Name: imaging of intracellular atp in organotypic tissue slices of the mouse brain using the fret-based sensor ateam1.03yemk
Uploaded: 2019-12-19T13:00:00
Duration: 11 min 20 s
Description: Neuronal activity in the central nervous system (CNS) evokes a high demand on cellular energy provided by the breakdown of adenosine triphosphate (ATP). A ...

The authors wish to thank Claudia Roderigo and Simone Durry for expert technical assistance. We thank Dr. Niklas J. Gerkau and M.Sc. Joel Nelson for assistance in the preparation of the organotypic slice cultures. Research in the author&#39;s laboratory was funded by the German Research Association (DFG; FOR 2795: Ro 2327/13-1 and SPP 1757: Ro 2327/8-2 to CRR; and SPP 1757: Young Glia Start-Up funding to RL).

The authors declare no competing interests. The authors received financial support enabling open access publication by Nikon Microscope Solutions, D&#252;sseldorf, Germany, which produces instruments used in the video article. The company was neither involved in designing the experiments presented here, nor in their execution, nor in the data handling, nor in the manuscript writing.

Here, we demonstrate a procedure for the cell-type specific expression of ATeam1.03YEMK, a FRET-based, genetically encoded nanosensor14, for measurement of changes in ATP levels in astrocytes or neurons in organotypic tissue slice cultures of the mouse brain15. In exemplary recordings, we show that an increase in the extracellular potassium concentration does not result in a change in ATP concentrations in neurons, while astrocytic ATP levels rise in response to this manipulation. Moreover, our results demonstrate that upon inhibition of cellular metabolism, ATeam1.03YEMK FRET ratio rapidly drops in both cell types, indicating a rapid decrease in intracellular ATP.
Expression of ATeam1.03YEMK in organotypic slice cultures requires maintenance of the tissue in culture under controlled conditions for at least 7-10 days. Alternatively, ATeam1.03YEMK can also be employed for measurement of ATP in acutely isolated brain tissue slices and in optic nerves of mice13,15. Measurements in acutely isolated tissue, however, necessitate the generation of transgenic animals or a stereotactic application of viral vectors into the brain, involving animal experimentation and strict animal care protocols. In this regard, ATeam1.03YEMK expression in organotypic tissue slice cultures represents a useful and valuable alternative22,23. For many years now, organotypic tissue slice cultures serve as an established model system to study neural properties, connectivity and development24,25,26. They not only maintain the general tissue architecture and lamination (Figure 1), but also host the preferential properties of cell cultures such as superior accessibility and direct control of experimental conditions. Organotypic tissue slice cultures are also routinely employed to express foreign genes by using viral vectors27. Several types of viral vectors have been reported to deliver transgenes into brain tissue16,28. Adenoviral vectors induce high expression in glial cells, but not hippocampal neurons16, and might generate glial reactivity17. Adeno-associated viral vectors as used here emerge as a good alternative15, and their effectiveness has also been shown in vivo29.
While mainly being used for the study of neuronal properties, recent studies have established that organotypic tissue slice cultures can also be employed for the analysis of astrocytes. Cultured slices are usually covered by a layer of reactive astrocytes19,30 (Figure 5), but astrocytes exhibit a more native, non-reactive morphology and cytoarchitecture in deeper layers19,30 (Figure 5). In the present study, we describe a procedure for the mechanical removal of the outer glial scar, which results in a better experimental and optical accessibility of native astrocytes within the proper organotypic tissue layers. Moreover, its removal improves the expression efficacy in deeper layers of the organotypic slices; if the glial scar is not removed, transduction by AAVs might tend to be restricted to the superficial cell layers.
Several external mechanical factors need to be considered when performing experiments in tissue slices. A variation in bath perfusion velocity can induce movements of the entire preparation and/or induce changes in focus, resulting in artificial transient changes of the sensor signal. Moreover, both astrocytes and neurons have been reported to respond to mechanical deformation such as imposed by high perfusion rates32,33. In our hands, using a reliable peristaltic pump, together with maintaining small and stable volumes of saline between the tissue and the objective (meniscus) results in a stable FRET-signal under baseline conditions at the perfusion velocity used here (1.5-2.5 mL/min; Figure 8).
In the present study, we also demonstrate that FRET-based imaging with ATeam1.03YEMK can be employed to monitor ATP levels in neurons and astrocytes. An alternative means introduced earlier for measurement of cellular ATP is the so-called luciferin-luciferase assay34,35,36,37. This approach, however, is based on imaging of bioluminescence and only provides a rather low temporal and spatial resolution partly due to high background noise levels. Another method routinely employed in recent years was the imaging of changes in the intracellular magnesium concentration using the ion-sensitive fluorophore magnesium green38,39,40. This approach relates to the observation that a consumption of ATP results in the release of its co-factor magnesium. Imaging with magnesium green thereby only provides a secondary estimate of changes in ATP levels. Moreover, magnesium green is also sensitive to changes in intracellular calcium, introducing another difficulty when interpreting results obtained with this method.
The recent development of genetically-encoded nanosensors for direct imaging of cellular metabolites, therefore, represented a big step forward11,12. Several different sensors were generated that can be employed for measurement of intracellular ATP36,41,42,43. Among those are the ratiometric fluorescent ATP indicator &#34;QUEEN&#34;41 as well as PercevalHR, which senses the ATP:ADP ratio42. While the latter probe is a valuable tool for the study of the energy status of cells, it requires simultaneous measurement of changes in pH42.
ATeam is a nanosensor of which several variants exist, which &#8212; among others &#8212; differ in their binding affinity for ATP14. In vitro, ATeam1.03YEMK exhibits a Kd of 1.2 mM at 37 &#176;C, which is close to cellular ATP levels determined in different neuronal cell types, ranging from hypothalamus and cerebellum34 to hippocampus37,44,45. In cuvette measurements, lowering the temperature by 10 &#176;C resulted in a significant decrease in the binding affinity of ATeam1.03YEMK to ATP, suggesting that it might not be ideal for cellular imaging at room temperature14. Our earlier study15, however, demonstrated that the behavior and response of ATeam1.03YEMK expressed in neurons and astrocytes to different manipulations is similar at near physiological and at room temperature, indicating that the sensor allows reliable determination of intracellular ATP levels under both conditions. In addition, our previous experiments addressed the pH-sensitivity of ATeam1.03YEMK expressed inside cells15, showing that it is insensitive to changes in intracellular pH by about 0.1-0.2 pH units. If the Kd in the low mM range is a concern, alternative ATeam variants might be used14, among them red-shifted variants of ATeam (&#34;GO-ATeam&#34;)43.
Our experiments using ATeam1.03YEMK demonstrate that an increase in the extracellular potassium concentration by a few mM only (from 3 to 8 mM) results in a transient increase in the ATeam1.03YEMK ratio in astrocytes in organotypic slice culture. This observation confirms earlier studies15,46 and clearly indicates that astrocytes respond to the release of potassium by active neurons with an increase in their ATP production, mostly likely as a consequence of a stimulation of the Na+/K+-ATPase and the Na+/HCO3- cotransporter, respectively47,48. In contrast to this, neurons did not show a response, which is in line with previous work as well15. Both cell types, however, rapidly and strongly reacted to inhibition of cellular glycolysis and mitochondrial respiration as shown before15. Under conditions of chemical ischemia, ATeam FRET ratios fell to a new stable level, indicating a nominal depletion of cellular ATP. The latter result suggests that both neurons as well as astrocytes exhibit a relevant consumption of ATP also under steady-state conditions without additional stimulation by synaptic activation or application of neurotransmitters. Taken together, we conclude that FRET-based imaging with genetically encoded nanosensors, among them ATeam1.03YEMK, will provide a valuable approach to elucidate the cellular processes that are responsible for changes in intracellular ATP levels and cellular ATP consumption under different conditions.

Excitatory electrical activity of neurons is largely based on the flux of cations such as sodium (Na+) and potassium (K+) across their plasma membranes. Maintenance of the electrochemical gradients of these two ions is thus required for signaling. This is accomplished by the cellular Na+/K+-ATPase (NKA), an ubiquitously expressed electrogenic transmembrane pump, which extrudes 3 Na+ out of the cell in exchange for 2 K+ from the extracellular space, requiring the consumption of one molecule of ATP per transport cycle1. In addition to the NKA, there are several other ATP-consuming ion transporters including the plasma membrane Ca2+-ATPase, which is vital for intracellular Ca2+ homeostasis and its export following activity-induced influx2. In presynaptic vesicles, a vacuolar-type H+-ATPase (v-ATPase) creates the proton gradient necessary for neurotransmitter uptake into this compartment3.
While activity of neurons thus requires a substantial amount of ATP4, they do not exhibit a significant capacity for storage of energy. Instead, they seem to rely on metabolic interactions with neighboring astrocytes, the major glycogen stores in the brain5. It has been suggested that astrocytic glycogen indeed plays an important role in supporting neuronal energy needs; and a key phenomenon in this proposed neuro-metabolic coupling between the two cell types is the capacity of astrocytes to increase their ATP production in response to neuronal activity6,7,8. This hypothesis, known as astrocyte-neuron lactate shuttle (ANLS), is still under debate, because other work has provided evidence that neurons may also increase their own rate of glycolysis in response to stimulation9,10, reflecting the necessity of further methods and approaches to study neuro-glia interaction.
Investigation of cellular energy metabolism and ATP levels in neurons and astrocytes to elucidate neuro-glia metabolic interactions has long been hampered by the lack of suitable probes for the detection of changes in metabolite concentrations in living cells in brain tissue. The last decade, however, has provided a surge in the development of new tools and new genetically encoded fluorescent probes for different metabolites, including sensors for ATP, lactate, pyruvate and others11,12. Using these tools, it is now possible to directly address questions related to cellular ATP consumption and changes in cellular energy levels at the single cell level and in a cell-type specific manner in intact brain tissue13.
In the present work, we describe a procedure to visualize cytosolic ATP dynamics on neurons and astrocytes of cultured organotypic brain slices. We show how to employ adeno-associated viral vectors (AAV) for cell-type specific expression of the genetically encoded ATP-nanosensor ATeam1.03YEMK (14) in neurons and astrocytes of slices of mouse brain that can be maintained in cell culture for several weeks15. A procedure of how to remove the glial scar that covers cultured tissue slices is described, which improves optical accessibility and imaging of cells in the organotypic tissue layers underneath. Finally, we show how ATeam1.03YEMK can be used to perform FRET-based imaging of changes in cellular ATP levels in this preparation. This method hosts the major advantages that it does not require surgical brain procedures, provides high levels of expression of the sensor and cell type specificity in cultured brain slices, reducing invasiveness or stress in the cells as compared with other methods, like transfection by electroporation or transduction with other viral vectors10,16,17. In addition, this protocol can be applied to other FRET based nanosensors, among them variants of ATeam1.03 that provide lower binding affinity for ATP14.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>2-deoxyglucose</td>
			<td>Alfa Aesar</td>
			<td>L07338</td>
			<td>Non-metabolizable glucose analog</td>
		</tr>
		<tr>
			<td>36-IMA-410-019 Argon laser</td>
			<td>Melles Griot</td>
			<td></td>
			<td>488 nm wavelength argon</td>
		</tr>
		<tr>
			<td>Ascorbic acid</td>
			<td>Carl Roth</td>
			<td>3525.1</td>
			<td>Antioxidant, Vitamin C</td>
		</tr>
		<tr>
			<td>band pass filters 483/32</td>
			<td>AHF Analysentechnik AG</td>
			<td></td>
			<td>Splitter compatible emmision filter</td>
		</tr>
		<tr>
			<td>band pass filters 542/27</td>
			<td>AHF Analysentechnik AG</td>
			<td></td>
			<td>Splitter compatible emmision filter</td>
		</tr>
		<tr>
			<td>Beamsplitter T 455 LP</td>
			<td>AHF Analysentechnik AG</td>
			<td></td>
			<td>Excitation dichroic mirror</td>
		</tr>
		<tr>
			<td>Beamsplitter T 505 LPXR</td>
			<td>AHF Analysentechnik AG</td>
			<td></td>
			<td>Splitter dichroic</td>
		</tr>
		<tr>
			<td>Confocal laser scannig microscope C1</td>
			<td>Nikon Microscope Solutions</td>
			<td></td>
			<td>Modular confocal microscope system C1</td>
		</tr>
		<tr>
			<td>Data processing Origin Pro 9.0.0 (64-bit)</td>
			<td>OriginLab corporation</td>
			<td></td>
			<td>Scientific graphing and data analysis software</td>
		</tr>
		<tr>
			<td>D-glucose monohydrate</td>
			<td>Caelo</td>
			<td>2580-1kg</td>
			<td></td>
		</tr>
		<tr>
			<td>DPBS</td>
			<td>GIBCO/Life</td>
			<td>14190250</td>
			<td>Dulbecco&#39;s phosphate-buffered saline</td>
		</tr>
		<tr>
			<td>Eclipse E 600FN upright microscope</td>
			<td>Nikon Microscope Solutions</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Eclipse FN1 upright microscope</td>
			<td>Nikon Microscope Solutions</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Experimental chamber</td>
			<td>custom build</td>
			<td></td>
			<td>Perfusion chamber for live-cell imaging</td>
		</tr>
		<tr>
			<td>EZ-C1 Silver Version 3.91</td>
			<td>Nikon Microscope Solutions</td>
			<td></td>
			<td>Imaging software for confocal microscope</td>
		</tr>
		<tr>
			<td>Hanks&#39; Balanced Salt solution</td>
			<td>Sigma-Aldrich</td>
			<td>H9394</td>
			<td>With Phenol Red for pH monitoring</td>
		</tr>
		<tr>
			<td>HERAcell 150</td>
			<td>Thermo Scientific</td>
			<td></td>
			<td>CO2 incubator HERAcell &#174; 150 with decontamination routine</td>
		</tr>
		<tr>
			<td>HERAsafe KS/KSP</td>
			<td>Thermo Scientific</td>
			<td></td>
			<td>Safety Cabinet</td>
		</tr>
		<tr>
			<td>Horse serum</td>
			<td>GIBCO/Life</td>
			<td>26050088</td>
			<td>Heat inactivated</td>
		</tr>
		<tr>
			<td>Huygens Professional</td>
			<td>SVI Imaging</td>
			<td></td>
			<td>Deconvolution software</td>
		</tr>
		<tr>
			<td>Image J 1.52i</td>
			<td>Wayne Rasban national Institute of Health</td>
			<td></td>
			<td>Image processing Software available in the public domain</td>
		</tr>
		<tr>
			<td>Insulin</td>
			<td>Sigma-Aldrich</td>
			<td>I6634</td>
			<td>Insulin from bovine pancreas</td>
		</tr>
		<tr>
			<td>IP serie peristaltic pump</td>
			<td>Ismatec</td>
			<td></td>
			<td>High-precisionmulti-channel pump</td>
		</tr>
		<tr>
			<td>Layout software, Illustrator CS6</td>
			<td>Adobe</td>
			<td></td>
			<td>Vector graphics editor</td>
		</tr>
		<tr>
			<td>L-glutamine</td>
			<td>GIBCO/Life</td>
			<td>25030024</td>
			<td></td>
		</tr>
		<tr>
			<td>Microm HM 650 V</td>
			<td>Thermo Scientific</td>
			<td></td>
			<td>Vibration microtome. Thermo scientific discontinued the production of the device in the meantime. Any other slicer or tissue chopper siutable for slicing living tissue is fine, too.</td>
		</tr>
		<tr>
			<td>Microscope stage</td>
			<td>custom build</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Microsoft Excel 16</td>
			<td>Microsoft</td>
			<td></td>
			<td>Spreadsheet software for basic data processing</td>
		</tr>
		<tr>
			<td>Millicell culture insert</td>
			<td>Merck Millipore</td>
			<td>PICM0RG50</td>
			<td>Hydrophilized PTFE, pore size 0.4 &#956;m</td>
		</tr>
		<tr>
			<td>Minimum Essential Medium Eagle</td>
			<td>Sigma-Aldrich</td>
			<td>M7278</td>
			<td>Synthetic cell culture media</td>
		</tr>
		<tr>
			<td>Monochromator Polychrome V</td>
			<td>Thermo Scientific/FEI</td>
			<td></td>
			<td>Ultra fast switching monochromator</td>
		</tr>
		<tr>
			<td>NaN3 (Sodium Azide)</td>
			<td>Sigma-Aldrich</td>
			<td>S-8032</td>
			<td>Mitochondrial inhibitor (complex IV inhibitor). CAUTION: Azide is toxic. Be aware not to accidentally ingest or inhale it, and prevent ist absoption through the skin.</td>
		</tr>
		<tr>
			<td>Nikon Fluor 40x / 0.80 W DIC M &#8734;/0 WD 2.0</td>
			<td>Nikon Microscope Solutions</td>
			<td></td>
			<td>Water Immersion Microscope Objective</td>
		</tr>
		<tr>
			<td>NIS Elements 4.50 advanced Research</td>
			<td>Nikon Microscope Solutions</td>
			<td></td>
			<td>Imaging software. Upgraded version for FRET imaging</td>
		</tr>
		<tr>
			<td>ORCA-Flash4.0</td>
			<td>Hamamatsu Photonics</td>
			<td></td>
			<td>Digital CMOS camera</td>
		</tr>
		<tr>
			<td>Perfusion tubing</td>
			<td>Pro Liquid GmbH</td>
			<td></td>
			<td>Tygon tubing, 1.52 x 322 mm (Wd: 0.85)</td>
		</tr>
		<tr>
			<td>Photoshop CS 6 Version 13.0</td>
			<td>Adobe</td>
			<td></td>
			<td>Image processing software</td>
		</tr>
		<tr>
			<td>Sodium L-lactate</td>
			<td>Sigma-Aldrich</td>
			<td>71718-10G</td>
			<td></td>
		</tr>
		<tr>
			<td>ssAAV-2/2-hSyn1-ATeam1.03YEMK-WPRE-hGHp(A)</td>
			<td>ETH Z&#252;rich</td>
			<td>v244</td>
			<td>Single-stranded AAV vector that induces the expression of ATeam1.03YEMK under the control of the human synapsin 1 promoter fragment hSyn1.</td>
		</tr>
		<tr>
			<td>ssAAV-5/2-hGFAP-hHBbI/E-ATeam1.03YEMK-WPRE-bGHp(A)</td>
			<td>ETH Z&#252;rich</td>
			<td>v307</td>
			<td>Single-stranded AAV vector that induces the expression of ATeam1.03YEMK under the control of the human glial fibrillary acidic protein promoter fragment ABC1D.</td>
		</tr>
		<tr>
			<td>WVIEW GEMINI optic system</td>
			<td>Hamamatsu Photonics</td>
			<td></td>
			<td>Emission Image Splitter</td>
		</tr>
	</tbody>
</table>

imaging of intracellular atp in organotypic tissue slices of the mouse brain using the fret-based sensor ateam1.03yemk

Neuronal activity in the central nervous system (CNS) evokes a high demand on cellular energy provided by the breakdown of adenosine triphosphate (ATP). A large share of ATP is needed to re-install ion gradients across plasma membranes degraded by electrical signaling of neurons. There is evidence that astrocytes - while not generating fast electrical signals themselves - undergo increased production of ATP in response to neuronal activity and support active neurons by providing energy metabolites to them. The recent development of genetically encoded sensors for different metabolites now enables the study of such metabolic interactions between neurons and astrocytes. Here, we describe a protocol for cell-type specific expression of the ATP-sensitive Fluorescence Resonance Energy Transfer- (FRET-) sensor ATeam1.03YEMK in organotypic tissue slice cultures of the mouse hippocampus and cortex using adeno-associated viral vectors (AAV). Furthermore, we demonstrate how this sensor can be employed for dynamic measurement of changes in cellular ATP levels in neurons and astrocytes upon increases in extracellular potassium and following induction of chemical ischemia (i.e., an inhibition of cellular energy metabolism).

AAV vectors are a reliable tool to selectively express foreign genes in cells within living tissue16. Direct application of AAVs containing the sequence cassette of ATeam1.03YEMK and a specific promoter results in a high expression of the sensor in the chosen cell type. At DIV 14 (~10 days after a transduction), neurons expressing ATeam under the human synapsin promoter are found at high density in the neocortex of cultured tissue slices at depths of up to 50 &#181;m below the slice surface (Figure 5A). Comparable results can be achieved in the hippocampus (Figure 5B).
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60294/60294fig5a.jpg" /> 
Figure 5: Visualization of neurons expressing ATeam1.03YEMK in cultured parasagittal organotypic brain slices. Images on the left correspond to extended focus projections of 43 optical sections (1.05 &#181;m each) of cortical tissue (A) and (B) of 70 optical sections (0.6 &#181;m each) of hippocampal tissue. Images on the right represent the volume view of the same projection. Cells are color-coded according to their depth relative to the slice surface as indicated by the color scale on the right. <a href="https://www.jove.com/files/ftp_upload/60294/60294fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
For the measurement of ATP levels in astrocytes, ATeam1.03YEMK is expressed under the control of the human glial fibrillary acidic protein (GFAP) promoter. This results in efficient transduction of cells in both neocortex and hippocampus of cultured tissue slices (Figure 6). Notably, two different morphological phenotypes can be distinguished, depending on the depth relative to the surface of slice preparations. In the first, superficial layer, cells are characterized by thick primary processes that are predominately arranged in parallel to the surface. These cells exhibit strongly overlapping domains, creating a dense meshwork of apparently reactive astrocytes (Figure 6). In deeper layers (30-60 &#181;m from the surface), transduced astrocytes exhibit fine cellular processes that form largely spherical domains and their morphology resembles that of astrocytes in situ as reported earlier19,20,21 (Figure 6). To obtain better transduction of deeper-layer astrocytes as well as better optical access to these deeper layers, the glial scar tissue can be removed as described in Step 4.
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/60294/60294fig6a.jpg" /> 
Figure 6: Visualization of astrocytes expressing ATeam1.03YEMK in cultured parasagittal organotypic brain slices. The image on the left corresponds to an extended focus projection of 191 optical sections (0.45 &#181;m each). For illustration purposes, the glial scar was excluded from the projection of astrocytes. Images on the right represent the volume view of the same projection before and after removal of the glial scar. Cells are color-coded according to their depth relative to the slice surface as indicated by the color scales on the right. <a href="https://www.jove.com/files/ftp_upload/60294/60294fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Successful expression of ATeam1.03YEMK allows the dynamic measurement of changes in ATP levels in neurons or astrocytes, depending on the promoter used (see above). Experiments were performed in an experimental bath constantly perfused with E-ACSF (bubbled with 95% O2/5% CO2). In organotypic slices expressing ATeam1.03YEMK in hippocampal neurons, regions of interest (ROIs) were selected before starting the recording, representing the somata of pyramidal cells (Figure 7A). Moreover, a region for background subtraction was chosen (Figure 7A). Emission of Venus as well as eCFP fluorescence was then collected for each of these ROIs separately and depicted as fluorescence emission level over time (Figure 7B). After recording fluorescence under control conditions for several minutes to ensure a stable baseline, cellular metabolism was inhibited by exposing the slice preparation to a glucose-free saline, to which 5 mM sodium azide (NaN3) was added for one minute (Figure 7B). This manipulation induced opposite changes in the emission intensity of the FRET pair (Figure 7B, left panels), with a decrease of Venus (527 nm) and an increase of eCFP (475 nm) emission. Calculating the FRET ratio by dividing the fluorescence emission of Venus by that of eCFP (FVenus/FeCFP) resulted in signals that reflect the relative changes in the intracellular ATP levels, the so-called &#34;ATeam FRET ratio&#34; (Figure 7B, right panel). In all recorded neurons (n = 70 cells in N = 5 slices), NaN3 caused a reversible decrease in the ATeam FRET ratio, indicating a reversible decrease in intracellular ATP levels upon inhibition of cellular metabolism.
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/60294/60294fig7a.jpg" /> 
Figure 7: Demonstration of time lapse ATeam FRET ratio imaging. (A) Top left: Wide-field fluorescence image of the pyramidal layer and stratum radiatum of the CA1 region of a cultured organotypic hippocampal slice expressing ATeam1.03YEMK in neurons. Top right: Enlarged view of boxed section as indicated on the left. White lines delineate regions of interest (ROIs) 1-3 representing cell bodies of CA1 pyramidal neurons chosen for analysis in (B). BG represents the ROI chosen for background correction. Bottom: Pseudo-colored images representing fluorescence emission of Venus (green), eCFP (purple) and the ratio of Venus/eCFP. (B) Time lapse recording in ROIs 1-3, representing neuronal cell bodies (see A). Traces on the left show normalized fluorescence emission of Venus (green) and eCFP (magenta). Traces on the right show the corresponding ATeam FRET ratio. Note that perfusion with 5 mM NaN3 in the absence of extracellular glucose for 1 minute (grey bar) induces a reversible decrease in the ATeam FRET ratio, indicating a decrease in intracellular ATP concentration. <a href="https://www.jove.com/files/ftp_upload/60294/60294fig7alarge.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
To ensure the stability of the preparation and the sensor under long-term experiment conditions, slices expressing ATeam in neurons or astrocytes were constantly perfused with ACSF for prolonged periods (&#62;50 min; n = 12 cells each, N = 3 OTCs from 3 brains). Under these conditions, the ATeam FRET ratio did not change (Figure 8). Exposing the preparations to CIS containing metabolic inhibitors (&#34;Chemical ischemia&#34;, see Table 1), in contrast, again resulted in the expected drop in the ATeam FRET ratio as observed above.
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/60294/60294fig8a.jpg" /> 
Figure 8: Baseline experiments employing ATeam. Long-term ATeam FRET ratio imaging in 14 different cells under baseline conditions in neurons (top) and astrocytes (bottom). Data were taken under comparable conditions as other experimental data. At the end of each measurement, chemical ischemia was elicited by perfusion with CIS as indicated by the arrow. NOTE: Baseline ATeam FRET ratios are stable over time under baseline conditions. <a href="https://www.jove.com/files/ftp_upload/60294/60294fig8large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Next, we analyzed the responses of neurons and astrocytes expressing ATeam1.03YEMK to an increase in the extracellular potassium concentration. After establishing a stable baseline, neurons were perfused with a saline in which the potassium concentration was increased from 3 to 8 mM for 3 minutes (Figure 9A). This manipulation did, however, not result in a detectable change in the ATeam FRET ratio (n = 56 cells in N = 5 slices). To ensure that the sensor reacted to a change in ATP levels, slices were then again exposed to a sustained period of chemical ischemia elicited by replacing E-ACSF by CIS. Chemical ischemia resulted in a rapid decrease in the ATeam FRET ratio to a new, stable level, indicating nominal depletion of ATP after 2-3 min (Figure 9A).
<img alt="Figure 9" class="xfigimg" src="/files/ftp_upload/60294/60294fig9a.jpg" /> 
Figure 9: Representative experiments illustrating changes in ATP levels in neurons and astrocytes. (A,B): Images on the left show ATeam fluorescence from neurons and astrocytes located in the hippocampal CA1 region of organotypic slices. Traces on the right represent time lapse recordings of the ATeam FRET ratio obtained from a ROI positioned over a single cell body. In both experiments, slices were first subjected to an increase in the extracellular potassium concentration for 3 minutes (see bar), followed by a final exposure to chemical ischemia. Note that while neurons do not respond to the elevation of extracellular potassium (A), astrocytes react with an increase in ATP (B). <a href="https://www.jove.com/files/ftp_upload/60294/60294fig9large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
The same experimental protocol was performed with slices, in which ATeam1.03YEMK was expressed in astrocytes. In contrast to what was observed in neurons, astrocytes reacted to the increase in extracellular potassium by a reversible increase in the ATeam FRET ratio, indicating an increase in intracellular ATP levels (n = 70 cells in N = 5 slices) (Figure 9B). Subsequent exposure to chemical ischemia resulted, as expected, in a large drop in the ATeam FRET ratio, indicative of nominal depletion of intracellular ATP (Figure 9B).

We describe a protocol for cell-type specific expression of the genetically encoded FRET-based sensor ATeam1.03YEMK in organotypic slice cultures of the mouse forebrain. Furthermore, we show how to use this sensor for dynamic imaging of cellular ATP levels in neurons and astrocytes.

Imaging of Intracellular ATP in Organotypic Tissue Slices of the Mouse Brain using the FRET-based Sensor ATeam1.03YEMK

Neuronal activity in the central nervous system (CNS) evokes a high demand on cellular energy provided by the breakdown of adenosine triphosphate (ATP). A ...

Here we demonstrate how to employ the ATP-sensitive FRET sensor ATeam 1.03 YEMK for dynamic measurements of changes in neuronal and astrocytic ATP levels in organotypically cultured mouse hippocampal slices. The main advantage of this technique is that the one can study energy metabolism in living brain tissue in a controlled setting, an environment with high sensor expression levels. This technique may help in gaining insights into pathomechanisms, underlying diseases, or brain damage, for example, caused by energy deprivation, like ischemic stroke.It can, in principle, be used to study ATP levels, not only in brain tissue, but in other organs as well. To successfully run this experiment, it is essential to perform all steps involved in culturing the tissue extremely careful and to maintain sterility. Moreover, some knowledge of cellular fluorescent imaging is required.Tissue handling in most sophisticated approach and a visualization is vital to understand the proper procedures. The same is true for imaging experiments. Demonstrating the procedure will be Rodrigo Lerchundi, a postdoc, and Na Huang, a Master student from the laboratory.In an ice-cold Petri dish filled with ACSF, place the brain on a filter membrane. Separate the hemispheres and perform a parasagittal cut at an angle of 45 degrees. Fix one hemisphere at the Vibratome tissue stage with superglue, and immediately transfer the tissue block to the Vibratome bath containing ice-cold ACSF bubbled with 5%carbon dioxide and 95%oxygen.Align the tissue in the Vibratome bath. Keep the second hemisphere in ice-cold ACSF until slicing. Adjust the Vibratome to cut slices at 250 to 400 micrometers.After cutting the slice, identify the hippocampal formation based on its typical morphological appearance and isolate it using hypodermic needles, keeping the part of the cerebral cortex adjacent to the hippocampus. Place the slice on a mesh in ACSF, warmed to 34 degrees Celsius and bubbled with 5%carbon dioxide and 95%oxygen until all the slices are collected. Transfer the slices to the laminar flow cabinet to continue under sterile conditions.With an inverted, sterile, glass Pasteur pipette, gently transfer the slices from the ACSF into one of the pre-warmed Petri dishes filled with sterile Hanks'salt solution. Change the pipette and transfer the slices to the second culture dish. Repeat the process five times overall.Transfer as little HBSS as possible to the following culture plates. Using a pipette, gently place one slice at a time on the top of the culture insert. Avoid turbulences in the pipette, and wait until the slice descends to the tip of the Pasteur pipette.Repeat the process for each slice. Place two slices on a membrane. Carefully remove any excess Hank solution from the top of the insert by using a fine tip.Keep the cultures in a humidified incubator at 5%carbon dioxide and 37 degrees Celsius until the day of the experiment. Replace the medium every two to three days. Without touching the tissue, apply 0.5 microliters of the diluted vector directly to the top of each slice.Finally, place the slices back into the incubator and maintain them there for at least six more days. Just before starting an experiment, transfer an insert containing cultured slices into the sterile hood, and place it into a 30 millimeter dish containing one milliliter of organotypic slice culture medium, or minimal essential medium. Place the dish under the stereoscope and focus onto the surface of the slice.Use two sterile hypodermic needles to make a short cross-cut right on the narrow edges of a chosen slice, and in the upper layer without damaging the tissue underneath. To remove the prepared slice from the insert, hold the edges of the membrane with tweezers and use a sterile scalpel to make straight, parallel cuts to the membrane, forming a square or a triangle with the slice in the center. If the insert hosts additional slices, transfer it back to the original plate and into the incubator.The surface tension of the medium will prevent its leakage onto the surface of the membrane. Prepare experimental ACSF and obtain a pH of 7.4 by bubbling it with 95%oxygen and 5%carbon dioxide through an inserted tubing connected to the carbogen supply for at least thirty minutes. Keep the saline bubbled during the entire experiment.Then switch on the fluorescent light source of the monochrometer. Transfer the organotypic slice culture into the experimental chamber. Place a grid on top of the organotypic slice culture with the frame down, not touching the culture, and the threads up, touching the membrane.Place the chamber on the microscope stage and connect it to the perfusion system. Switch on the peristaltic pump at a flow rate of 1.5 to 2.5 milliliters per minute. Make sure there is no leaking of the perfusion system.Using transmission light, bring the cultured slice into focus and identify the area where experiments shall be performed. Before starting imaging experiments, wait at least 15 minutes to allow slices to adapt to the saline conditions, then switch on the camera and the imaging software. Excite the donor fluorescent protein at 435 nanometers.Set the exposure time to between 40 to 90 milliseconds. Then insert the dichroic mirror and the filters into the beam splitter unit. Split the fluorescence emission at 500 nanometers with an emission image splitter, and employ band pass filters at 482 plus or minus 16 and 542 plus or minus 13.5 nanometers to further isolate donor and acceptor fluorescence.Select a region of interest apparently devoid of cellular fluorescence for background subtraction. Circle single structures of labeled tissue in the image on the screen to create ROIs. Set the frequency of image acquisition and the overall recording time.For experiments longer than 30 minutes, an acquisition frequency of 0.2 to 0.5 Hertz is recommended to prevent phototoxicity. Subsequently, start the recording. To induce changes in intracellular ATP, switch the perfusion tube from standard ACSF to a saline containing metabolic inhibitors, for example, chemical ischemia solution.Alternatively, use a saline with elevated potassium concentration at eight millimolar to mimic release of potassium ion from active neurons. Directly after the recordings, exchange the experimental ACSF with heaps-buffered ACSF. Then transfer the recording chamber containing the slice culture to the confocal laser scan microscope.Take Z stack images at the highest Z resolution possible at the given optical configuration. In this protocol, ten days after a transduction, neurons expressing ATeam 1.03 YEMK were found at high density in the neocortex of cultured tissue slices at depths of up to 50 micrometers below the slice surface. Comparable results were achieved in the hippocampus.For astrocytes, ATeam 1.03 YEMK was expressed under the control of the human glial fibrolary acidic protein promoter, and organotypic slices expressing ATeam 1.03 YEMK in the hippocampal neurons selected ROIs represent the somata of pyramidal cells. After exposing the slice to 5 millimolar sodium azide in the absence of extracellular glucose for one minute, opposite changes were induced in the emission intensity of the FRET pair. A reversible decrease in the ATeam FRET ratio was also observed.Long-term ATeam FRET ratio in 14 different cells under baseline conditions in neurons and astrocytes show that the ATeam is a reliable and stable sensor. Please note that the chemical ischemia resulted in the expected strong drop in the ATeam ratio in both cell types at the end of this experiment. An increase in the extracellular potassium concentration from three to eight millimolar for three minutes did not result in a detectable change in the neurons expressing ATeam 1.03 YEMK.In contrast, astrocytes reacted to the increase in extracellular potassium by a reversible increase in the ATeam FRET ratio, indicating an increase in intracellular ATP levels. Again, chemical ischemia caused an immediate decrease in the ATeam ratio, demonstrating that the sensor can detect changes in ATP. When attempting FRET-based cellular imaging as described here, it's important to keep in mind that the production of high-quality slices in organotypic cultures is a key step.Moreover, careful removal of the glial scar and expert-level imaging are required. Following this procedure, one should also be able to perform experiment incorporating different other FRET-based nanosensor for cellular metabolites. For example, those for glucose or lactate.Last, but not least, it needs to be emphasized that relevant acts for protection of animals must always be observed. This is also true for the effective laws governing the handling of genetically modified organisms.

Research

JoVE Journal

Neuroscience

Time-lapse Imaging of Neuroblast Migration in Acute Slices of the Adult Mouse Forebrain

There is a substantial body of evidence indicating that new functional neurons are constitutively generated from an endogenous pool of neural stem cells ...

Imaging Calcium Responses in GFP-tagged Neurons of Hypothalamic Mouse Brain Slices

Despite an enormous increase in our knowledge about the mechanisms underlying the encoding of information in the brain, a central question concerning the ...

Paired Whole Cell Recordings in Organotypic Hippocampal Slices

Pair recordings involve simultaneous whole cell patch clamp recordings from two synaptically connected neurons, enabling not only direct ...

Regioselective Biolistic Targeting in Organotypic Brain Slices Using a Modified Gene Gun

Transfection of DNA has been invaluable for biological sciences and with recent advances to organotypic brain slice preparations, the effect of various ...

Imaging Serotonergic Fibers in the Mouse Spinal Cord Using the CLARITY/CUBIC Technique

Long descending fibers to the spinal cord are essential for locomotion, pain perception, and other behaviors. The fiber termination pattern in the spinal ...

In-depth Physiological Analysis of Defined Cell Populations in Acute Tissue Slices of the Mouse Vomeronasal Organ

In most mammals, the vomeronasal organ (VNO) is a chemosensory structure that detects both hetero- and conspecific social cues. Vomeronasal sensory ...

Time-lapse Confocal Imaging of Migrating Neurons in Organotypic Slice Culture of Embryonic Mouse Brain Using In Utero Electroporation

In utero electroporation is a rapid and powerful approach to study the process of radial migration in the cerebral cortex of developing mouse embryos. It ...

Recording Synaptic Plasticity in Acute Hippocampal Slices Maintained in a Small-volume Recycling-, Perfusion-, and Submersion-type Chamber System

Even though experiments on brain slices have been in use since 1951, problems remain that reduce the probability of achieving a stable and successful ...

Visualization and Live Imaging of Oligodendrocyte Organelles in Organotypic Brain Slices Using Adeno-associated Virus and Confocal Microscopy

Neurons rely on the electric insulation and trophic support of myelinating oligodendrocytes. Despite the importance of oligodendrocytes, the advanced ...

Two-photon Imaging of Microglial Processes' Attraction Toward ATP or Serotonin in Acute Brain Slices

Microglial cells are resident innate immune cells of the brain that constantly scan their environment with their long processes and, upon disruption of ...