We thank Wendy Herbst and Lindsay Schleifer for their contributions to the development of these protocols. This work was supported by grants from the National Institutes of Health (1R15NS067566-01, 1R15HD077624-01 and 1R15HD096415-01) to MSS.

<ol>
	<li>Humeau, J., 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=Calcium+signaling+and+cell+cycle:+Progression+or+death.">Calcium signaling and cell cycle: Progression or death.</a> Cell Calcium. 70, 3-15 (2017).</li><li>Kim, J. M., Lee, M., Kim, N., Heo, W. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optogenetic+toolkit+reveals+the+role+of+Ca2++sparklets+in+coordinated+cell+migration.">Optogenetic toolkit reveals the role of Ca2+ sparklets in coordinated cell migration.</a> PNAS. 112 (21), 5951-5957 (2016).</li><li>Orrenius, S., Zhivotovsky, B., Nicotera, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Regulation+of+cell+death:+the+calcium-apoptosis+link.">Regulation of cell death: the calcium-apoptosis link.</a> Nature Reviews Molecular Cell Biology. 4 (7), 552-565 (2003).</li><li>Pham, K., 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=Ca2++and+Mg2++module+conformational+dynamics+and+stability+of+downstream+regulatory+element+antagonist+modulator.">Ca2+ and Mg2+ module conformational dynamics and stability of downstream regulatory element antagonist modulator.</a> Protein Science. 24 (5), 741-751 (2015).</li><li>Smedler, E., Uhl&#233;n, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Frequency+decoding+of+calcium+oscillations.">Frequency decoding of calcium oscillations.</a> Biochimica et Biophysica Acta. 1840 (3), 964-969 (2014).</li><li>Moreau, M., N&#233;ant, I., Webb, S. E., Miller, A. L., Riou, J. F., Leclerc, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ca(2+)+coding+and+decoding+strategies+for+the+specification+of+neural+and+renal+precursor+cells+during+development.">Ca(2+) coding and decoding strategies for the specification of neural and renal precursor cells during development.</a> Cell Calcium. 59 (2-3), 75-83 (2016).</li><li>Tomida, T., Hirose, K., Takizawa, A., Shibasaki, F., Iino, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=NFAT+functions+as+a+working+memory+of+Ca2++signals+in+decoding+Ca2++oscillation.">NFAT functions as a working memory of Ca2+ signals in decoding Ca2+ oscillation.</a> EMBO. 22 (15), 3825-3832 (2003).</li><li>Hannanta-Anan, P., Chow, B. Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optogenetic+Control+of+Calcium+Oscillation+Waveform+Defines+NFAT+as+an+Integrator+of+Calcium+Load.">Optogenetic Control of Calcium Oscillation Waveform Defines NFAT as an Integrator of Calcium Load.</a> Cell Systems. 2 (4), 283-288 (2016).</li><li>Li, L., Stefan, M. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Le+Nov&#232;re+N.+Calcium+input+frequency,+duration+and+amplitude+differentially+module+the+relative+activation+of+calcineurin+and+CaMKII.">Le Nov&#232;re N. Calcium input frequency, duration and amplitude differentially module the relative activation of calcineurin and CaMKII.</a> PLoS One. 7 (9), 43810 (2012).</li><li>Romano, D. R., Pharris, M. C., Patel, N. M., Kinzer-Ursem, T. 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D., Faber, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> The stages of Xenopus embryonic development. Normal Table of Xenopus laevis. , (1994).</li><li>Paudel, S., 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=Calcium+Activity+Dynamics+Correlate+with+Neuronal+Phenotype+at+a+Single+Cell+Level+and+in+a+Threshold-Dependent+Manner.">Calcium Activity Dynamics Correlate with Neuronal Phenotype at a Single Cell Level and in a Threshold-Dependent Manner.</a> International Journal of Molecular Science. 20 (8), 1880 (2019).</li><li>Marken, J. P., 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+Markovian+Entropy+Measure+for+the+Analysis+of+Calcium+Activity+Time+Series.">A Markovian Entropy Measure for the Analysis of Calcium Activity Time Series.</a> PLoS One. 11 (12), 0168342 (2016).</li><li>Eilers, P. H. C., Boelens, H. F. 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No conflicts of interest declared.

Characteristic patterns of calcium activity have been observed in the cells that make up the developing nervous system, with specific types of activity associated with distinct neurodevelopmental processes. However, further understanding of the mechanisms by which these information-dense activity patterns are translated into transcriptional responses requires information about calcium activity and gene expression to be collected with single-cell resolution. While systems that exhibit more stereotypical calcium activity, such as mature neurons, can be reasonably assayed on a bulk level, the irregular patterns that characterize the embryonic nervous system are easily masked by less precise recordings.
The experimental framework established in this protocol is easily adaptable to a wide variety of cell types and fluorescent reporters. Tissue containing virtually any cell type or combination of cell types can be dissected from a model organism of interest and plated for single-cell imaging. In addition to allowing cell identification and isolating the effect of cell-autonomous processes, a primary cell culture approach allows the experimenter to define media components as desired. For example, experiments comparing the activity of neuronal precursors in 2 mM Ca2+ solution have been performed to investigate whether the relationships between spike frequency and neurotransmitter phenotype in the embryonic spinal cord can be recapitulated without the influence of cell-cell interactions13,20.
While this protocol leverages the fluorescent marker Fluo4-AM to detect intracellular calcium activity, depending upon the selection criteria, users can select other commercially available markers21, including genetically encoded calcium indicators. Similarly, alternative markers could be used to monitor dynamic changes to concentration of an ion of interest (including K+, Na+, and Zn2+), membrane potential, or cellular pH. Imaging settings and image duration can be modified as necessary.
Although we correlated calcium activity and neuronal phenotype as a specific application, this method is also applicable for a variety of other cellular properties. For example, fluorescence in situ hybridization can be performed with probes against any gene of interest, including the neuronal marker ChAT or the transcription factor Engrailed, allowing sensitive detection of a customizable panel of mRNA species. These probes can be designed to be isoform-specific, supporting additional target specificity if desired. Double FISH can be performed using probes conjugated to several two different fluorophores, allowing the simultaneous assessment of the expression of multiple genes. However, the additional washes required by this type of experiment are associated with an increased chance of cell loss or movement and require experience and delicacy to be performed successfully.
Regardless of any experiment-specific modifications made to this protocol, there are several key steps that require careful attention. Dissections should be performed with care to remove all contaminating tissues or cell populations; because spatial patterning is lost when the explants are dissociated, any remaining cells from neighboring tissues will become interspersed with and indistinguishable from the cells of interest. After cells are plated, samples should be handled as gently as possible to prevent cells from being dislodged. Most importantly, this means that all solution changes should be performed slowly and carefully, with the pipette placed at the edge of plate when solution is being removed and added. This will ensure that cells can be confidently identified in both calcium and FISH images. If cells are disrupted during processing, it may be impossible to identify some or all of the corresponding cells between the two images. We advise erring on the side of caution with these assignments, such that only unambiguously corresponding cells are used for further analysis.
Depending on the biological question being addressed, a variety of analysis approaches may be appropriate. Time-series calcium activity can be processed and quantified in a variety of ways, with experimenter flexibility in choosing de-trending parameters, analysis metrics, and analysis parameters (for example, the % of baseline threshold used to define a calcium spike). Correlations between calcium activity and level of gene expression can be drawn by analyzing gene expression as an absolute or relative fluorescence value extracted from the FISH image. Alternatively, correlations between calcium activity and gene expression (presence/absence) can be drawn by defining a fluorescence threshold for positive gene expression signal and assigning &#39;yes&#39; or &#39;no&#39; identifiers to individual cells. As a whole, this experimental schema provides an incredibly flexible pipeline for the collection and preliminary analysis of time-series data in conjunction with cell-matched gene expression data. Such experiments will be critical for better understanding the complex relationships between cellular dynamics and transcriptional changes, as exemplified by the identification of calcium activity patterns characteristic of inhibitory-fated and excitatory-fated neuronal precursors in embryonic Xenopus laevis.

Free cytosolic calcium is critical to a variety of biological processes, ranging from cell proliferation and migration to apoptosis and autophagy1,2,3. Within these pathways, calcium can exert downstream effects on gene expression by interacting with calcium-binding domains to induce conformational changes that modulate protein activity and interactions. For example, a neuronal calcium sensor known as the Downstream Regulatory Element Antagonist Modulator (DREAM) is held in an unfolded intermediate conformation when bound by calcium, preventing it from interacting with its protein and DNA targets4. Beyond serving as a simple signaling molecule, however, the dynamic nature of intracellular calcium transients allows these activity patterns to encode more complex amplitude- or frequency-based signals5,6. Nuclear translocation of the transcription factor nuclear factor of activated T-cells (NFAT) is enhanced by high-frequency calcium oscillations but inhibited by low-frequency oscillations7. Compellingly, recent work has suggested that NFAT may actually responsive to cumulative calcium exposure8. Both calcineurin and Ca2+/calmodulin-dependent protein kinase II (CaMKII) also exhibit distinct responses to calcium transients of a specific frequency, duration, or amplitude9. To add an additional level of regulatory complexity, computational models suggest that many downstream calcium-binding proteins become more or less frequency-dependent in response to the presence or absence of binding competitors10,11.
Within the developing nervous system, two main classes of calcium activity behaviors have been defined and associated with specific biological processes. Calcium influxes are classified as &#34;spikes&#34; if they occur within individual cells, reach a peak intensity of ~400% of baseline within five seconds, and exhibit double exponential decay12. This type of signal is associated primarily with neurotransmitter phenotype specification13. In contrast, &#34;waves&#34; are defined as slower, less extreme calcium transients in which a cell&#39;s intracellular calcium concentration rises to ~200% of baseline over a period of thirty seconds or more, then decays over several minutes12. These signals often propagate across multiple neighboring cells, and their presence has been associated with neurite outgrowth and cell proliferation14,15. However, although these two classes have been defined based on characteristic kinetic profiles, it remains unclear exactly which characteristics of these patterns are actually being detected by cells and translated by downstream effectors.
Understanding the relationship between intracellular calcium oscillations and gene expression would provide crucial insight into one of the regulatory mechanisms that ensures appropriate development and patterning of the nervous system. To this end, studies of the embryonic spinal cord have demonstrated that increased calcium spike activity during development is associated with higher levels of inhibitory neurons, while decreased calcium spike activity is associated with higher levels of excitatory neurons13. However, these population-level assays have not been used to associate calcium activity with gene expression on a single-cell level.
Approaching these questions on the level of the single cell offers several distinct advantages over previous work. For one, the ability to assess calcium activity and gene expression in many cells individually allows the full repertoire of distinct activity patterns to be observed without being obfuscated by a bulk-level measurement. Additionally, studying these relationships in single-cell primary culture means that cell-autonomous links between calcium activity and gene expression will be maintained, while interactions requiring cell-cell communication will be abrogated. Therefore, this approach allows these cell-autonomous mechanisms to be studied in isolation. However, it also allows the role of non-cell-autonomous calcium activity to be elucidated and interrogated. For example, cells can be dissected from an embryo at the neural plate stage, cultured until sibling controls reach the neural tube stage, and then compared to cells that have been freshly dissected from a neural-tube-stage embryo. This allows direct comparison of cells that retained cell-cell communication across a key developmental period to those in which cell-cell communication was abolished.
In seeking to address the limitations of previous experimental approaches, we developed a protocol that would enable the assessment of both calcium activity and gene expression in individual neural progenitor cells, facilitating the correlation of specific activity patterns with subsequent differentiation programs. Neural tissue was dissected from Xenopus laevis at various stages of neural development, dissociated into single cells, and imaged via confocal microscopy in the presence of a fluorescent calcium indicator. Following live-cell imaging, samples were fixed and assayed via fluorescence in situ hybridization (FISH) to detect expression of a gene or isoform of interest. Importantly, individual cells can be tracked across both imaging experiments, meaning that a cell&#39;s calcium activity profile and its gene expression level can be associated with one another (Figure 1). The protocol reported here is intended to probe relationships between calcium activity patterns and gene expression across embryonic neurodevelopment in Xenopus laevis. However, the broader experimental framework (single-cell time-course imaging followed by FISH and image coregistration) can be modified and applied to virtually any cell type, fluorescent reporter, and gene of interest.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>For Animal Husbandry &#38; Cell Culture</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CHORULON (chorionic gonodotropin)</td>
			<td>Merck Animal Health</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Gentamycin sulfate salt</td>
			<td>Millipore Sigma</td>
			<td>G1264</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin (10,000 U/mL)</td>
			<td>Thermo Fisher Scientific</td>
			<td>15140122</td>
			<td></td>
		</tr>
		<tr>
			<td>Pyrex petri dishes, 100 mm x 20 mm</td>
			<td>Millipore Sigma</td>
			<td>CLS3160102</td>
			<td></td>
		</tr>
		<tr>
			<td>Corning Falcon Easy-Grip Tissue Culture Dishes, 35mm</td>
			<td>Fisher Scientific</td>
			<td>08-772A</td>
			<td></td>
		</tr>
		<tr>
			<td>Corning Falcon Easy-Grip Tissue Culture Dishes, 60mm</td>
			<td>Fisher Scientific</td>
			<td>08-772F</td>
			<td></td>
		</tr>
		<tr>
			<td>Falcon Standard Tissue Culture Dishes</td>
			<td>Fisher Scientific</td>
			<td>08-772E</td>
			<td></td>
		</tr>
		<tr>
			<td>Thermo Scientifc Nunc Cell Culture / Petri Dishes, 35x10mm Dish, Nunclon Delta</td>
			<td>Fisher Scientific</td>
			<td>12-565-90</td>
			<td></td>
		</tr>
		<tr>
			<td>Fisherbrand Standard Disposable Transfer Pipettes, Nongraduated; Length: 5.875 in.; Capacity: 7.7 mL</td>
			<td>Fisher Scientific</td>
			<td>13-711-7M</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethyl 3-aminobenzoate methanesulfonate</td>
			<td>Millipore Sigma</td>
			<td>E10521</td>
			<td></td>
		</tr>
		<tr>
			<td>Collagenase B</td>
			<td>Millipore Sigma</td>
			<td>11088807001</td>
			<td></td>
		</tr>
		<tr>
			<td>Dumont #55 Forceps, Dumostar</td>
			<td>Fine Science Tools</td>
			<td>11295-51</td>
			<td></td>
		</tr>
		<tr>
			<td>Dumont #5 Forceps, Dumostar</td>
			<td>Fine Science Tools</td>
			<td>11295-00</td>
			<td></td>
		</tr>
		<tr>
			<td>Cellattice Micro-Ruled Cell Culture Surface</td>
			<td>Nexcelom Bioscience</td>
			<td>CLS5-25D-050</td>
			<td></td>
		</tr>
		<tr>
			<td>For Calcium Imaging</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Fluo-4, AM, cell permeant</td>
			<td>Thermo Fisher Scientific</td>
			<td>F14201</td>
			<td></td>
		</tr>
		<tr>
			<td>Pluronic F-127, 0.2 &#181;m filtered (10% Solution in Water)</td>
			<td>Thermo Fisher Scientific</td>
			<td>P6866</td>
			<td></td>
		</tr>
		<tr>
			<td>For RNA Probe Generation</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>PureYield Plasmid Miniprep System</td>
			<td>Promega</td>
			<td>A1222</td>
			<td></td>
		</tr>
		<tr>
			<td>rATP</td>
			<td>Promega</td>
			<td>P1132</td>
			<td></td>
		</tr>
		<tr>
			<td>rCTP</td>
			<td>Promega</td>
			<td>P1142</td>
			<td></td>
		</tr>
		<tr>
			<td>rGTP</td>
			<td>Promega</td>
			<td>P1152</td>
			<td></td>
		</tr>
		<tr>
			<td>rUTP</td>
			<td>Promega</td>
			<td>P1162</td>
			<td></td>
		</tr>
		<tr>
			<td>Digoxigenin-11-UTP</td>
			<td>Millipore Sigma</td>
			<td>3359247910</td>
			<td></td>
		</tr>
		<tr>
			<td>Rnase Inhibitor</td>
			<td>Thermo Fisher Scientific</td>
			<td>N8080119</td>
			<td></td>
		</tr>
		<tr>
			<td>T3 RNA Polymerase</td>
			<td>Promega</td>
			<td>P2083</td>
			<td></td>
		</tr>
		<tr>
			<td>T7 RNA Polymerase</td>
			<td>Promega</td>
			<td>P2075</td>
			<td></td>
		</tr>
		<tr>
			<td>SP6 RNA Polymerase</td>
			<td>Promega</td>
			<td>P1085</td>
			<td></td>
		</tr>
		<tr>
			<td>RQ1 Rnase-Free Dnase</td>
			<td>Promega</td>
			<td>M6101</td>
			<td></td>
		</tr>
		<tr>
			<td>LiCl Precipitation Solution (7.5 M)</td>
			<td>Thermo Fisher Scientific</td>
			<td>AM9480</td>
			<td></td>
		</tr>
		<tr>
			<td>For Fluorescence In Situ Hybridization</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Acetic Anhydride</td>
			<td>Thermo Fisher Scientific</td>
			<td>320102</td>
			<td></td>
		</tr>
		<tr>
			<td>Blocking Reagent</td>
			<td>Millipore Sigma</td>
			<td>11096176001</td>
			<td></td>
		</tr>
		<tr>
			<td>Anti-Digoxigenin-POD, Fab fragments</td>
			<td>Millipore Sigma</td>
			<td>11207733910</td>
			<td></td>
		</tr>
		<tr>
			<td>Cy3 Mono-Reactive NHS Ester</td>
			<td>Millipore Sigma</td>
			<td>GEPA13105</td>
			<td></td>
		</tr>
		<tr>
			<td>Solution Components</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Calcium chloride, 96% extra pure, powder, anhydrous, ACROS Organixs</td>
			<td>Fisher Scientific</td>
			<td>AC349610</td>
			<td></td>
		</tr>
		<tr>
			<td>Calcium chloride dihydrate</td>
			<td>Millipore Sigma</td>
			<td>C3306</td>
			<td></td>
		</tr>
		<tr>
			<td>CHAPS hydrate</td>
			<td>Millipore Sigma</td>
			<td>C3023</td>
			<td></td>
		</tr>
		<tr>
			<td>Denhardt&#39;s Solution (50X)</td>
			<td>Thermo Fisher Scientific</td>
			<td>750018</td>
			<td></td>
		</tr>
		<tr>
			<td>DTT, Molecular Grade (DL-Dithiothreitol)</td>
			<td>Promega</td>
			<td>P1171</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethylenediaminetetraacetic Acid, Disodium Salt Dihydrate</td>
			<td>Fisher Scientific</td>
			<td>S311</td>
			<td></td>
		</tr>
		<tr>
			<td>Ethylene glycol-bis(2-aminoethylether)-N,N,N&#39;,N&#39;-tetraacetic acid</td>
			<td>Millipore Sigma</td>
			<td>E3889</td>
			<td></td>
		</tr>
		<tr>
			<td>Formamide (Deionized)</td>
			<td>Thermo Fisher Scientific</td>
			<td>AM9342</td>
			<td></td>
		</tr>
		<tr>
			<td>Herparin sodium salt from porcine intestinal mucosa</td>
			<td>Millipore Sigma</td>
			<td>H3393</td>
			<td></td>
		</tr>
		<tr>
			<td>HEPES (Ultra Pure)</td>
			<td>Thermo Fisher Scientific</td>
			<td>11344041</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen peroxide solution</td>
			<td>Millipore Sigma</td>
			<td>H1109</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Cysteine</td>
			<td>Millipore Sigma</td>
			<td>168149</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnesium chloride, pure, ACROS Organics</td>
			<td>Fisher Scientific</td>
			<td>AC223211000</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnesium sulfate, 97% pure, ACROS Organixs, anhydrous</td>
			<td>Fisher Scientific</td>
			<td>AC413480050</td>
			<td></td>
		</tr>
		<tr>
			<td>Maleic Acid, 99%, ACROS Organics</td>
			<td>Fisher Scientific</td>
			<td>ACS125231000</td>
			<td></td>
		</tr>
		<tr>
			<td>MOPS (Fine White Crystals/Molecular Biology), Fisher BioReagents</td>
			<td>Fisher Scientific</td>
			<td>BP308</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium chloride</td>
			<td>Millipore Sigma</td>
			<td>P9541</td>
			<td></td>
		</tr>
		<tr>
			<td>Ribonucleic acid from torula yeast, Type IX</td>
			<td>Millipore Sigma</td>
			<td>R3629</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium chloride</td>
			<td>Millipore Sigma</td>
			<td>S7653</td>
			<td></td>
		</tr>
		<tr>
			<td>Triethanolamine</td>
			<td>Millipore Sigma</td>
			<td>90279</td>
			<td></td>
		</tr>
		<tr>
			<td>Tris</td>
			<td>Millipore Sigma</td>
			<td>GE17-1321-01</td>
			<td></td>
		</tr>
		<tr>
			<td>TWEEN 20</td>
			<td>Millipore Sigma</td>
			<td>P9416</td>
			<td></td>
		</tr>
		<tr>
			<td>Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Laminar Flow Hood</td>
			<td>model of choice</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Dissecting Microscope</td>
			<td>model of choice</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Inverted Fluorescence Microscope</td>
			<td>Nikon</td>
			<td>TE200</td>
			<td></td>
		</tr>
		<tr>
			<td>NIS-Elements Imaging Software</td>
			<td>Nikon</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Shaking Incubator</td>
			<td>model of choice</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Refrigerated Centrifuge</td>
			<td>model of choice</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Miscellaneous</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Corning bottle-top vaccum filter system, 0.22 &#956;m pore, 500 mL bottle capacity</td>
			<td>Millipore Sigma</td>
			<td>CLS430769</td>
			<td></td>
		</tr>
		<tr>
			<td>Falcon 50mL Conical Centrifuge Tubes</td>
			<td>Fisher Scientific</td>
			<td>14-432-22</td>
			<td></td>
		</tr>
	</tbody>
</table>

fluorescent calcium imaging and subsequent in situ hybridization for neuronal precursor characterization in xenopus laevis

Spontaneous intracellular calcium activity can be observed in a variety of cell types and is proposed to play critical roles in a variety of physiological processes. In particular, appropriate regulation of calcium activity patterns during embryogenesis is necessary for many aspects of vertebrate neural development, including proper neural tube closure, synaptogenesis, and neurotransmitter phenotype specification. While the observation that calcium activity patterns can differ in both frequency and amplitude suggests a compelling mechanism by which these fluxes might transmit encoded signals to downstream effectors and regulate gene expression, existing population-level approaches have lacked the precision necessary to further explore this possibility. Furthermore, these approaches limit studies of the role of cell-cell interactions by precluding the ability to assay the state of neuronal determination in the absence of cell-cell contact. Therefore, we have established an experimental workflow that pairs time-lapse calcium imaging of dissociated neuronal explants with a fluorescence in situ hybridization assay, allowing the unambiguous correlation of calcium activity pattern with molecular phenotype on a single-cell level. We were successfully able to use this approach to distinguish and characterize specific calcium activity patterns associated with differentiating neural cells and neural progenitor cells, respectively; beyond this, however, the experimental framework described in this article could be readily adapted to investigate correlations between any time-series activity profile and expression of a gene or genes of interest.

A successful example of dissociated cells prepared for calcium imaging can be seen in Figure 2A. Cells are densely plated, allowing the maximum amount of information to be collected from each image, but not so densely plated that individual cells cannot be confidently distinguished form one another. Fluorescence is detected for each defined cell over the 2 h imaging period. Visualization of a composite plot containing the traces for all cells recorded in an experiment reveals the degree to which bulk or population measurements can obscure more nuanced patterns of spiking behavior (Figure 2B). When the recorded profiles of individual cells are isolated, examples of the irregular spiking activity characteristic of neural progenitor cells can be clearly identified. Unlike mature neurons, embryonic neuronal cells exhibit irregular, highly variable and complex nature of calcium activity (Figure 2B). In order to quantify this complexity, application of different data analysis methods have been applied17,18, including diverse parameters to define a spike (Figure 2C).
Successful fluorescence in situ hybridization, including successful design and synthesis of an antisense mRNA probe, can be assessed by comparing the experimental plate against a background control incubated with a non-binding sense RNA control (Figure 3A,B). A positive probe control can also be performed by processing a cell type known to express the target mRNA at detectable levels.
Identification of the same cell across calcium and FISH imaging requires that cells retain roughly the same position during probe hybridization and processing. If plates are handled roughly or washes are performed too forcefully, cells can be dislodged from the plate surface and either lost when solution is discarded or deposited on a different location on the plate, making it impossible for them to be matched across images (Figure 4A). If this disruption affects only some of the cells in the field of view, it may still be possible to detect and assign some cells within the image (Figure 4B). However, the maximum amount of data is gained from an experiment in which FISH is performed carefully and few cells are lost or repositioned between images (Figure 4C).
Once data has been collected to describe both the calcium activity and gene expression of a reasonable number of cells at the developmental stage(s) of interest, further analyses can be performed to assess correlations between these two features (Figure 1). A number of metrics have been applied to quantify calcium activity patterns, including spike counting/frequency, average power, Hurst exponent estimation, and Markovian entropy measurement17,18. Gene expression can be defined quantitatively by absolute fluorescence level or graded on a binary (yes/no) scale, depending on the experimental questions being addressed.
Results from experiments collating calcium activity with the expression of neural progenitor marker genes revealed numerous associations between specific patterns of calcium activity and neurotransmitter phenotypes. At the neural plate stage (Stage 14), GABAergic cells expressing the inhibitory neuron marker gad1.1 exhibit calcium activity that is more regular and higher-amplitude than that of cells that lack gad1.1 expression (Figure 5A). Furthermore, while these gad1.1-expressing cells are associated with higher levels of high-amplitude spiking, low-amplitude spiking is more frequent in glutamatergic cells expressing the excitatory neuron marker slc17a7.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60726/60726fig1a.jpg" /> 
Figure 1: Schematic of experimental workflow. Scale bar = 100 &#956;m. Images in panels 3-5 were taken from Paudel et al. (2019)17. <a href="https://www.jove.com/files/ftp_upload/60726/60726fig1alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60726/60726fig2b.jpg" /> 
Figure 2: Calcium imaging and example activity profiles. (A) Intracellular calcium activity as reported by Fluo4-AM. Each 2 h image is composed of 901 frames, with one representative frame show here. (B) Composite plot of fluorescence intensity over time in all cells within the imaged field of view. The traces clearly indicate photobleaching of indicator dye (Fluo4) overtime. Raster plot on top left shows representative traces of calcium activity after application of a de-trending algorithm developed by Eilers and Boelen19, where cells shown here exhibit diverse pattern of spiking behavior. (C) Application of different thresholds (150% and 200% of baseline, where the baseline is the average of de-trended fluorescent intensity) to define a spike (green and blue arrows). Scale bar = 100 &#956;m. <a href="https://www.jove.com/files/ftp_upload/60726/60726fig2blarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60726/60726fig3a.jpg" /> 
Figure 3: FISH imaging. (A) FISH performed with a non-binding sense RNA probe as a negative control. Imaging settings have been adjusted so that no cells appear fluorescent. Some fields of view may include non-cell debris with some fluorescence, such as seen in the top right and bottom right corners of (A); these can be ignored for the purpose of background setting. (B) The same imaging settings are then used to image an experimental plate (antisense RNA probe). Fluorescence under these conditions corresponds to gene expression above background. Scale bar = 100 &#956;m. <a href="https://www.jove.com/files/ftp_upload/60726/60726fig3alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60726/60726fig4a.jpg" /> 
Figure 4: Image overlay and coregistration. Schematic representations of a sample imaged for calcium activity (cells represented by filled green circles) and after FISH (cells represented by shaded red circles). (A) Cells that have moved significantly during sample handling and processing cannot be reliably identified across the two images. (B) Cell disruption may affect only some cells in the field of view. Some cells can be clearly identified in both images, while others cannot be confidently matched. (C) If samples are handled carefully, most cells will remain undisturbed and can be identified in both images. <a href="https://www.jove.com/files/ftp_upload/60726/60726fig4alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60726/60726fig5b.jpg" /> 
Figure 5: An example of application of this method, boxplots showing associations between calcium activity and gene expression (GABA and Glut for genes gad1.1 and slc17a7 respectively) in neural plate stage Xenopus laevis. At stage 14, gad1.1-postive cells (GABA) exhibit higher-amplitude and more regular calcium activity as defined by (A) Markovian entropy18 and (B) spike counts using thresholds 125%, 150%, 200% and 300% of the average of the de-trended fluorescent intensity (baseline)17 than slc17a7 positive cells (Glut). Stars indicate statistically significant differences according to both Bonferroni-corrected two-sample Kolmogorov&#8211;Smirnov Test (p &#60; 0.05) and Cohen&#39;s d statistics for effect size (n = 5 cultures and &#62;100 cells; * 0.2 &#8804; |d| &#60; 0.5). The figure was redrawn and adapted from data set obtained from Paudel et al.17. <a href="https://www.jove.com/files/ftp_upload/60726/60726fig5blarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Fluorescent Calcium Imaging and Subsequent In Situ Hybridization for Neuronal Precursor Characterization in Xenopus laevis at JoVE.com

Fluorescent Calcium Imaging and Subsequent In Situ Hybridization for Neuronal Precursor Characterization in Xenopus laevis

We present a two-part protocol that combines fluorescent calcium imaging with in situ hybridization, allowing the experimenter to correlate patterns of calcium activity with gene expression profiles on a single-cell level.

Fluorescent Calcium Imaging and Subsequent In Situ Hybridization for Neuronal Precursor Characterization in Xenopus laevis

Spontaneous intracellular calcium activity can be observed in a variety of cell types and is proposed to play critical roles in a variety of physiological ...

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Developmental Biology

7.8K Views.  Harvard University. We present a two-part protocol that combines fluorescent calcium imaging with in situ hybridization, allowing the experimenter to correlate patterns of calcium activity with gene expression profiles on a single-cell level.

Video: Fluorescent Calcium Imaging and Subsequent In Situ Hybridization for Neuronal Precursor Characterization in Xenopus laevis

Understanding Early Organogenesis Using a Simplified In Situ Hybridization Protocol in Xenopus

Organogenesis is the study of how organs are specified and then acquire their specific shape and functions during development. The Xenopuslaevis embryo is very useful for studying organogenesis because their large size makes them very suitable for identifying organs at the earliest steps in organogenesis. At this time, the primary method used for identifying a specific organ or primordium is whole mount in situ hybridization with labeled antisense RNA probes specific to a gene that is expressed in the organ of interest. In addition, it is relatively easy to manipulate genes or signaling pathways in Xenopus and in situ hybridization allows one to then assay for changes in the presence or morphology of a target organ. Whole mount in situ hybridization is a multi-day protocol with many steps involved. Here we provide a simplified protocol with reduced numbers of steps and reagents used that works well for routine assays. In situ hybridization robots have greatly facilitated the process and we detail how and when we utilize that technology in the process. Once an in situ hybridization is complete, capturing the best image of the result can be frustrating. We provide advice on how to optimize imaging of in situ hybridization results. Although the protocol describes assessing organogenesis in Xenopus laevis, the same basic protocol can almost certainly be adapted to Xenopus tropicalis and other model systems.

Understanding Early Organogenesis Using a Simplified In Situ Hybridization Protocol in Xenopus

The Xenopus laevis embryo continues to be exceptionally useful in the study of early development due to its large size and ease of manipulation. A simplified protocol for whole mount in situ hybridization protocol is provided that can be used in the identification of specific organs in this model system.

Organogenesis is the study of how organs are specified and then acquire their specific shape and functions during development.  The Xenopuslaevis embryo ...

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 ...

Using Confocal Analysis of Xenopus laevis to Investigate Modulators of Wnt and Shh Morphogen Gradients

This protocol describes a method to visualise ligands distributed across a field of cells. The ease of expressing exogenous proteins, together with the large size of their cells in early embryos, make Xenopus laevis a useful model for visualising GFP-tagged ligands. Synthetic mRNAs are efficiently translated after injection into early stage Xenopus embryos, and injections can be targeted to a single cell. When combined with a lineage tracer such as membrane tethered RFP, the injected cell (and its descendants) that are producing the overexpressed protein can easily be followed. This protocol describes a method for the production of fluorescently tagged Wnt and Shh ligands from injected mRNA. The methods involve the micro dissection of ectodermal explants (animal caps) and the analysis of ligand diffusion in multiple samples. By using confocal imaging, information about ligand secretion and diffusion over a field of cells can be obtained. Statistical analyses of confocal images provide quantitative data on the shape of ligand gradients. These methods may be useful to researchers who want to test the effects of factors that may regulate the shape of morphogen gradients.

Using Confocal Analysis of Xenopus laevis to Investigate Modulators of Wnt and Shh Morphogen Gradients

The manuscript here provides a simple set of methods for analysing the secretion and diffusion of fluorescently tagged ligands in Xenopus. This provides a context for testing the ability of other proteins to modify ligand distribution and allowing experiments that may give insight into mechanisms regulating morphogen gradients.

This protocol describes a method to visualise ligands distributed across a field of cells. The ease of expressing exogenous proteins, together with the ...

Imaging Subcellular Structures in the Living Zebrafish Embryo

In vivo imaging provides unprecedented access to the dynamic behavior of cellular and subcellular structures in their natural context. Performing such imaging experiments in higher vertebrates such as mammals generally requires surgical access to the system under study. The optical accessibility of embryonic and larval zebrafish allows such invasive procedures to be circumvented and permits imaging in the intact organism. Indeed the zebrafish is now a well-established model to visualize dynamic cellular behaviors using in vivo microscopy in a wide range of developmental contexts from proliferation to migration and differentiation. A more recent development is the increasing use of zebrafish to study subcellular events including mitochondrial trafficking and centrosome dynamics. The relative ease with which these subcellular structures can be genetically labeled by fluorescent proteins and the use of light microscopy techniques to image them is transforming the zebrafish into an in vivo model of cell biology. Here we describe methods to generate genetic constructs that fluorescently label organelles, highlighting mitochondria and centrosomes as specific examples. We use the bipartite Gal4-UAS system in multiple configurations to restrict expression to specific cell-types and provide protocols to generate transiently expressing and stable transgenic fish. Finally, we provide guidelines for choosing light microscopy methods that are most suitable for imaging subcellular dynamics.

Imaging the dynamic behavior of organelles and other subcellular structures in vivo can shed light on their function in physiological and disease conditions. Here, we present methods for genetically tagging two organelles, centrosomes and mitochondria, and imaging their dynamics in living zebrafish embryos using wide-field and confocal microscopy.

In vivo imaging provides unprecedented access to the dynamic behavior of cellular and subcellular structures in their natural context. Performing such ...

Technique to Target Microinjection to the Developing Xenopus Kidney

The embryonic kidney of Xenopus&#160;laevis (frog), the pronephros, consists of a single nephron, and can be used as a model for kidney disease. Xenopus embryos are large, develop externally, and can be easily manipulated by microinjection or surgical procedures. In addition, fate maps have been established for early Xenopus embryos. Targeted microinjection into the individual blastomere that will eventually give rise to an organ or tissue of interest can be used to selectively overexpress or knock down gene expression within this restricted region, decreasing secondary effects in the rest of the developing embryo. In this protocol, we describe how to utilize established Xenopus fate maps to target the developing Xenopus kidney (the pronephros), through microinjection into specific blastomere of 4- and 8-cell embryos. Injection of lineage tracers allows verification of the specific targeting of the injection. After embryos have developed to stage 38 - 40, whole-mount immunostaining is used to visualize pronephric development, and the contribution by targeted cells to the pronephros can be assessed. The same technique can be adapted to target other tissue types in addition to the pronephros.

Technique to Target Microinjection to the Developing Xenopus Kidney

Here, we present a protocol to use fate maps and lineage tracers to target injections into individual blastomeres that give rise to the kidney of Xenopus laevis embryos.

The embryonic kidney of Xenopus&#160;laevis (frog), the pronephros, consists of a single nephron, and can be used as a model for kidney disease. Xenopus ...

Multi-target Chromogenic Whole-mount In Situ Hybridization for Comparing Gene Expression Domains in Drosophila Embryos

To analyze gene regulatory networks active during embryonic development and organogenesis it is essential to precisely define how the different genes are expressed in spatial relation to each other in situ. Multi-target chromogenic whole-mount in situ hybridization (MC-WISH) greatly facilitates the instant comparison of gene expression patterns, as it allows distinctive visualization of different mRNA species in contrasting colors in the same sample specimen. This provides the possibility to relate gene expression domains topographically to each other with high accuracy and to define unique and overlapping expression sites. In the presented protocol, we describe a MC-WISH procedure for comparing mRNA expression patterns of different genes in Drosophila embryos. Up to three RNA probes, each specific for another gene and labeled by a different hapten, are simultaneously hybridized to the embryo samples and subsequently detected by alkaline phosphatase-based colorimetric immunohistochemistry. The described procedure is detailed here for Drosophila, but works equally well with zebrafish embryos.

Multi-target Chromogenic Whole-mount In Situ Hybridization for Comparing Gene Expression Domains in Drosophila Embryos

We describe a multi-target chromogenic whole-mount in situ hybridization (MC-WISH) procedure in intact Drosophila embryos allowing simultaneous and specific detection of three different mRNA distribution patterns by contrasting color precipitates.

To analyze gene regulatory networks active during embryonic development and organogenesis it is essential to precisely define how the different genes are ...

Imaging Calcium in Drosophila at Egg Activation

Egg activation is a universal process that includes a series of events to allow the fertilized egg to complete meiosis and initiate embryonic development. One aspect of egg activation, conserved across all organisms examined, is a change in the intracellular concentration of calcium (Ca2+) often termed a 'Ca2+ wave'. While the speed and number of oscillations of the Ca2+ wave varies between species, the change in intracellular Ca2+ is key in bringing about essential events for embryonic development. These changes include resumption of the cell cycle, mRNA regulation, cortical granule exocytosis, and rearrangement of the cytoskeleton.
In the mature Drosophila egg, activation occurs in the female oviduct prior to fertilization, initiating a series of Ca2+-dependent events. Here we present a protocol for imaging the Ca2+ wave in Drosophila. This approach provides a manipulable model system to interrogate the mechanism of the Ca2+ wave and the downstream changes associated with it.

Imaging Calcium in Drosophila at Egg Activation

This article describes an adaptable ex vivo protocol for visualizing Ca2+ during egg activation in Drosophila.

Egg activation is a universal process that includes a series of events to allow the fertilized egg to complete meiosis and initiate embryonic development. ...

Visualizing Multiciliated Cells in the Zebrafish Through a Combined Protocol of Whole Mount Fluorescent In Situ Hybridization and Immunofluorescence

In recent years, the zebrafish embryo has emerged as a popular model to study developmental biology due to traits such as ex utero embryo development and optical transparency. In particular, the zebrafish embryo has become an important organism to study vertebrate kidney organogenesis as well as multiciliated cell (MCC) development. To visualize MCCs in the embryonic zebrafish kidney, we have developed a combined protocol of whole-mount fluorescent in situ hybridization (FISH) and whole mount immunofluorescence (IF) that enables high resolution imaging. This manuscript describes our technique for co-localizing RNA transcripts and protein as a tool to better understand the regulation of developmental programs through the expression of various lineage factors.

Visualizing Multiciliated Cells in the Zebrafish Through a Combined Protocol of Whole Mount Fluorescent In Situ Hybridization and Immunofluorescence

Cilia development is vital to proper organogenesis. This protocol describes an optimized method to label and visualize ciliated cells of the zebrafish.

In recent years, the zebrafish embryo has emerged as a popular model to study developmental biology due to traits such as ex utero embryo development and ...

Wholemount In Situ Hybridization for Astyanax Embryos

In recent years, a draft genome for the blind Mexican cavefish (Astyanax mexicanus) has been released, revealing the sequence identities for thousands of genes. Prior research into this emerging model system capitalized on comprehensive genome-wide investigations that have identified numerous quantitative trait loci (QTL) associated with various cave-associated phenotypes. However, the ability to connect genes of interest to the heritable basis for phenotypic change remains a significant challenge. One technique that can facilitate deeper understanding of the role of development in troglomorphic evolution is whole-mount in situ hybridization. This technique can be implemented to directly compare gene expression between cave- and surface-dwelling forms, nominate candidate genes underlying established QTL, identify genes of interest from next-generation sequencing studies, or develop other discovery-based approaches. In this report, we present a simple protocol, supported by a flexible checklist, that can be widely adapted for use well beyond the presented study system. It is hoped that this protocol can serve as a broad resource for the Astyanax community and beyond.

Wholemount In Situ Hybridization for Astyanax Embryos

This protocol enables visualization of gene expression in embryonic Astyanax cavefish. This approach has been developed with the goal of maximizing gene expression signal, while minimizing non-specific background staining.

In recent years, a draft genome for the blind Mexican cavefish (Astyanax mexicanus) has been released, revealing the sequence identities for thousands of ...

Genotyping and Quantification of In Situ Hybridization Staining in Zebrafish

In situ hybridization (ISH) is an important technique that enables researchers to study mRNA distribution in situ and has been a critical technique in developmental biology for decades. Traditionally, most gene expression studies relied on visual evaluation of the ISH signal, a method that is prone to bias, particularly in cases where sample identities are known a priori. We have previously reported on a method to circumvent this bias and provide a more accurate quantification of ISH signals. Here, we present a simple guide to apply this method to quantify the expression levels of genes of interest in ISH-stained embryos and correlate that with their corresponding genotypes. The method is particularly useful to quantify spatially restricted gene expression signals in samples of mixed genotypes and it provides an unbiased and accurate alternative to the traditional visual scoring methods.

Gene editing technologies have enabled researchers to generate zebrafish mutants to investigate gene function with relative ease. Here, we provide a guide to perform parallel embryo genotyping and quantification of in situ hybridization signals in zebrafish. This unbiased approach provides greater accuracy in phenotypical analyses based on in situ hybridization.

In situ hybridization (ISH) is an important technique that enables researchers to study mRNA distribution in situ and has been a critical technique in ...