Amount20 µL 1 M Tris-HCl pH 8.0 (Invitrogen AM9855G, final 20 mM)
  • Amount10 µL 1 M MgCl2 (Thermo Fisher AM9530G, final 10 mM)
  • 400 ul 40% (w/w) Polyethylene glycol 8000 (Sigma P1458, final 16% (w/w) )
  • Amount570 µL nuclease-free H2O
Store at -20℃ if needed



Pick single cells.
Use mouth pipette or Fluorescence-activated cell sorting (FACS) to pick single cell into single PCR tubes or 96-well plates containing Amount2 µL cell lysis buffer.

Cell lysis.
Incubate as:
50℃, 1 hr
65℃, 1 hr
70℃, 15 min
Stored at -80℃ or proceed to transposon-based amplification.
Transposition (Amount8 µL for each well, below recipe for 1 96-well plate).
  1. Prepare Transposition Mix
  • Amount440 µL 2 X Transposition Buffer
  • Add META transposome* to a final concentration of 0.3 nM
  • Add nuclease-free H2O to Amount880 µL
* Transposome: META transposome is optional. Alternatively, Nextera transposome can be used (if using Nextera procedure, please refer to "Dip-C (Part 2: Whole-genome Amplification with Nextera)") on protocol.io.
2. Add Amount8 µL Transposition Mix to each well with a multi-channel pipette, vortex to mix.
3. Incubate at 55℃ for 10 min.



Tn5 Stop.
  • Prepare Tn5 Removal Mix (250 mM NaCl, 37.5 mM EDTA, 2 mg/mL QIAGEN protease).
  • Add Amount2 µL Tn5 Removal Mix to each tube, and mix thoroughly.
  • Incubate as:
50℃, 30 min
70℃, 15 min

Pre-amplification.




Prepare Preamplification Mix (Amount16 µL each well, below recipe for 1 96-well plate)
  • Amount1540 µL 2 X Q5 High-fidelity Master Mix (NEB M0492L)
  • Amount98.6 µL 50 µM First Step PCR Primers (final 0.1 µM each)
  • Amount55 µL 100 mM MgCl2
  • Amount66.4 µL nuclease-free H2O
Add Amount16 µL Preamplification Mix each well with a multi-channel pipette, and vortex to mix.

Amplified as:
72℃, 5 min
98℃, 30 s
12 cycles of [98℃ for 10 s, 62℃ for 30 s, 72℃, 2 min]
65℃, 5 min.
Cell Barcoding PCR.
  • Add Amount1 µL individual 50 μM barcoded META16-ADP1 Primer and Amount1 µL individual 50 μM indexed META16-ADP2 Primer Mix to each well with a multi-channel pipette.
  • Incubate as:
98℃, 30 s
3 cycles of [98℃ for 10 s, 62℃ for 30 s, 72℃, 2 min]
65℃, 5 min
Illustration of barcode addition.

Purification.
  • Pool a 96-well plate to purify.
  • Purify with clean column (ZYMO, D4014).
  • Quantify with Qubit Fluorometer.
Library preparation.
  • Take 400 ng amplified product for each plate as input.
  • Prepare Library Preparation Mix (Amount50 µL 2 X Q5 High-fidelity Mater Mix, template (400 ng), Amount5 µL 10 µM Indexed i5 primer, Amount5 µL 10 µM Indexed i7 primer (Vazyeme N321/N322), nuclease-free H2O to Amount100 µL )
  • Amplified as:
98℃, 30 s
2 cycles of [98℃, 10 s, 68℃, 30 s, 72℃, 2 min]
72℃, 5 min
  • Purify with PCR clean column (ZYMO D4014), then purify with 0.75 x AMPUre XP beads to select > 300 bp fragments for sequencing.




Sequencing.
META library only has 20% valid fragments having both P7 and P5 on either side, need to RUN qPCR quantify before loading.