Name: imaging ca2+ responses during shigella infection of epithelial cells
Uploaded: 2018-05-24T14:45:00
Description: Watch this Scientific Journal Video about Imaging Ca2+ Responses During Shigella Infection of Epithelial Cells at JoVE.com

We thank Jenny-Lee Thomassin for her help in editing the manuscript. The work was supported by the ANR grants MITOPATHO and PATHIMMUN, grants from the Labex Memolife and PSL IDEX Shigaforce. Chunhui Sun is a recipient of a Ph.D. grant from the China Scholarship Council. Laurent Combettes and Guy Tran Van Nhieu are recipients of a WBI-France exchange Tournesol program N&#176;31268YG (Wallonie-Bruxelles International, Fonds de la Recherche Scientifique, Minist&#232;re Fran&#231;ais des Affaires &#233;trang&#232;res et europ&#233;ennes, Minist&#232;re de l&#39;Enseignement sup&#233;rieur et de la Recherche dans le cadre des Partenariats Hubert Curien).

1. Preparations
<ol>
	<li>Bacteria preparation
	<ol>
		<li>Plate the bacteria&#8212;Shigella wild-type strain expressing the AfaE adhesin (M90T-AfaE)&#8212;onto a trypticase soy (TCS) agar plate containing 0.01% Congo red (CR) and incubate them for 18 h at 37 &#176;C. 
		Note: To increase their reproducibility, the Shigella plates obtained in step 1.1.1 are stored at 4 &#176;C and should be used within a week, because all colonies on CR medium will eventually turn red over time.<...

<ol>
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This manuscript describes the protocol that we engineered to follow local Ca2+ signals during the relatively short kinetics of a Shigella invasion, as well as global Ca2+ responses during the extended kinetics of Shigella. Below, key issues can be found that need to be addressed to optimize the detection of Ca2+ signals while minimizing any interference with the biological processes.
Chemical vs. genetically encoded Ca2+ probe...

Ca2+ regulates all known cell processes, including cytoskeletal reorganization, inflammatory responses, and cell death pathways related to host-pathogen interactions1,2,3. Under physiological conditions, basal cytosolic Ca2+ concentrations are low, in the hundreds of nM range, but can be subjected to transient increases upon agonist stimulation. These variations often show oscillatory behavior through the action of pumps and channels at the plasma and endoplasmic reticulum membranes. The pattern of these oscillations is characterized by the period, duration, and amplitude of Ca2+ increases, and is decrypted by cells which, in turn, trigger specific responses in what is known as the Ca2+ code4,5. A sustained increase in the cytosolic Ca2+ concentration under pathological conditions may lead to cell death associated with the permeabilization of mitochondrial membranes and the release of pro-apoptotic or necrotic factors6,7.
Shigella, the causative agent of bacillary dysentery, invades epithelial cells by injecting effectors into host cells using a type III secretion system (T3SS)8,9. A Shigella invasion of host cells is associated with local and global Ca2+ signals elicited by the T3SS. As for pore-forming toxins, the T3SS translocon that inserts into host cell membranes and is required for the injection of T3SS effectors is likely responsible for the activation of PLC and the inositol (1, 4, 5) trisphosphate (InsP3)-dependent Ca2+ release. The combination of the localized PLC stimulation and the accumulation of polymerized actin at sites of a Shigella invasion result in an atypically long-lasting InsP3-dependent Ca2+ release10. The type III effector IpgD, a phosphatidyl 4,5 bisphosphate (PIP2)-4-phosphatase, limits the local amount of PIP2, thereby controlling the quantity of available substrate for PLC to generate InsP3, which contributes to the confinement of local Ca2+ responses at bacterial invasion sites11,12. These local Ca2+ responses likely contribute to the actin polymerization at Shigella invasion sites10. Global Ca2+ responses that are also elicited by Shigella, however, are dispensable for the bacterial invasion process but trigger the opening of connexin hemichannels at the plasma membrane and the release of ATP in the extracellular compartment. Released ATP acting in a paracrine manner, in turn, stimulates Ca2+ oscillatory responses in cells next to the infected cell. IpgD is also responsible for shaping global Ca2+ responses into erratic isolated responses with slow dynamics. Eventually, upon a prolonged bacterial infection, IpgD leads to the inhibition of InsP3-mediated Ca2+ signals. Through its interference with Ca2+ signaling, IpgD delays a Ca2+-dependent calpain activation leading to the disassembly of focal adhesion structures and the premature detachment of infected cells13.
While Ca2+ signals are involved in critical aspects of pathogenesis, the use of a microorganism raises a number of technical challenges that are not encountered in classical agonist studies. The protocols described here use the commonly used fluorescent Ca2+ chemical indicator Fluo-4 that we engineered to characterize local Ca2+ signals during a Shigella infection. Steps critical for the detection of these signals are discussed, as well as procedures implemented for their quantitative analysis that is required to characterize the role of bacterial effectors in Ca2+ signaling.

<table border="0" cellpadding="0" cellspacing="0" style="width:693px;" width="694">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Fluo-4 AM</td>
			<td style="width:120px;">Invitrogen</td>
			<td style="width:136px;">F14201</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:247px;">Metamorph version 7.7</td>
			<td style="width:120px;">Universal Imaging</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">CoolLED illumination system pE-2</td>
			<td style="width:120px;">Roper Scientific</td>
			<td style="width:136px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">micro-dish 35 mm, high&#160;</td>
			<td style="width:120px;">IBIDI</td>
			<td>81156</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Trypticase Soy (TCS) broth</td>
			<td style="width:120px;">Thermofisher</td>
			<td style="width:136px;"></td>
			<td>B11768</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">TCS agar</td>
			<td style="width:120px;">Thermofisher</td>
			<td style="width:136px;"></td>
			<td>B11043</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Congo red</td>
			<td style="width:120px;">Sigma-Aldrich</td>
			<td style="width:136px;"></td>
			<td>75768</td>
		</tr>
		<tr height="85">
			<td height="85" style="height:85px;width:247px;">M90T-AfaE</td>
			<td style="width:120px;">Sun et al. 2017</td>
			<td style="width:136px;">Shigella flexneri serotype V. expressing the AfaE adhesin</td>
			<td></td>
		</tr>
		<tr height="85">
			<td height="85" style="height:85px;width:247px;">ipgD-AfaE</td>
			<td style="width:120px;">Sun et al. 2017</td>
			<td style="width:136px;">isogenic ipgD mutant strain expressing the AfaE adhesin</td>
			<td></td>
		</tr>
	</tbody>
</table>

imaging ca2+ responses during shigella infection of epithelial cells

Ca2+ is a ubiquitous ion involved in all known cellular processes. While global Ca2+ responses may affect cell fate, local variations in free Ca2+ cytosolic concentrations, linked to release from internal stores or an influx through plasma membrane channels, regulate cortical cell processes. Pathogens that adhere to or invade host cells trigger a reorganization of the actin cytoskeleton underlying the host plasma membrane, which likely affects both global and local Ca2+ signaling. Because these events may occur at low frequencies in a pseudo-stochastic manner over extended kinetics, the analysis of Ca2+ signals induced by pathogens raises major technical challenges that need to be addressed.
Here, we report protocols for the detection of global and local Ca2+ signals upon a Shigella infection of epithelial cells. In these protocols, artefacts linked to a prolonged exposure and photodamage associated with the excitation of Ca2+ fluorescent probes are troubleshot by stringently controlling the acquisition parameters over defined time periods during a Shigella invasion. Procedures are implemented to rigorously analyze the amplitude and frequency of global cytosolic Ca2+ signals during extended infection kinetics using the chemical probe Fluo-4.

Shigella invasion is associated with atypical long-lasting local Ca2+ responses:
Following the protocol mentioned above, Fluo-4-loaded HeLa cells were challenged with WT Shigella and stream acquisitions were performed to analyze Ca2+ signals. A representative experiment is shown in Figure 1, with time-lapse images series of the fluorescence intensity of the Fluo-...

Watch this Scientific Journal Video about Imaging Ca2+ Responses During Shigella Infection of Epithelial Cells at JoVE.com

Imaging Ca2+ Responses During Shigella Infection of Epithelial Cells

Here, we present protocols to visualize calcium (Ca2+) responses elicited by HeLa cells infected by Shigella. By optimizing the parameters of bacterial infection and imaging with Ca2+ fluorescent probes, atypical global and local Ca2+ signals induced by bacteria over a large range of infection kinetics are characterized.

Imaging Ca2+ Responses During Shigella Infection of Epithelial Cells

Ca2+ is a ubiquitous ion involved in all known cellular processes. While global Ca2+ responses may affect cell fate, local variations in free Ca2+ ...

The overall goal of this fluorescent microscopy analysis using chemical probes is to analyze local and global calcium responses elicited by host cells during infection by pathogenic Shigella. This method can help answer key questions in the field of cellular microbiology such as pathogen-induced signaling leading to cytoskeletal reorganization or cell death. The main advantage of this technique is that by using chemical probes one can follow local calcium signals associated with local processes diverted by the pathogen.Generally individuals new to this method will struggle because of the setting of parameters that enable to follow local calcium signals while preserving the integrity of biological samples. To begin, plate wild-type Shigella expressing the AfaE adhesion onto a trypticase soy or TCS agar plate containing 0.01%Congo red and incubate the plates at 37 degrees Celsius for 18 hours. Prepare pre-cultures from the streaked plates by picking three red colonies and inoculating TCS broth with 75 micrograms per milliliter of ampicillin.Grow the liquid cultures in a shaking incubator at 37 degrees Celsius and 200 rpm for 16 hours. Next, inoculate TCS with the bacterial pre-culture at a 1:100 dilution. Incubate the culture at 37 degrees Celsius in a shaking incubator at 200 rpm for two hours.Ensure that the optical density at 600 nanometers is 0.2 to 0.4. Centrifuge the culture at 13, 000 g and 21 degrees Celsius for two minutes and re-suspend the pellet in an equivalent volume of EM medium. Dilute the bacterial suspension in EM buffer at a final OD 600 of 0.1 and use it immediately or store it at 21 degrees Celsius and use it within the next 60 minutes.Grow HeLa cells in DMEM with one gram per liter of glucose supplemented with 10%FCS and grow the culture at 37 degrees Celsius with 10%CO2. Grow TC7 cells in DMEM with 4.5 grams per liter of glucose, 10%FCS, and non-essential amino acids and culture at 37 degrees Celsius and 10%CO2. For cell maintenance, split the cells regularly to avoid growth contact inhibition linked to confluent states.This is critical for high calcium probe loading and transfection efficiency. Trypsinize and count the TC7 cells. The day before the experiment with HeLa cells, plate the cells onto sterile, 25-millimeter diameter circular cover slips in six-well plates at a density of three times 10 to the five cells per well.Then, to allow for polarization before the experiments, seed the cells at four times 10 to the five cells per well and incubate them for five to seven days, replacing the medium every day. On the day of the experiment, remove the medium and use one milliliter of EM medium to wash the cells three times. Load the cells with three micromolar Fluo-4 AM and EM buffer and incubate the plates at 21 degrees Celsius for 30 minutes.With EM buffer, wash the samples three times, then add one milliliter of EM buffer, and incubate the cells at 21 degrees Celsius to allow for the hydrolysis of the AM moiety. Place the coverslip containing the Fluo-4 loaded cells in an imaging or glass-bottom chamber. Use EM buffer to wash the sample three times to remove compounds potentially resulting from cell lysis.Then, add one milliliter of EM buffer. Place the chamber on an inverted fluorescence microscope stage heated at 33 degrees Celsius. Select a microscopy field and set up acquisition parameters including exposure time and binning if necessary to optimize the Fluo-4 fluorescent signal.To add bacteria to the sample, carefully remove 500 microliters of EM buffer from the chamber and add 500 microliters of the bacterial suspension to obtain a final OD 600 of 0.05. Immediately perform an acquisition or wait 10 minutes to allow the bacteria to sediment onto the cells. At the end of the acquisition stream, acquire a phase-contrast image of the selected field to visualize the bacteria contacting the cells, and the membrane ruffles associated with bacterial invasion sites.Repeat the acquisition procedure at intervals that are amenable to the duration of the image acquisitions, file savings, and selection of a new field to cover the whole process. At the end of the acquisition procedure, add to the sample two micromolar of the calcium ionophore ionomycin to determine the maximal amplitude of the calcium signals. Acquire images every three to five seconds until the signal stabilizes, usually for less than 10 minutes.Follow by adding the calcium chelator EGTA to a final concentration of 10 millimolar to determine the fluorescent signal in the absence of calcium. Acquire images every three to five seconds until signal stabilization, usually for less than 10 minutes. Carry out image analysis according to the text protocol.In this experiment, Fluo-4 loaded HeLa cells were challenged with wild-type Shigella and stream acquisitions were performed to analyze calcium signals. A time-lapse image series of the fluorescence intensity of the Fluo-4 probe is averaged in a region of interest for a single cell, and the corresponding phase-contrast image is shown. Atypical local increases in free cytosolic calcium are observed at the Shigella invasion site with a varying amplitude and durations ranging from 2.5 to five seconds, followed by global increases in the infected cell.In this example, an isogenic mutant strain, deficient for the type III effector IpgD, a phosphatidyl 4, 5-bisphosphate phosphatase, induced more global and less atypical local responses with long durations as compared to the wild-type Shigella strain. As seen in this experiment, the imaging of global calcium responses over extended infection kinetics shows a decrease in the frequency of responses 30 minutes following the challenge with wild-type Shigella. Finally, the quantification of dose-dependent cell responses to the calcium agonist histamine illustrates the inhibition of an inositol triphosphate dependent calcium release at late stages of a Shigella infection.Following this procedure, all the methods simultaneously combining compatible fluids and imaging can be performed in order to address the role of calcium signals in a given process. Don't forget that working with pathogens can be extremely hazardous, and precautions such as waste disposal, cleaning of contaminated surfaces or equipments should always be taken while performing this procedure.

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