Name: microprobe capillary electrophoresis mass spectrometry for single-cell metabolomics in live frog (xenopus laevis) embryos
Uploaded: 2017-12-22T15:00:00
Description: Watch this Scientific Journal Video about Microprobe Capillary Electrophoresis Mass Spectrometry for Single-cell Metabolomics in Live Frog Xenopus laevis Embryos at JoVE.com

This work was supported by National Institutes of Health Grants GM114854 (to P.N.) and CA211635 (to P.N.), the Arnold and Mabel Beckman Foundation Beckman Young Investigator grant (to P.N.), the DuPont Young Professor award (to P.N.), the American Society for Mass Spectrometry Research Award (to P.N.), and COSMOS Club Foundation fellowships (to R.M.O. and E.P.P.). The opinions and conclusions expressed in this publication are solely those of the authors and do not necessarily represent the official views of the funding sources.

All protocols related to the maintenance and handling of Xenopus laevis were approved by the Institutional Animal Care and Use Committee at the George Washington University (IACUC no. A311).
1. Preparation of Sampling Instruments, Media, Solvents, and Sampling Dishes
<ol>
	<li>Prepare 1x Steinberg&#39;s solution (SS) by dissolving the following salts in ultrapure water (~18.2 M&#937;.cm at 25 &#176;C) in the following order and at the indicated concentrations following a standard pr...

Microprobe CE-ESI-MS enables the direct characterization of metabolites in single cells in live, freely developing embryos. At the heart of the approach are two technical subcomponents, namely in situ capillary microsampling and high-sensitivity CE-ESI-MS. Compared to whole-cell dissection, capillary microsampling has the advantage of fast operation (few seconds vs. 5 min/cell by dissection), compatibility with the complex three-dimensional morphology of embryos, and scalability to smaller cells that form at lat...

A comprehensive understanding of embryonic development requires characterization of all molecular changes that unfold in every cell of the developing organism. While Next-Generation Sequencing with molecular amplification enables deep measurement of single-cell transcriptomes1 in developing systems2,3, considerably less is known about the suite of smaller molecules produced in single embryonic cells, including proteins and, especially, metabolites (molecular mass &#60;~1,500 Da). With a fast and dynamic response to intrinsic and extrinsic events, the metabolome serves as a powerful descriptor of a cell&#39;s molecular state. The single-cell metabolome, therefore, raises the potential to track the spatial and temporal development of cell heterogeneity in the early embryo and to identify new molecules for functional studies. However, without molecular amplification available for these molecules, detection of the metabolome demands exceptional sensitivity using mass spectrometry (MS), which is the technology of choice for metabolite analysis.
Single-cell MS is a collection of technologies with sufficient sensitivity to measure metabolites in single cells (see reviews 4,5,6,7,8,9,10,11,12,13,14,15). Reproducible sampling of cells and efficient extraction of metabolites are essential to the successful detection of metabolites in single cells. Whole-cell dissection of identified cells from Xenopus embryos has enabled the characterization of small molecules and peptides16. Other approaches employ micropipettes to sample individual live cells followed by detection using electrospray ionization (ESI) MS. For example, metabolites were measured in plant or mammalian cells by single-cell video MS17, pressure probe18, single probe19, and fluidic force microscopy20, among other techniques21,22,23,24. Additionally, incorporation of chemical separation prior to ionization into the single-cell MS workflow efficiently simplifies the metabolome, thus alleviating potential interferences during ion generation before detection. Importantly, separation also provides compound-specific information to assist in molecular identifications. Capillary electrophoresis (CE) has been used to detect metabolites in single dissected25,26 or microsampled27neurons, capturing small-molecule differences between neuron phenotypes. We recently adapted CE to ESI tandem MS to enable the trace-level detection of hundreds of metabolites in individual cells that were dissected from early embryos of Xenopus laevis16,28. These studies revealed surprising metabolic differences between embryonic cells at an early stage of development and led to the discovery of metabolites with previously unknown developmental impacts16.
Here we provide a protocol that enabled the detection of metabolites in single cells directly in a live vertebrate embryo using microprobe single-cell CE-ESI-MS29,30. The model organism chosen is the 8-to-32-cell X. laevis embryo, although the approach is also applicable to later stages of development and other types of model organisms. This protocol uses sharpened capillaries with multi-axis translational control under guidance by a high-resolution imaging system to aspirate an ~10 nL portion of identified cells in situ in the morphologically complex developing embryo. This microprobe is scalable to smaller cells and operates within seconds, which is sufficiently fast to track cell lineages in the embryo. After extracting polar or apolar small molecules, such as metabolites and peptides, from the collected sample in ~4-5 &#181;L extraction solution, a ~10 nL of the resulting extract is analyzed in a custom-built CE platform hyphenated to an ESI mass spectrometer. Construction and operation of the CE-ESI-MS platform builds on protocols described elsewhere.31,32 The co-axial CE-ESI interface is constructed as described elsewhere.31 This platform is maintained in the cone-jet spraying regime to achieve trace-level sensitivity with a capability for quantification over a 4-5 log-order dynamic range (relative28,29,30 or absolute16). The CE-ESI-MS platform offers a 60-amol lower limit of detection with 8% relative standard deviation (RSD) in quantitation over a tested range of 10 nM to 1 &#181;M for small molecules16, which are sufficient to characterize endogenous metabolites in X. laevis cells. Microprobed cells continue to divide as the embryo progresses through development30, allowing for temporally and spatially resolved analysis of cellular metabolism. Indeed, single-cell CE-ESI-MS can be used to find metabolic differences between cells that occupy the dorsal-ventral16,29, animal-vegetal16, and left-right28 developmental axes as well as cells that form the neural-tissue fated dorsal lineage from a common progenitor cell in X. laevis30. Besides querying metabolic differences between individual embryonic cells at different developmental stages of the X. laevis embryo30, we anticipate that the protocols described here are applicable to a broad array of biomolecules and single cells microsampled from different stages of embryonic development as well as other types of cells and model organisms. Additionally, the microprobe could be used for microsampling while a different platform compatible with miniscule samples could be used for separation and/or characterization of biomolecules.

<table>
	<tbody>
		<tr>
			<td>Reagents for Embryo Culture Media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Potasium chloride</td>
			<td>Fisher Scientific</td>
			<td>BP 366-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnesium sulfate</td>
			<td>Fisher Scientific</td>
			<td>M 65-3</td>
			<td></td>
		</tr>
		<tr>
			<td>Calcium nitrate</td>
			<td>Sigma Aldrich</td>
			<td>C1396</td>
			<td></td>
		</tr>
		<tr>
			<td>Cysteine</td>
			<td>MP Biomedicals</td>
			<td>101444</td>
			<td></td>
		</tr>
		<tr>
			<td>Trizma hydrochloride</td>
			<td>Sigma Aldrich</td>
			<td>T3253</td>
			<td></td>
		</tr>
		<tr>
			<td>Trizma base</td>
			<td>Sigma Aldrich</td>
			<td>T1503</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium chloride</td>
			<td>Fisher Scientific</td>
			<td>5641-212</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Metabolite Extraction Solvents</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Acetonitrile (LC-MS-grade)</td>
			<td>Fisher Scientific</td>
			<td>A955</td>
			<td></td>
		</tr>
		<tr>
			<td>Methanol (LC-MS-grade)</td>
			<td>Fisher Scientific</td>
			<td>A456</td>
			<td></td>
		</tr>
		<tr>
			<td>Water (LC-MS-grade)</td>
			<td>Fisher Scientific</td>
			<td>W6</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Solvents and Standards for CE-ESI-MS</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Formic acid (LC-MS-grade)</td>
			<td>Fisher Scientific</td>
			<td>A11710X1-AMP</td>
			<td></td>
		</tr>
		<tr>
			<td>Methanol (LC-MS-grade)</td>
			<td>Fisher Scientific</td>
			<td>A456-4</td>
			<td></td>
		</tr>
		<tr>
			<td>Water (LC-MS-grade)</td>
			<td>Fisher Scientific</td>
			<td>W6</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium chloride</td>
			<td>Fisher Scientific</td>
			<td>5641-212</td>
			<td></td>
		</tr>
		<tr>
			<td>Acetylcholine chloride</td>
			<td>Acros Organics</td>
			<td>159170050</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Microprobe Fabrication Setup</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Micropippette puller</td>
			<td>Sutter Instrument Co.</td>
			<td>P-1000</td>
			<td></td>
		</tr>
		<tr>
			<td>Borosilicate capillaries</td>
			<td>Sutter Instrument Co.</td>
			<td>B100-50-10</td>
			<td></td>
		</tr>
		<tr>
			<td>Fine sharp forceps: Dumont #5, Biologie/Dumoxel</td>
			<td>Fine Science Tools (USA) Inc</td>
			<td>11252-30</td>
			<td>Corrosion resitant and autoclavable.</td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Microprobe Sampling Setup</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Micromanipulator</td>
			<td>Eppendorf, Hauppauge, NY</td>
			<td>TransferMan 4r</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereomicroscope</td>
			<td>Nikon</td>
			<td>SMZ18</td>
			<td>Should be vibrationally isolated.</td>
		</tr>
		<tr>
			<td>Illuminator e.g. Goosenecks</td>
			<td>Nikon</td>
			<td>C-FLED2</td>
			<td></td>
		</tr>
		<tr>
			<td>Microinjector</td>
			<td>Warner Instrument, Handem, CT</td>
			<td>PLI-100A</td>
			<td></td>
		</tr>
		<tr>
			<td>Transfer pipettes (Plastic, disposable)</td>
			<td>Fisher Scientific</td>
			<td>13-711-7M</td>
			<td></td>
		</tr>
		<tr>
			<td>Petri dish 60 mm and 80 mm</td>
			<td>Fisher Scientific</td>
			<td>S08184</td>
			<td></td>
		</tr>
		<tr>
			<td>Glass Pasteur Pipets ( Borosilicate, disposable)</td>
			<td>Fisher Scientific</td>
			<td>13-678-20A</td>
			<td></td>
		</tr>
		<tr>
			<td>Centrifuge</td>
			<td>Thermo Scientific</td>
			<td>Sorvall Legend X1R</td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>CE-ESI-MS Setup</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>High voltage power supply</td>
			<td>Spellman</td>
			<td>CZE1000R</td>
			<td>The HVPS may be controlled remotely using a low-voltage program generated by a personal computer. Caution: High voltage presents electrical shock hazard; all connective parts must be grounded or carefully shielded to prevent users from accidental exposure.</td>
		</tr>
		<tr>
			<td>Syringe pumps (2)</td>
			<td>Harvard Apparatus</td>
			<td>704506</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereomicroscope</td>
			<td>Amscope</td>
			<td>SM-3BZZ</td>
			<td>Stereomicroscope capable of 4.5&#215; magnification, equipped with an illuminator to monitor the spraying mode of the CE-ESI interface.</td>
		</tr>
		<tr>
			<td>XYZ translation stage</td>
			<td>Thorlabs</td>
			<td>PT3</td>
			<td></td>
		</tr>
		<tr>
			<td>XYZ translation stage</td>
			<td>Custom-built</td>
			<td></td>
			<td>This platform is capable of loading nanoliter-amounts of sample into the separation capillary via hydrodynamic injection and supplying the BGE for CE. Both interfaces described in this work were able to inject 6&#8211;10 nL of sample within 1 min into a 1 m separation capillary</td>
		</tr>
		<tr>
			<td>Stainless steel sample vials</td>
			<td>Custom-built</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Stainless steel BGE vial</td>
			<td>Custom-built</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Fused silica capillary (40 &#181;m/105 &#181;m ID/OD; 100 cm)</td>
			<td>Polymicro technologies</td>
			<td>TSP040105</td>
			<td></td>
		</tr>
		<tr>
			<td>Fused silica capillary (75 &#181;m/360 &#181;m ID/OD; 100 cm)</td>
			<td>Polymicro technologies</td>
			<td>TSP075375</td>
			<td></td>
		</tr>
		<tr>
			<td>Stainless steel emitter with blunt tips (130/260 &#181;m ID/OD)</td>
			<td>Hamilton Co.</td>
			<td>21031A</td>
			<td>For better performance, laser-cleave and fine-polish the emitter tip.</td>
		</tr>
		<tr>
			<td>Syringes (gas-tight): 500 - 1000 &#181;L</td>
			<td>Hamilton Co.</td>
			<td>1750TTL</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital multimeter</td>
			<td>Fluke</td>
			<td>Fluke 117</td>
			<td></td>
		</tr>
		<tr>
			<td>High-resolution Mass Spectrometer</td>
			<td>Bruker Daltonics</td>
			<td>Maxis Impact HD</td>
			<td>High-resolution tandem mass spectrometer equipped with an atmospheric-pressure interface configured for ESI</td>
		</tr>
		<tr>
			<td>Tunning mixture for mass spectrometer calibration</td>
			<td>Agilent technologies</td>
			<td>ESI-L G1969-85000</td>
			<td></td>
		</tr>
		<tr>
			<td>Data Analysis ver. 4.3 software</td>
			<td>Bruker Daltonics</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>Ancillary Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Vacuum concentrator capable of operation at 4&#8211;10&#176;C</td>
			<td>Labconco</td>
			<td>7310022</td>
			<td></td>
		</tr>
		<tr>
			<td>Analytical microbalance (XSE105DU)</td>
			<td>Fisher Scientific</td>
			<td>01911005</td>
			<td></td>
		</tr>
		<tr>
			<td>Freezer (-20 &#176;C)</td>
			<td>Fisher Scientific</td>
			<td>97-926-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Freezer (-80 &#176;C)</td>
			<td>Fisher Scientific</td>
			<td>88300ASP</td>
			<td></td>
		</tr>
		<tr>
			<td>Refrigerated Incubator</td>
			<td>Fisher Scientific</td>
			<td>11475126</td>
			<td></td>
		</tr>
		<tr>
			<td>Vortex-mixer</td>
			<td>Benchmark</td>
			<td>BS-VM-1000</td>
			<td></td>
		</tr>
	</tbody>
</table>

microprobe capillary electrophoresis mass spectrometry for single-cell metabolomics in live frog (xenopus laevis) embryos

The quantification of small molecules in single cells raises new potentials for better understanding the basic processes that underlie embryonic development. To enable single-cell investigations directly in live embryos, new analytical approaches are needed, particularly those that are sensitive, selective, quantitative, robust, and scalable to different cell sizes. Here, we present a protocol that enables the in situ analysis of metabolism in single cells in freely developing embryos of the South African clawed frog (Xenopus laevis), a powerful model in cell and developmental biology. This approach uses a capillary microprobe to aspirate a defined portion from single identified cells in the embryo, leaving neighboring cells intact for subsequent analysis. The collected cell content is analyzed by a microscale capillary electrophoresis electrospray ionization (CE-ESI) interface coupled to a high-resolution tandem mass spectrometer. This approach is scalable to various cell sizes and compatible with the complex three-dimensional structure of the developing embryo. As an example, we demonstrate that microprobe single-cell CE-ESI-MS enables the elucidation of metabolic cell heterogeneity that unfolds as a progenitor cell gives rise to descendants during development of the embryo. Besides cell and developmental biology, the single-cell analysis protocols described here are amenable to other cell sizes, cell types, or animal models.

We recently employed microprobe single-cell CE-ESI-MS to characterize metabolites in individual identified cells in freely developing Xenopus laevis embryos29,30. The microprobe enables fast (~5 sec/cell), in situ aspiration of ~10 nL from an individual cell, multiple aspirations of the same cell, or several different cells within the same or later stages of development of the live embryo (Fi...

Watch this Scientific Journal Video about Microprobe Capillary Electrophoresis Mass Spectrometry for Single-cell Metabolomics in Live Frog Xenopus laevis Embryos at JoVE.com

Microprobe Capillary Electrophoresis Mass Spectrometry for Single-cell Metabolomics in Live Frog Xenopus laevis Embryos

We describe steps that enable fast in situ sampling of a small portion of an individual cell with high precision and minimal invasion using capillary-based micro-sampling, to facilitate chemical characterization of a snapshot of metabolic activity in live embryos using a custom-built single cell capillary electrophoresis and mass spectrometry platform.

Microprobe Capillary Electrophoresis Mass Spectrometry for Single-cell Metabolomics in Live Frog (Xenopus laevis) Embryos

The quantification of small molecules in single cells raises new potentials for better understanding the basic processes that underlie embryonic ...

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