We thank Madeleine Lu for constructing securin-mCherry plasmid, Lap Man Lee,&#160;Kenneth Ho and Allen P Liu&#160;for discussions about droplet generation, Jeremy B. Chang and James E. Ferrell Jr for providing GFP-NLS construct. This work was supported by the National Science Foundation (Early CAREER Grant #1553031), the National Institutes of Health (MIRA #GM119688), and a Sloan Research Fellowship.

All methods described here have been approved by the Institutional Animal Care and Use Committee (IACUC) of University of Michigan.
1. Preparation of Materials for Cell Cycle Reconstitution and Detection
<ol>
	<li>Gibson Assembly cloning for the plasmid DNA construction and mRNA purification of securin-mCherry
	<ol>
		<li>Prepare three DNA fragments including a pMTB2 vector backbone, securin, and mCherry through polymerase chain reaction (PCR) and gel purification21,22.</li>
		<li>Measure the concentrations of fragments using a fluorospectrometer. Combine 100 ng of backbones with securin and mCherry inserts at a 1:1:1 molecular ratio and add deionized water to adjust the total volume of reaction to 5 &#181;L.</li>
		<li>Prepare 6 mL of 5x isothermal assembly reaction buffer with 3 mL of 1 M Tris-HCl (pH 7.5), 300 &#181;L of 1 M MgCl2, 600 &#181;L of 10 mM dNTP, 300 &#181;L of 1 M DTT, 1.5 g of PEG-8000, and 20 mg of NAD23.</li>
		<li>Add the combined fragments to a 15 &#181;L of 1x isothermal assembly reaction buffer aliquot. Incubate the reaction in a PCR reaction tube at 50 &#176;C for 15 minutes.</li>
		<li>Make a 0.8% agarose gel and run with a voltage of 10 V/cm to verify the annealing efficiency of these fragments.</li>
		<li>Purify the constructed plasmid and elute using 15 &#181;L of RNase-free water. Transform 1 &#181;L of the purified plasmid DNA into 50 &#181;L of DH5&#945; competent E. coli cells24 for future use.</li>
		<li>Extract and purify the plasmid DNA of securin-mCherry from the DH5&#945; competent E. coli cells.</li>
		<li>Transcribe the linearized securin-mCherry plasmid DNA into mRNA and purify the mRNA. Use a spectrophotometer to verify that the concentration of mRNA is more than 100 ng/&#181;L and the ratio of absorbance at 260 nm and 280 nm is about 2.0.</li>
	</ol>
	</li>
	<li>Preparation of demembranated Xenopus laevis sperm DNA 
	NOTE: Adapted from Murray 19911.
	<ol>
		<li>Euthanize adult male Xenopus laevis25 and dissect testes from four male frogs26,27.</li>
		<li>Place testes from four frogs in the same 100 mL Petri dish. Rinse the testes three times in 50 mL of cold 1x MMR solution (0.1 M NaCl, 2 mM KCl, 1 mM of MgCl2, 2 mM CaCl2, 5 mM HEPES, 0.1 mM EDTA).</li>
		<li>Remove excess blood and fat from the testes in a 100 mL petri dish and then wash twice in cold nuclear preparation buffer (NPB) (250 mM sucrose, 15 mM HEPES, pH 7.4, 1 mM EDTA, pH 8.0, 0.5 mM spermidine trihydrochloride, 0.2 mM spermine tetrahydrochloride).</li>
		<li>Remove the nuclear preparation buffer (NPB) and cut testes into pieces with lengths smaller than 0.2 cm using sharp dissecting scissors in a 100 mL Petri dish. Add 8 mL of cold NPB to the testes for further breakdown.</li>
		<li>Use nylon mesh fabrics with a pore size of 70 &#181;m to filter the testes and collect sperm sample into a 15 mL conical tube. Spin down the collected sperms at 1,730 x g for 10 minutes at 4 &#176;C and remove supernatant.</li>
		<li>Supply the sperm cells with 1 mL of NPB and 50 &#181;L of 10 mg/mL lysolecithin and incubate at room temperature for 5 min to demembranate sperm cells.</li>
		<li>Wash the demembranated sperm DNA with 10 mL of cold NPB with 3% BSA solution three times.</li>
		<li>Measure the density of sperm DNA with a hemocytometer.</li>
		<li>Freeze the sperm DNA in 5 &#181;L aliquots in liquid nitrogen and store at -80 &#176;C. The optimal concentration of sperm DNA is approximately 105 nuclei/&#181;L.</li>
	</ol>
	</li>
	<li>Protein purification of GFP-Nuclear Localization Signal (GFP-NLS)
	<ol>
		<li>Transform the GST-tagged GFP-NLS plasmid into BL21 (DE3) competent E. coli cells5.</li>
		<li>Incubate the cells without antibiotics at 37 &#176;C for 1 h and then spread on a LB agarose plate with 100 &#181;g/mL ampicillin.</li>
		<li>Incubate the plate at 37 &#176;C for 14 to 18 hours to grow the cells into single colonies. Pick and inoculate single colonies into 4 mL of LB media with 100 &#181;g/mL ampicillin. Shake the cells at 37 &#176;C for 12 hours.</li>
		<li>Prepare the autoclaved YT media (16 g of Bacto-tryptone, 10 g of Bacto-yeast extract, 5 g of NaCl) and heat the YT media to 37 &#176;C.</li>
		<li>Inoculate 1 mL of the overnight incubated cells in LB media into 250 mL of preheated YT media with 100 &#181;g/mL ampicillin and shake for 2 hours to increase cell density.</li>
		<li>Measure cell optical density (OD) every 30 minutes, using a spectrophotometer at a wavelength of 600 nm. Add 0.1 mM IPTG into cells to induce GFP-NLS protein expression once the OD value reaches 0.1.</li>
		<li>Shake the cells at 18 &#176;C overnight to reduce cell growth rate and allow better protein expression and synthesis.</li>
		<li>Spin down the cells at 34,524 x g at 4 &#176;C for 5 minutes and remove supernatant.</li>
		<li>Resuspend the cell pellets in 40 mL of PBS buffer (supplied with 800 &#181;L of 50 mg/mL lysozyme, 100 &#181;L of 0.2 M PMSF, 13.2 &#181;L of a 10 mg/mL mix of leupeptin, pepstatin, and chymostatin, and 8 &#181;L of 0.5 M EDTA) and incubate at 4 &#176;C for 20 minutes.</li>
		<li>Break down cells using a sonicator at a frequency of 20 kHz for 4 x 30 s, with 30 s intervals between each sonication, to release expressed GFP-NLS protein.</li>
		<li>Spin at 20,000 x g for 35 minutes to remove the broken cells.</li>
		<li>Use affinity chromatography to extract and purify GFP-NLS protein.</li>
		<li>Wash the 1.5 mL bead slurry three times with 15 mL of PBS buffer and incubate 250 mL of lysate with beads for 4 hours at 4 &#176;C with inversion.</li>
		<li>Spin down the beads at 2000 x g for 5 min and add 10 mL of wash solution (25 mM Tris base, pH 7.5, 500 mM NaCl, and 5 mM DTT) to beads. Incubate beads in 700 &#181;L of elution buffer (50 mM Tris-HCl, pH 8.8, 20 mM reduced glutathione, and 5 mM DTT) with rotation at 4 &#176;C. Collect an aliquot of the elution with a volume of 50 &#181;L.</li>
		<li>Measure the concentration of collected protein by running a Coomassie blue gel28.</li>
		<li>Elute the proteins using PD-10 columns with 3 mL of storage buffer (20 mM HEPES, 150 mM KCl, 10% glycerol, pH 7.7) through buffer exchange.</li>
	</ol>
	</li>
</ol>
2. Preparation of Cycling Xenopus Extracts
NOTE: The overall procedure of preparing the extracts is illustrated in Figure 1A. Adapted from Murray 19911.
<ol>
	<li>Inject three mature female Xenopus laevis with 66 IU pregnant mare serum gonadotropin (PMSG) 10 days before laying eggs.</li>
	<li>Inject these Xenopus frogs with 500 IU of human chorionic gonadotropin (HCG) to induce egg laying 18 hours before the preparation of cycling extracts.</li>
	<li>Discard laid eggs overnight. Based on experiments (data not shown), extracts prepared from the freshly squeezed eggs generate longer-lasting activities than extracts prepared from laid eggs from the same female frog. Squeeze the eggs of each female frog into 100 mm Petri dishes containing 10 mL of 0.2x MMR buffer and inspect under a stereo microscope.</li>
	<li>Discard the batches of eggs that have unclear boundaries between animal and vegetal poles or have irregular white spots on top of animal poles.</li>
	<li>Choose the batch of eggs from one female frog presenting a homogeneous population of eggs with clearly differentiated animal and vegetal poles and transfer the eggs into a 600 mL beaker.</li>
	<li>Pour out excess 0.2x MMR buffer and gently add 250 mL of 2 g per 100 mL cysteine (pH 7.7) in 1x extract buffer (100 mM KCl, 0.1 mM CaCl2, 1 mM MgCl2, 10 mM HEPES, 50 mM sucrose, pH 7.7) to eggs.</li>
	<li>Shake the eggs vigorously by hand and remove the jelly coats of eggs over three washes with a total volume of 800 mL of cysteine buffer for 3 minutes or until eggs aggregate and settle at the bottom of the beaker, indicating that jelly coat removal is successful.</li>
	<li>Wash eggs with 1 L of 0.2x MMR solution over four times.</li>
	<li>Pick and discard eggs that turn white using a glass transfer pipette.</li>
	<li>Pour out the MMR buffer and supply eggs with 200 mL of 0.1 &#181;g/mL calcium ionophore A23187 solution in 0.2x MMR buffer for activation.</li>
	<li>Use the wide opening side of a glass pipette to stir the eggs gently until the eggs are activated and remove calcium ionophore solution.</li>
	<li>Check the activation efficiency after eggs settle down at the bottom of the beaker. Well-activated eggs have approximately 25% of the animal pole and 75% of the vegetal pole.</li>
	<li>Wash the activated eggs twice with 50 mL of 1x extract buffer supplemented with protease inhibitors (10 &#181;g/mL each of leupeptin, pepstatin, and chymostatin).</li>
	<li>Carefully transfer eggs to a 0.4 mL snap-cap microtube and then pack eggs through centrifugation at 200 x g for 60 seconds, and then 600 x g for 30 seconds.</li>
	<li>Remove extra buffer on the top of eggs using a glass Pasteur pipette to reduce the dilution of extracts.</li>
	<li>Crush eggs at 15,000 x g for 10 minutes at 4 &#176;C.</li>
	<li>Cut the tubes with a razor blade to separate three layers of crushed eggs and keep the crude cytoplasm in the middle layer.</li>
	<li>Transfer the crude cytoplasm into new ultracentrifuge tubes, and supplement with protease inhibitors and cytochalasin B at 10 &#181;g/mL each.</li>
	<li>Centrifuge the crude cytoplasm again at 15,000 x g at 4&#176; C for 5 minutes to further purify cytoplasm by removing excess lipid and yolk.</li>
	<li>Keep freshly prepared extracts on ice.</li>
</ol>
3. Droplet Generation
<ol>
	<li>2D glass chamber preparation 
	NOTE: The rectangle glass tubes serve as chambers that confine the droplets in a single 2-dimensional plane, allowing for easier observation and data collection.
	<ol>
		<li>Cut rectangle glass tubes with an inner dimension of 2 mm x 100 &#181;m into 4 mm long pieces. Use the sharp edge of a whetstone, mark the desired boundaries on the outer surface of the glass tube with light etchings, then gently apply pressure on both sides of the etchings to break off pieces of glass tubes.</li>
		<li>Pre-heat a dry bath incubator to 95 &#176;C. Take out the heating block and place it into a polycarbonate vacuum desiccator.</li>
		<li>Place a 1.5 mL microcentrifuge tube containing 30 &#181;L of trichloro(1H,1H,2H,2H-perfluorooctyl)silane in the heating block. Lay the cut glass tubes inside the vacuum desiccator at approximately 5 cm to the heating block.</li>
		<li>Connect the desiccator to a vacuum pump. Turn on the pump for 1 minute to reach the desired vacuum level, then close the 3-way valve to seal the desiccator and let the trichloro(1H,1H,2H,2H-perfluorooctyl)silane vapor coat the inner wall of the glass tubes. This will create a hydrophobic environment for stabilizing the droplets.</li>
		<li>Leave the glass tubes inside the desiccator for overnight coating. The tubes will be ready for use the next day.</li>
	</ol>
	</li>
	<li>Droplet generation and imaging 
	NOTE: The procedures of generating droplets and imaging are illustrated in Figures 1B and 1C.
	<ol>
		<li>Supply extracts prepared in step 2 with 10 ng/&#181;L securin-mCherry mRNA for the simple cytoplasmic-only cell cycle experiments. Alternatively, supply extracts with demembranated sperm chromatin (250 per &#181;L of extract), 10 &#181;M GFP-NLS protein, and 10 ng/&#181;L securin-mCherry mRNA to create droplets exhibiting periodic nuclei morphology changes.</li>
		<li>In a 1.5 mL microcentrifuge tube, mix 20 &#181;L of extract supplied with recombinant protein or mRNA with 200 &#181;L of 2% perfluoropolyether-polyethylene glycol (PFPE-PEG) fluorosurfactant in HFE7500 fluorinated oil.</li>
		<li>Generate droplets using a vortex mixer. The range of droplet sizes can be modulated by vortex speed and duration. To create droplets with radii ranging from 50 to 200 &#181;m, set the vortex speed at 56 x g (3,200 rpm with a radius of 4.9 mm) and gently press the microcentrifuge tube containing the extract and surfactant mixture on the mixer for 3.5 seconds. Visually inspect if the droplets are uniform in size.</li>
		<li>Use a 20 &#181;L pipette to transfer the droplets floating on top and an equal volume of PFPE-PEG surfactant into a PCR tube. Dip the glass tube prepared from step into the droplet layer in 1.5 mL microcentrifuge tube and wait until droplets have filled approximately 75% of the tube, then push the tube down into the surfactant layer until the tube is completely filled. This will lower the droplet density and allow the droplets to form one single layer inside the chamber without overlapping or shape distortion caused by overcrowding.</li>
		<li>Fill a glass bottom dish with a diameter of 40 mm with mineral oil. Immerse the droplet-filled tubes in oil by gently pushing them down using a fine tweezer.</li>
		<li>After loading all samples, use a glass pipette to remove any visible debris or leaked droplets. Add more mineral oil if the tubes are not or will not be completely immersed in the imaging process.</li>
		<li>Conduct imaging of droplets on an epifluorescence microscope equipped with a motorized x-y stage (see Table of Materials). Use a 4X air objective for all imaging. For fluorescence imaging, use a 130 W mercury vapor short arc lamp as fluorescence light source for illumination, and a GFP filter set (excitation 482/35 nm and emission 514/LP nm) and an mCherry filter set (excitation 562/40 nm and emission 641/75 nm) for imaging GFP-NLS and securin-mCherry signals.</li>
		<li>Record time-lapse videos in the bright field and multiple fluorescence channels every 6 to 9 minutes until the droplets lose their activity.</li>
	</ol>
	</li>
</ol>
4. Image Processing and Data Analysis
<ol>
	<li>Segmentation and tracking of droplets
	<ol>
		<li>Import recorded data in the format of &#34;.tif&#34; or &#34;.oif&#34; into image analysis software (see Table of Materials).</li>
		<li>Segment single droplets using the bright field channel. The bright field image will show droplets as bright circles with dark boundaries. Apply thresholding to the image. Add a tracking surface to each droplet by tuning the threshold between dark and bright pixels so that each surface covers the whole area of the corresponding droplet.</li>
		<li>Automatically track the segments between adjacent frames using the autoregressive motion algorithm. Tune the acceptable maximum traveling distance of a droplet between two adjacent frames to be smaller than the radius of most droplets to accurately track small droplets.</li>
		<li>Visually check all tracks across the entire video to detect incorrect segmentations that segment the empty space between droplets instead of actual droplets. Manually delete incorrect tracks.</li>
		<li>Segment and track background area without any droplets to obtain the background fluorescence intensity. To improve signal to noise, subtract background intensity from the fluorescence intensity of droplets at each time frame.</li>
		<li>Export measured parameters for each track over time, including the mean and standard deviation of the fluorescence intensity and the droplet area into &#34;.csv&#34; files.</li>
	</ol>
	</li>
	<li>Data analysis of droplet size and oscillation period
	<ol>
		<li>Load the &#34;.csv&#34; files into R to create plots of fluorescence intensity over time for each droplet. Record droplet ID into a &#34;.csv&#34; file.</li>
		<li>Import the &#34;.csv&#34; files exported from the analysis software and the &#34;.csv&#34; file with a list of droplet ID into Matlab and save as &#34;.mat&#34; files for further data analysis.</li>
		<li>Calculate the volume of a compressed droplet based on the exported droplet area information after segmentation and the height of glass chamber19. Use the volume to calculate the radius of droplet as a sphere.</li>
		<li>Extract the time points of peaks and troughs using peak/trough detection algorithm. Perform manual corrections on the position of peaks and troughs to ensure they are accurately detected.</li>
		<li>To calculate the cell-cycle period, calculate the interval time between two adjacent peaks. Take the average of all intervals for each droplet as its cell-cycle period.</li>
	</ol>
	</li>
</ol>

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M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Isolating+Xenopus+laevis+Testes.">Isolating Xenopus laevis Testes.</a> Cold Spring Harbor Protocols. 2007, (2007).</li><li>Showell, C., Conlon, F. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Egg+Collection+and+In+vitro+Fertilization+of+the+Western+Clawed+Frog+Xenopus+tropicalis.">Egg Collection and In vitro Fertilization of the Western Clawed Frog Xenopus tropicalis.</a> Cold Spring Harbor Protocols. 2009, (2009).</li><li>Wilson, C. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Methods in Enzymology. 91, 236-247 (1983).</li><li>Schutze, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+streamlined+protocol+for+emulsion+polymerase+chain+reaction+and+subsequent+purification.">A streamlined protocol for emulsion polymerase chain reaction and subsequent purification.</a> Analytical Biochemistry. 410, 155-157 (2011).</li><li>Weitz, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diversity+in+the+dynamical+behaviour+of+a+compartmentalized+programmable+biochemical+oscillator.">Diversity in the dynamical behaviour of a compartmentalized programmable biochemical oscillator.</a> Nature Chemistry. 6, 295-302 (2014).</li><li>Ho, K. K., Lee, J. W., Durand, G., Majumder, S., Liu, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Protein+aggregation+with+poly(vinyl)+alcohol+surfactant+reduces+double+emulsion-encapsulated+mammalian+cell-free+expression.">Protein aggregation with poly(vinyl) alcohol surfactant reduces double emulsion-encapsulated mammalian cell-free expression.</a> PloS One. 12, e0174689 (2017).</li><li>Nakajima, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reconstitution+of+circadian+oscillation+of+cyanobacterial+KaiC+phosphorylation+in+vitro.">Reconstitution of circadian oscillation of cyanobacterial KaiC phosphorylation in vitro.</a> Science. 308, 414-415 (2005).</li><li>Guan, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+robust+and+tunable+mitotic+oscillator+in+artificial+cells.">A robust and tunable mitotic oscillator in artificial cells.</a> eLife. 7, (2018).</li></ol>

We have nothing to disclose.

We have presented a novel method for developing a high-throughput artificial cell system that enables in vitro reconstitution and long-term tracking of self-sustained cell-cycle oscillations at the single-cell level. There are several critical steps that make this method successful. First, freshly squeezed Xenopus eggs with a good quality, compared with laid eggs, tend to produce extracts with longer-lasting oscillation activity. Second, encapsulation of extracts within the surfactant-stabilized microen...

Cytoplasmic extracts prepared from Xenopus laevis eggs represent one of the most predominant models for the biochemical study of cell cycles, given the large volume of oocytes, the rapid cell cycle progression, and the capability of reconstituting mitotic events in vitro1,2. This system has allowed the initial discovery and mechanistic characterization of essential cell-cycle regulators like maturation-promoting factor (MPF) as well as downstream mitotic processes including spindle assembly and chromosome segregation1,2,3,4,5,6,7,8,9,10,11. The Xenopus egg extracts have also been used for detailed dissection of the regulatory networks of the cell cycle clock8,12,13,14 and for studies of the DNA damage/replication checkpoint15 and the mitotic spindle assembly checkpoint16,17,18.
These studies of cell cycles using the Xenopus egg extracts have mainly been based on bulk measurements. However, conventional bulk reaction assays may not mimic real cell behaviors, given a major discrepancy in their dimensions and subcellular spatial compartmentalization of reaction molecules. Moreover, bulk measurements of mitotic activities are prone to giving a limited number of cycles before quickly damping8. These disadvantages of bulk reactions have prevented the extract system to provide further understanding of complex clock dynamic properties and functions. Recent studies have encapsulated cell-free cytostatic factor-arrested (CSF) Xenopus extracts19,20 into size-defined cell-like compartments, which have helped elucidate how spindle size is modulated by the cytoplasmic volume. However, this in vitro system is arrested at metaphase of meiosis II by the action of cytostatic factor1, and a system capable of long-term sustained oscillations at the single-cell level is needed for further investigation of the cell cycle oscillator.
To study cell cycle oscillations with single-cell resolution, we have developed a cell-scale, high-throughput system for reconstitution and simultaneous measurement of multiple self-sustained mitotic oscillatory processes in individual microemulsion droplets. In this detailed video protocol, we demonstrate the creation of the artificial mitotic oscillation system by encapsulating cycling Xenopus laevis egg cytoplasm in microemulsions of sizes ranging from 10 to 300 &#181;m. In this system, mitotic oscillations including chromosome condensation and de-condensation, nuclear envelope breakdown and reformation, and the degradation and synthesis of anaphase substrates (e.g., securin-mCherry in this protocol) were successfully reconstituted.

<table><tbody><tr><td>&lt;em&gt;Xenopus laevis&lt;/em&gt; frogs</td><td>Xenopus-I Inc.</td><td></td><td></td></tr><tr><td>QIAprep spin miniprep kit</td><td>QIAGEN</td><td>27104</td><td></td></tr><tr><td>QIAquick PCR Purification Kit (250)</td><td>QIAGEN</td><td>28106</td><td></td></tr><tr><td>mMESSAGE mMACHINE SP6 Transcription Kit</td><td>Ambion</td><td>AM1340</td><td></td></tr><tr><td>BL21 (DE3)-T-1 competent cell</td><td>Sigma-Aldrich</td><td>B2935</td><td></td></tr><tr><td>Calcium ionophore</td><td>Sigma-Aldrich</td><td>A23187</td><td></td></tr><tr><td>Hoechst 33342</td><td>Sigma-Aldrich</td><td>B2261</td><td>Toxic</td></tr><tr><td>Trichloro</td><td>Sigma-Aldrich</td><td>448931</td><td>Toxic</td></tr><tr><td>(1H,1H,2H,2H-perfluorooctyl) silane</td><td></td><td></td><td></td></tr><tr><td>PFPE-PEG surfactant</td><td>Ran Biotechnologies</td><td>008-FluoroSurfactant-2wtH-50G</td><td></td></tr><tr><td>GE Healthcare Glutathione Sepharose 4B beads</td><td>Sigma-Aldrich</td><td>GE17-0756-01</td><td></td></tr><tr><td>PD-10 column</td><td>Sigma-Aldrich</td><td>GE17-0851-01</td><td></td></tr><tr><td>VitroCom miniature hollow glass tubing</td><td>VitroCom</td><td>5012</td><td></td></tr><tr><td>Olympus SZ61 Stereo Microscope</td><td>Olympus</td><td></td><td></td></tr><tr><td>Olympus IX83 microscope</td><td>Olympus</td><td></td><td></td></tr><tr><td>Olympus FV1200 confocal microscope</td><td>Olympus</td><td></td><td></td></tr><tr><td>NanoDrop spectrophotometer</td><td>Thermofisher</td><td>ND-2000</td><td></td></tr><tr><td>0.4 mL Snap-Cap Microtubes</td><td>E&amp;amp;K Scientific</td><td>485050-B</td><td></td></tr><tr><td>PureLink RNA Mini Kit</td><td>ThermoFisher (Ambion)</td><td>12183018A</td><td></td></tr><tr><td>Fisherbrand Analog Vortex Mixer</td><td>Fisher Scientific</td><td>2215365</td><td></td></tr><tr><td>Imaris</td><td>Bitplane</td><td>Version 7.3</td><td>Image analysis software</td></tr></tbody></table>

reconstitution of cell-cycle oscillations in microemulsions of cell-free xenopus egg extracts

Real-time measurement of oscillations at the single-cell level is important to uncover the mechanisms of biological clocks. Although bulk extracts prepared from Xenopus laevis eggs have been powerful in dissecting biochemical networks underlying the cell-cycle progression, their ensemble average measurement typically leads to a damped oscillation, despite each individual oscillator being sustained. This is due to the difficulty of perfect synchronization among individual oscillators in noisy biological systems. To retrieve the single-cell dynamics of the oscillator, we developed a droplet-based artificial cell system that can reconstitute mitotic cycles in cell-like compartments encapsulating cycling cytoplasmic extracts of Xenopus laevis eggs. These simple cytoplasmic-only cells exhibit sustained oscillations for over 30 cycles. To build more complicated cells with nuclei, we added demembranated sperm chromatin to trigger nuclei self-assembly in the system. We observed a periodic progression of chromosome condensation/decondensation and nuclei envelop breakdown/reformation, like in real cells. This indicates that the mitotic oscillator functions faithfully to drive multiple downstream mitotic events. We simultaneously tracked the dynamics of the mitotic oscillator and downstream processes in individual droplets using multi-channel time-lapse fluorescence microscopy. The artificial cell-cycle system provides a high-throughput framework for quantitative manipulation and analysis of mitotic oscillations with single-cell resolution, which likely provides important insights into the regulatory machinery and functions of the clock.

In Figure 2, we show that this protocol produces mitotic oscillations in both simple, nuclear-free cells as well as complicated cells with nuclei, where the oscillator drives the cyclic alternation of nuclei formation and deformation. The nuclei-free droplets generate mitotic oscillations up to 30 undamped cycles over the time span of 92 hours, as indicated by the periodic synthesis and degradation of a fluorescence reporter securin-mCherry (<strong class="xf...

Watch this Scientific Journal Video about Reconstitution of Cell-cycle Oscillations in Microemulsions of Cell-free Xenopus Egg Extracts at JoVE.com

Reconstitution of Cell-cycle Oscillations in Microemulsions of Cell-free Xenopus Egg Extracts

We present a method for the generation of in vitro self-sustained mitotic oscillations at the single-cell level by encapsulating egg extracts of Xenopus laevis in water-in-oil microemulsions.

Reconstitution of Cell-cycle Oscillations in Microemulsions of Cell-free Xenopus Egg Extracts

Real-time measurement of oscillations at the single-cell level is important to uncover the mechanisms of biological clocks. Although bulk extracts ...

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Research

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Biochemistry

8.1K Views.  University of Michigan, Ann Arbor. We present a method for the generation of in vitro self-sustained mitotic oscillations at the single-cell level by encapsulating egg extracts of Xenopus laevis in water-in-oil microemulsions.

Video: Reconstitution of Cell-cycle Oscillations in Microemulsions of Cell-free Xenopus Egg Extracts

Manipulation and In Vitro Maturation of Xenopus laevis Oocytes, Followed by Intracytoplasmic Sperm Injection, to Study Embryonic Development

Amphibian eggs have been widely used to study embryonic development. Early embryonic development is driven by maternally stored factors accumulated during oogenesis. In order to study roles of such maternal factors in early embryonic development, it is desirable to manipulate their functions from the very beginning of embryonic development. Conventional ways of gene interference are achieved by injection of antisense oligonucleotides (oligos) or mRNA into fertilized eggs, enabling under- or over-expression of specific proteins, respectively. However, these methods normally require more than several hours until protein expression is affected, and, hence, the interference of gene functions is not effective during early embryonic stages. Here, we introduce an experimental system in which expression levels of maternal proteins can be altered before fertilization. Xenopus laevis oocytes obtained from ovaries are defolliculated by incubating with enzymes. Antisense oligos or mRNAs are injected into defolliculated oocytes at the germinal vesicle (GV) stage. These oocytes are in vitro matured to eggs at the metaphase II (MII) stage, followed by intracytoplasmic sperm injection (ICSI). By this way, up to 10% of ICSI embryos can reach the swimming tadpole stage, thus allowing functional tests of specific gene knockdown or overexpression. This approach can be a useful way to study roles of maternally stored factors in early embryonic development.

Manipulation and In Vitro Maturation of Xenopus laevis Oocytes, Followed by Intracytoplasmic Sperm Injection, to Study Embryonic Development

We describe methods of manipulating Xenopus laevis immature oocytes, in vitro maturation of oocytes to eggs, and intracytoplasmic sperm injection. This protocol allows degradation of some maternal proteins and overexpression of genes of interest at fertilization, and hence is valuable to study roles of specific factors in early embryonic development.

Amphibian eggs have been widely used to study embryonic development. Early embryonic development is driven by maternally stored factors accumulated during ...

Developmental Biology

Functional Cloning Using a Xenopus Oocyte Expression System

Identification of genes responsible for embryonic induction poses a number of challenges; to name a few, secreted molecules of interest may be low in abundance, may not be secreted but tethered to the signaling cell(s), or may require the presence of binding partners or upstream regulatory molecules. Thus in a search for gene products capable of eliciting an early lens-inductive response in competent ectoderm, we utilized an expression cloning system that would allow identification of paracrine or juxtacrine factors as well as transcriptional or other regulatory proteins. Pools of mRNA were injected into Xenopus oocytes, and responding tissue placed directly on the oocytes and co-cultured. Following functional cloning of ldb1 from a neural plate stage cDNA library based on its ability to elicit the expression of the early lens placode marker foxe3 in lens-competent animal cap ectoderm, we characterized the mRNA expression pattern, and assayed developmental progression following overexpression or knockdown of ldb1. This system is suitable in a very wide variety of contexts where identification of an inducer or its upstream regulatory molecules is sought using a functional response in competent tissue.

Functional Cloning Using a Xenopus Oocyte Expression System

We describe a Xenopus oocyte and animal cap system for the expression cloning of genes capable of inducing a response in competent ectoderm, and discuss techniques for the subsequent analysis of such genes. This system is useful in the functional identification of a wide range of gene products.

Identification of genes responsible for embryonic induction poses a number of challenges; to name a few, secreted molecules of interest may be low in ...

Preparation and Fractionation of Xenopus laevis Egg Extracts

Crude and fractionated Xenopus egg extracts can be used to provide ingredients for reconstituting cellular processes for morphological and biochemical analysis. Egg lysis and differential centrifugation are used to prepare the crude extract which in turn in used to prepare fractionated extracts and light membrane preparations.

Crude and fractionated Xenopus egg extracts can be used to provide ingredients for reconstituting cellular processes for morphological and biochemical analysis. Egg lysis and differential centrifugation are used to prepare the crude extract which in turn in used to prepare fractionated extracts and light membrane preparations.

Crude and fractionated Xenopus egg extracts can be used to provide ingredients for reconstituting cellular processes for morphological and biochemical ...

Biology

Live-cell Imaging and Quantitative Analysis of Embryonic Epithelial Cells in Xenopus laevis

Embryonic epithelial cells serve as an ideal model to study morphogenesis where multi-cellular tissues undergo changes in their geometry, such as changes in cell surface area and cell height, and where cells undergo mitosis and migrate. Furthermore, epithelial cells can also regulate morphogenetic movements in adjacent tissues1. A traditional method to study epithelial cells and tissues involve chemical fixation and histological methods to determine cell morphology or localization of particular proteins of interest. These approaches continue to be useful and provide "snapshots" of cell shapes and tissue architecture, however, much remains to be understood about how cells acquire specific shapes, how various proteins move or localize to specific positions, and what paths cells follow toward their final differentiated fate. High resolution live imaging complements traditional methods and also allows more direct investigation into the dynamic cellular processes involved in the formation, maintenance, and morphogenesis of multicellular epithelial sheets. Here we demonstrate experimental methods from the isolation of animal cap tissues from Xenopus laevis embryos to confocal imaging of epithelial cells and simple measurement approaches that together can augment molecular and cellular studies of epithelial morphogenesis.

Live-cell Imaging and Quantitative Analysis of Embryonic Epithelial Cells in Xenopus laevis

Xenopus embryonic epithelia are an ideal model system to study cell behaviors such as polarity development and shape change during epithelial morphogenesis. Traditional histology of fixed samples is increasingly being complemented by live-cell confocal imaging. Here we demonstrate methods to isolate frog tissues and visualize live epithelial cells and their cytoskeleton using live-cell confocal microscopy.

Embryonic epithelial cells serve as an ideal model to study morphogenesis where multi-cellular tissues undergo changes in their geometry, such as changes ...

A Microfluidics Approach for the Functional Investigation of Signaling Oscillations Governing Somitogenesis

Periodic segmentation of the presomitic mesoderm of a developing mouse embryo is controlled by a network of signaling pathways. Signaling oscillations and gradients are thought to control the timing and spacing of segment formation, respectively. While the involved signaling pathways have been studied extensively over the last decades, direct evidence for the function of signaling oscillations in controlling somitogenesis has been lacking. To enable the functional investigation of signaling dynamics, microfluidics is a previously established tool for the subtle modulation of these dynamics. With this microfluidics-based entrainment approach endogenous signaling oscillations are synchronized by pulses of pathway modulators. This enables modulation of, for instance, the oscillation period or the phase-relationship between two oscillating pathways. Furthermore, spatial gradients of pathway modulators can be established along the tissue to study how specific changes in the signaling gradients affect somitogenesis.
The present protocol is meant to help establish microfluidic approaches for the first-time users of microfluidics. The basic principles and equipment needed to set up a microfluidic system are described, and a chip design is provided, with which a mold for chip generation can conveniently be prepared using a 3D printer. Finally, how to culture primary mouse tissue on a microfluidic chip and how to entrain signaling oscillations to external pulses of pathway modulators are discussed.
This microfluidic system can also be adapted to harbor other in vivo and in vitro model systems such as gastruloids and organoids for functional investigation of signaling dynamics and morphogen gradients in other contexts.

A protocol for the culture and manipulation of mouse embryonic tissue on a microfluidic chip is provided. By applying pulses of pathway modulators, this system can be used to externally control signaling oscillations for the functional investigation of mouse somitogenesis.

Periodic segmentation of the presomitic mesoderm of a developing mouse embryo is controlled by a network of signaling pathways. Signaling oscillations and ...

Production of Xenopus tropicalis Egg Extracts to Identify Microtubule-associated RNAs

Many organisms localize mRNAs to specific subcellular destinations to spatially and temporally control gene expression. Recent studies have demonstrated that the majority of the transcriptome is localized to a nonrandom position in cells and embryos. One approach to identify localized mRNAs is to biochemically purify a cellular structure of interest and to identify all associated transcripts. Using recently developed high-throughput sequencing technologies it is now straightforward to identify all RNAs associated with a subcellular structure. To facilitate transcript identification it is necessary to work with an organism with a fully sequenced genome. One attractive system for the biochemical purification of subcellular structures are egg extracts produced from the frog Xenopus laevis. However, X. laevis currently does not have a fully sequenced genome, which hampers transcript identification. In this article we describe a method to produce egg extracts from a related frog, X. tropicalis, that has a fully sequenced genome. We provide details for microtubule polymerization, purification and transcript isolation. While this article describes a specific method for identification of microtubule-associated transcripts, we believe that it will be easily applied to other subcellular structures and will provide a powerful method for identification of localized RNAs.

Production of Xenopus tropicalis Egg Extracts to Identify Microtubule-associated RNAs

We describe the collection of unfertilized Xenopus tropicalis eggs and production of a meiosis II-arrested egg extract. This egg extract can be used to purify microtubules and microtubule-associated RNAs.

Many  organisms localize mRNAs to specific subcellular destinations to spatially and  temporally control gene expression. Recent studies have demonstrated ...

Reconstitution Of &#946;-catenin Degradation In Xenopus Egg Extract

Xenopus laevis egg extract is a well-characterized, robust system for studying the biochemistry of diverse cellular processes. Xenopus egg extract has been used to study protein turnover in many cellular contexts, including the cell cycle and signal transduction pathways1-3. Herein, a method is described for isolating Xenopus egg extract that has been optimized to promote the degradation of the critical Wnt pathway component, &beta;-catenin. Two different methods are described to assess &beta;-catenin protein degradation in Xenopus egg extract. One method is visually informative ([35S]-radiolabeled proteins), while the other is more readily scaled for high-throughput assays (firefly luciferase-tagged fusion proteins). The techniques described can be used to, but are not limited to, assess &beta;-catenin protein turnover and identify molecular components contributing to its turnover. Additionally, the ability to purify large volumes of homogenous Xenopus egg extract combined with the quantitative and facile readout of luciferase-tagged proteins allows this system to be easily adapted for high-throughput screening for modulators of &beta;-catenin degradation.

Reconstitution Of &#946;-catenin Degradation In Xenopus Egg Extract

A method is described for analyzing protein degradation using radiolabeled and luciferase-fusion proteins in Xenopus egg extract and its adaptation for high-throughput screening for small molecule modulators of protein degradation.

Xenopus laevis egg extract is a well-characterized, robust system for studying the biochemistry of diverse cellular processes. Xenopus egg extract has ...

Xenopus laevis as a Model to Identify Translation Impairment

Protein synthesis is a fundamental process to gene expression impacting diverse biological processes notably adaptation to environmental conditions. The initiation step, which involves the assembly of the ribosomal subunits at the mRNA initiation codon, involved initiation factor including eIF4G1. Defects in this rate limiting step of translation are linked to diverse disorders. To study the potential consequences of such deregulations, Xenopus laevis oocytes constitute an attractive model with high degrees of conservation of essential cellular and molecular mechanisms with human. In addition, during meiotic maturation, oocytes are transcriptionally repressed and all necessary proteins are translated from preexisting, maternally derived mRNAs. This inexpensive model enables exogenous mRNA to become perfectly integrated with an effective translation. Here is described a protocol for assessing translation with a factor of interest (here eIF4G1) using stored maternal mRNA that are the first to be polyadenylated and translated during oocyte maturation as a physiological readout. At first, mRNA synthetized by in vitro transcription of plasmids of interest (here eIF4G1) are injected in oocytes and kinetics of oocyte maturation by Germinal Vesicle Breakdown detection is determined. The studied maternal mRNA target is the serine/threonine-protein-kinase mos. Its polyadenylation and its subsequent translation are investigated together with the expression and phosphorylation of proteins of the mos signaling cascade involved in oocyte maturation. Variations of the current protocol to put forward translational defects are also proposed to emphasize its general applicability. In light of emerging evidence that aberrant protein synthesis may be involved in the pathogenesis of neurological disorders, such a model provides the opportunity to easily assess this impairment and identify new targets.

Xenopus laevis as a Model to Identify Translation Impairment

Protein synthesis control occurs mainly at the translation initiation step, deficiencies in which are linked to diverse disorders. To better understand their etiology, we described here a protocol using Xenopus laevis oocytes assessing the translation of mos transcript in the presence of a mutant of translation initiation factor eIF4G1.

Protein synthesis is a fundamental process to gene expression impacting diverse biological processes notably adaptation to environmental conditions. The ...

A Cell Free Assay to Study Chromatin Decondensation at the End of Mitosis

During the vertebrate cell cycle chromatin undergoes extensive structural and functional changes. Upon mitotic entry, it massively condenses into rod shaped chromosomes which are moved individually by the mitotic spindle apparatus. Mitotic chromatin condensation yields chromosomes compacted fifty-fold denser as in interphase. During exit from mitosis, chromosomes have to re-establish their functional interphase state, which is enclosed by a nuclear envelope and is competent for replication and transcription. The decondensation process is morphologically well described, but in molecular terms poorly understood: We lack knowledge about the underlying molecular events and to a large extent the factors involved as well as their regulation. We describe here a cell-free system that faithfully recapitulates chromatin decondensation in vitro, based on mitotic chromatin clusters purified from synchronized HeLa cells and X. laevis egg extract. Our cell-free system provides an important tool for further molecular characterization of chromatin decondensation and its co-ordination with processes simultaneously occurring during mitotic exit such as nuclear envelope and pore complex re-assembly.

The molecular mechanisms of the decondensation of highly compacted mitotic chromatin are ill-defined. We present a cell-free assay based on mitotic chromatin clusters isolated from HeLa cells and Xenopus laevis egg extract that faithfully reconstitutes the decondensation process in vitro.

During the vertebrate cell cycle chromatin undergoes extensive structural and functional changes. Upon mitotic entry, it massively condenses into rod ...

A Cell-Free Assay Using Xenopus laevis Embryo Extracts to Study Mechanisms of Nuclear Size Regulation

A fundamental question in cell biology is how cell and organelle sizes are regulated. It has long been recognized that the size of the nucleus generally scales with the size of the cell, notably during embryogenesis when dramatic reductions in both cell and nuclear sizes occur. Mechanisms of nuclear size regulation are largely unknown and may be relevant to cancer where altered nuclear size is a key diagnostic and prognostic parameter. In vivo approaches to identifying nuclear size regulators are complicated by the essential and complex nature of nuclear function. The in vitro approach described here to study nuclear size control takes advantage of the normal reductions in nuclear size that occur during Xenopus laevis development. First, nuclei are assembled in X. laevis egg extract. Then, these nuclei are isolated and resuspended in cytoplasm from late stage embryos. After a 30 - 90 min incubation period, nuclear surface area decreases by 20 - 60%, providing a useful assay to identify cytoplasmic components present in late stage embryos that contribute to developmental nuclear size scaling. A major advantage of this approach is the relative facility with which the egg and embryo extracts can be biochemically manipulated, allowing for the identification of novel proteins and activities that regulate nuclear size. As with any in vitro approach, validation of results in an in vivo system is important, and microinjection of X. laevis embryos is particularly appropriate for these studies.

A Cell-Free Assay Using Xenopus laevis Embryo Extracts to Study Mechanisms of Nuclear Size Regulation

Mechanisms of cellular and intra-cellular scaling remain elusive. The use of Xenopus embryo extracts has become increasingly common to elucidate mechanisms of organelle size regulation. This method describes embryo extract preparation and a novel nuclear scaling assay through which mechanisms of nuclear size regulation can be identified.

A fundamental question in cell biology is how cell and organelle sizes are regulated. It has long been recognized that the size of the nucleus generally ...

Reconstitution of Cell-cycle Oscillations in Microemulsions of Cell-free Xenopus Egg Extracts

Summary

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Chapters in this video

Manipulation and In Vitro Maturation of Xenopus laevis Oocytes, Followed by Intracytoplasmic Sperm Injection, to Study Embryonic Development

Functional Cloning Using a Xenopus Oocyte Expression System

Preparation and Fractionation of Xenopus laevis Egg Extracts

Live-cell Imaging and Quantitative Analysis of Embryonic Epithelial Cells in Xenopus laevis

A Microfluidics Approach for the Functional Investigation of Signaling Oscillations Governing Somitogenesis

Production of Xenopus tropicalis Egg Extracts to Identify Microtubule-associated RNAs

Reconstitution Of β-catenin Degradation In Xenopus Egg Extract

Xenopus laevis as a Model to Identify Translation Impairment

A Cell Free Assay to Study Chromatin Decondensation at the End of Mitosis

A Cell-Free Assay Using Xenopus laevis Embryo Extracts to Study Mechanisms of Nuclear Size Regulation