Rotavirus Calcium Dysregulation Manifests as Dynamic Calcium Signaling in the Cytoplasm and Endoplasmic Reticulum

Like many viruses, rotavirus (RV) dysregulates calcium homeostasis by elevating cytosolic calcium ([Ca2+]cyt) and decreasing endoplasmic reticulum (ER) stores. While an overall, monophasic increase in [Ca2+]cyt during RV infection has been shown, the nature of the RV-induced aberrant calcium signals and how they manifest over time at the single-cell level have not been characterized. Thus, we generated cell lines and human intestinal enteroids (HIEs) stably expressing cytosolic and/or ER-targeted genetically-encoded calcium indicators to characterize calcium signaling throughout RV infection by time-lapse imaging. We found that RV induces highly dynamic [Ca2+]cyt signaling that manifest as hundreds of discrete [Ca2+]cyt spikes, which increase during peak infection. Knockdown of nonstructural protein 4 (NSP4) attenuates the [Ca2+]cyt spikes, consistent with its role in dysregulating calcium homeostasis. RV-induced [Ca2+]cyt spikes were primarily from ER calcium release and were attenuated by inhibiting the store-operated calcium entry (SOCE) channel Orai1. RV-infected HIEs also exhibited prominent [Ca2+]cyt spikes that were attenuated by inhibiting SOCE, underlining the relevance of these [Ca2+]cyt spikes to gastrointestinal physiology and role of SOCE in RV pathophysiology. Thus, our discovery that RV increases [Ca2+]cyt by dynamic calcium signaling, establishes a new, paradigm-shifting understanding of the spatial and temporal complexity of virus-induced calcium signaling.

Eukaryotic signal transduction pathways employ a variety of signaling molecules to regulate cellular processes. Calcium (Ca 2+ ) is one of the most ubiquitous secondary messengers in the cell, which tightly regulates Ca 2+ movement through the coordinated function of Ca 2+ channels, transporters, and pumps. Since Ca 2+ signaling modulates a wide array of cellular processes, it is not surprising that many different viruses exploit Ca 2+ signaling to facilitate their replication, and the resulting dysregulation of Ca 2+ signaling causes pathogenesis. Rotavirus (RV), a member of the Reoviridae family, is one of the first viruses shown to elevate cellular Ca 2+ levels and has become a widely-used model system to characterize mechanisms by which viruses dysregulate host Ca 2+ homeostasis 1 . RV is a clinically important enteric virus that causes severe diarrhea and vomiting in children, resulting in over approximately 258 million diarrhea episodes and 198,000 deaths in 2016 2 . Hyperactivation of cyclic nucleotide (e.g., cAMP/cGMP) and Ca 2+ signaling pathways is a common strategy among enteric pathogens 3 . Thus, understanding how RV exploits Ca 2+ signaling is key to understanding and combating RV-induced diarrhea.
RV was first reported to elevate cytosolic [Ca 2+ ] by Michelangeli et al (1991), which stimulated subsequent research into how RV alters cellular Ca 2+ levels 4 . RV causes a 2-fold steady-state increase in cytosolic Ca 2+ , which is due to increased Ca 2+ release from the endoplasmic reticulum (ER) and increased Ca 2+ influx through host Ca 2+ channels in the plasma membrane (PM) 1,5 . Elevated cytosolic Ca 2+ activates autophagy, which is critical for RV replication, and has wide-ranging consequences to host cell functions, including disruption of the cytoskeleton and activation of chloride and serotonin secretion to cause diarrhea and vomiting 1,5 .

Microscopy and image analysis. To image viroplasms, we used a GE Healthcare DeltaVision LIVE High
Resolution Deconvolution with an Olympus IX-71 base and illumination provided by a xenon lamp. Images were captured with Plan Apo 60X Oil DIC objective and a pco.edge sCMOS camera. Images were acquired and deconvolved using SoftWoRx software and further analyzed with Fiji (ImageJ).
For Ca 2+ imaging, MA104 cells and HIEs were imaged with a widefield epifluorescence Nikon TiE inverted microscope using a SPECTRAX LED light source (Lumencor) and either a 20x Plan Fluor (NA 0.45) phase contrast or a 20X Plan Apo (NA 0.75) differential interference contrast (DIC) objective. Fluorescence and transmitted light images were recorded using an ORCA-Flash 4.0 sCMOS camera (Hamamatsu), and Nikon Elements Advanced Research v4.5 software was used for multipoint position selection, data acquisition, and image analysis.
Images were read-noise subtracted using an average of 10 no-light acquisitions of the camera. Single cells were selected as Regions of Interest (ROI) and fluorescence intensity measured for the experiment. 3D HIE's fluorescence was measured individually using threshold analysis adjusted to select each enteroid with the Fill Holes algorithm included. Enteroids that moved out of the field of view or could not be separated from adjacent enteroids were removed from analysis. The fluorescence intensity of whole field-of-view was measured for HIE monolayers.
Fluorescence intensity values were exported to Microsoft Excel and normalized to the baseline fluorescence. The number and magnitude of Ca 2+ spikes were calculated by subtracting each normalized fluorescence measurement from the previous measurement to determine the change in GECI fluorescence (ΔF) between each timepoint. Ca 2+ signals with a ΔF magnitude of >5% were counted as Ca 2+ spikes.

Calcium imaging. MA104-GECI cells. Confluent monolayers of MA104-GECI cells in 8-well chamber
slides (ibidi) were mock-or RV-infected in FBS-free media for 1 hr at the indicated multiplicity of infection (MOI). Then the inoculum was removed and replaced with FB-Plus, and for appropriate studies, with DMSO or drugs at indicated concentrations. The slide was mounted into an Okolab stage-top incubation chamber equilibrated to 37 °C with a humidified 5% CO 2 atmosphere. For each experiment, 3-5 positions per well were selected and imaged every 1 minute for ~18-20 hrs.
GECI HIEs. To test Ca 2+ response, 3D G6S-jHIEs were suspended in 25% Matrigel diluted in FB-Diff media and seeded into optical-bottom 10-well Cellview chamber slides (Greiner bio-one) thinly coated with Matrigel. After baseline imaging using the stage-top incubator, 200 µM carbachol in FB-Diff or FB-Diff alone was added to the well and imaging continued for 1 hour with 6-10 enteroids imaged every 10 s.
For RV infection in 3D HIEs, the jHIEs were split and grown in hW-CMGF + for 2 days followed by differentiation medium for 1 day. G6S-jHIEs were gently washed using ice cold 1XPBS and resuspended in inoculum of 50 µL MA104 cell lysate or RV (strain Ito) diluted with 150 µL CMGF-and incubated for 1 hr. Then HIEs were washed, resuspended in 25% Matrigel diluted in FB-Diff (with DMSO or 2-APB in indicated experiments) and pipetted onto 8-well chamber slides (Matek) pre-coated with Matrigel. Imaging positions were chosen so that between 20-50 enteroids were selected per experimental condition. Enteroids were imaged using the stage-top incubator with transmitted light and GFP fluorescence every 2-3 minutes for ~18 hrs. (2019) 9:10822 | https://doi.org/10.1038/s41598-019-46856-8 www.nature.com/scientificreports www.nature.com/scientificreports/ For RV infection in monolayers, G6S-jHIE differentiated monolayers were washed once with CMGF-and treated with an inoculum of 50 µL CMGF-plus 30 µL MA104 cell lysate or RV (strain Ito) and incubated for 2 hr. Then inoculum was removed, and monolayers were washed once with FB-Diff before adding FB-Diff with DMSO or 2-APB. Monolayers were transferred to the stage-top incubator for imaging with 4 fields of view chosen per well, and GFP fluorescence was measured every minute for ~18 hrs. store-operated calcium entry assay. G6S-jHIE monolayers after 4 days in differentiation media were washed and incubated in 0 mM Ca 2+ (0Ca 2+ ) Ringers solution (160 mM NaCl, 4.5 mM KCl, 1 mM MgCl 2 , 10 mM HEPES, pH = 7.4). Endoplasmic reticulum Ca 2+ stores were depleted by incubating cells with 500 nM thapsigargin in 0Ca 2+ with either 50 µM 2-APB or DMSO as a vehicle control. SOCE was measured using live-cell fluorescence imaging of the increase in GFP fluorescence after the addition of normal Ringers to bring the total Ca 2+ concentration to 2 mM. Western blot analysis. RV proteins were detected by immunoblot analysis as previously described, with the following modifications 35 . Cells were lysed using a 1X RIPA buffer solution [10 mM Tris-HCL pH 8.0, 1 mM EDTA, 1% Triton X-100, 0.1% sodium deoxycholate, 0.1% SDS, 140 mM NaCl, and 1 tablet complete mini protease inhibitor (Roche)] and passed through a Qiashredder (Qiagen). Samples were boiled for 10 min at 100 °C in SDS-PAGE sample buffer and separated on Tris-glycine 4-20% SDS-PAGE gels (BioRad). Detected protein bands for each blot were quantified using ImageJ software for gel densitometry measurements of NSP4:GAPDH.
Immunofluorescence. MA104 cells and HIEs were fixed using the Cytofix/Cytoperm kit (BD Biosciences) according to manufacturer instructions. Primary antibodies were diluted in 1X Perm/Wash overnight at 4 °C. The next day, the cells were washed three times with 1X Perm/Wash solution and then incubated with corresponding secondary antibodies for 1 hr at room temperature. Nuclei were stained with NucBlue Fixed Cell Stain (Life Technologies) for 5 min at room temperature and washed with 1X PBS for imaging. plaque assays. Plaque assays were performed as described previously with the following modifications 37 .
Briefly, MA104 cells or the MA104-shRNA expressing cells were seeded and grown to confluency in 6 wells plates. Wells were infected at 10-fold dilutions in duplicate for 1 hr and media replaced with an overlay of 1.2% Avicel in serum-free DMEM supplemented with DEAE dextran, and 1 μg/mL Worthington's Trypsin, and, for indicated experiments, DMSO vehicle or SOCE drugs 38 . The cells were incubated at 37 °C/5% CO 2 for 48-72 hrs before overlay was removed and cells stained with crystal violet to count plaques. RNA extraction, reverse transcription, and quantitative pCR. Total RNA was extracted from HIEs wells (in hW-CMGF+ or differentiation media for 4 days) or MA104 cells grown to confluency in a 6-well plate using TRIzol reagent (Ambion). Total RNA was treated with Turbo DNase I (Ambion) and cDNA was generated from 250 ng RNA using the SensiFAST cDNA synthesis kit (Bioline). Quantitative PCR was performed using Fast SYBR Green (Life Technologies) with primers designed using NCBI Primer-Blast (Table 1) and using a QuantStudio real time thermocycler (Applied Biosciences). Target genes were normalized to the housekeeping gene ribosomal subunit 18 s and relative expression was calculated using the ddCT method. statistical analysis. Biostatistical analyses were performed using GraphPad Prism (version 8.1) with results presented as mean ± standard deviation. Comparisons used an unpaired Student's t-test, the nonparametric Mann-Whitney test, or a One-way Analysis of Variance (ANOVA) with Tukey's post hoc multiple comparisons test where appropriate. Differences were considered statistically significant for p < 0.05. All authors had access to the study data, reviewed, and approved the final manuscript.

Results
Previous studies have shown that RV significantly increases cytosolic Ca 2+ over several hours during the peak of RV replication 18,39 ; however, the kinetics of this increase and whether it is a monophasic increases or manifests as discrete Ca 2+ transients are not known. To address these questions, we developed a series of cell lines stably expressing GECIs and used these cell lines to perform live-cell Ca 2+ imaging over the course of a RV www.nature.com/scientificreports www.nature.com/scientificreports/ infection 23 . For the long-term imaging experiments, MA104 cells stably expressing cytosolic GCaMP5G (MA104-GCaMP5G) were seeded into chamber slides and either mock-or RV-infected with strain SA114F (MOI 10), and GCaMP5G fluorescence imaged for ~18 hr (2-20 hpi) (Fig. 1). Mock-infected cells maintained a low fluorescence throughout the time course (Fig. 1A, upper panels), whereas RV-infected cells exhibited strongly increased fluorescence, indicating elevated Ca 2+ levels (Fig. 1A, lower panels and Supplementary Video 1). Since GECIs use an engineered calmodulin to sense Ca 2+ , overexpression of GCaMP5G could act as a Ca 2+ buffer and alter the kinetics of RV replication. To assess whether the MA104-GCaMP5G cells exhibited altered RV infection/protein synthesis, we analyzed NSP4 expression in parental MA104 cells and the MA104-GCaMP5G cells infected with SA114F (MOI 10) from 3-8 hpi by western blot (Fig. 1B). NSP4 expression was similar in both parental and GCaMP5G-expressing cells, indicating that stable GCaMP5G expression does not interfere with RV infection.
Next, we measured changes in cytosolic Ca 2+ by determining the relative GCaMP5G fluorescence (F/F 0 ) for the whole field-of-view (FOV) (~455 µm 2 ) for three replicate infections and time lapse images were acquired once per minute (Fig. 1C)  www.nature.com/scientificreports www.nature.com/scientificreports/ Ca 2+ signals (Fig. 1C, black and grey lines). In RV-infected cells, the steady-state Ca 2+ levels began to increase at ~6 hpi, and we observed many large amplitude, transient Ca 2+ signals that occurred concomitantly with the steady-state elevation in cytosolic Ca 2+ levels (Fig. 1C, arrow). Further, at 6-8 hpi the RV-infected cells had more low and moderate amplitude Ca 2+ signals than mock-infected cells (Fig. 1D, arrows), which occurred during the initial increase in steady-state Ca 2+ levels. A more detailed examination of the Ca 2+ signaling over a period of 5 mins at 480 minutes post-infection (~8 hpi) shows that the increase in cytosolic Ca 2+ manifests as discrete and dynamic Ca 2+ fluxes from individual or small groups of cells (Fig. 1E). The dynamic changes are exemplified by the two areas outlined (Fig. 1E, magenta or yellow box), showing substantial changes over the 5 min period. To compare our GECI-based Ca 2+ imaging of RV-induced Ca 2+ signaling to previous cell population-based studies, we determined the average GCaMP5G fluorescence from 18-19 hpi (Fig. 1F) and the slope of the fluorescence increase from 8-18 hpi (Fig. 1G). We found a similar ~2-fold increase in cytosolic Ca 2+ and a rate of Ca 2+ increase consistent with that found in studies using Ca 2+ indicator dyes 12,18,19 . Thus, the MA104-GCaMP5G cells exhibit the well-characterized hallmarks of RV-induced Ca 2+ dysregulation but have greater spatial and temporal resolution to study Ca 2+ in RV-infected cells. This has revealed a new dimension of the RV-induced Ca 2+ signaling, in that the cytosolic Ca 2+ increase manifests through highly dynamic and discrete Ca 2+ signaling events, which had not been previously observed.
RV-induces dynamic Ca 2+ signaling. Our long-term Ca 2+ imaging approach using MA104-GCaMP5G cells had sufficient resolution to enable analysis of Ca 2+ signaling at the single-cell level over the course of the RV infection. Three representative traces from mock-or RV-infected cells (MOI 10, imaged once per minute) show that while individual cells display unique characteristics of Ca 2+ signaling, they all exhibit similar patterns of Ca 2+ signaling ( Fig. 2A). Mock-infected cells maintain low cytosolic Ca 2+ with few low amplitude Ca 2+ signals ( Fig. 2A, black lines), but RV-infected cells display a large number of large amplitude Ca 2+ transients, as well as an overall increase in cytosolic Ca 2+ ( Fig. 2A, red lines). These Ca 2+ transients were the most prominent Ca 2+ signal during the infection and were infrequently detected in mock-infected cells. Finally, as RV is a lytic virus, we observed clear evidence of cell lysis late in infection, but this was associated with a loss of GCaMP5G signal and dynamics because the sensor diffused away from the ruptured cells (see Supplementary Video 1). We sought to determine a threshold to define these "Ca 2+ spikes" so that we could measure the number and amplitude of these Ca 2+ signals. To define a "Ca 2+ spike", we set a cutoff for the change in GCaMP5G fluorescence between two measurements to be greater than 5% (ΔF > 5%). The mean Ca 2+ transient amplitude of mock-infected cells was 0.3% (± 0.5% standard deviation) (data not shown; see Fig. 2D for a subset of this data). Thus the ΔF > 5% cutoff is more than 3 standard deviations above the mean, which establishes a stringent threshold for quantitating Ca 2+ spikes. Next, we determined the change in fluorescence between each data point and found that the majority of Ca 2+ spikes were captured in 1 image (Fig. 2B). This simple method enabled detection of Ca 2+ spikes with approximately 80% accuracy; however, it results in a 20% under-estimation of Ca 2+ spikes, which were captured in 2-3 images (Fig. 2B, red dots). Nevertheless, using this method of Ca 2+ spike analysis, we found that RV significantly increases the number of Ca 2+ spikes per cell (Fig. 2C). We then determined the amplitude for the top 150  www.nature.com/scientificreports www.nature.com/scientificreports/ Ca 2+ spikes per representative cell and found that while the amplitude was highly variable between RV-infected cells, these signals were significantly greater than mock-inoculated cells (Fig. 2D). Finally, we compared the RV-induced Ca 2+ spikes to the Ca 2+ response induced by 10 μM ATP (Fig. 2E). As expected, ATP induced a strong Ca 2+ flux that was similar to the amplitude of Ca 2+ spikes induced during RV infection. Although the ATP-induced response was significantly greater than the RV-induced Ca 2+ spikes, it is important to note that the amplitude of the ATP-induced signals represent the peak of the Ca 2+ response, whereas it is not possible to know how many of the RV-induced Ca 2+ spikes were captured at the peak of the signal. Thus, at the individual cell level, a high MOI RV infection induces up to hundreds of discrete and high amplitude Ca 2+ signaling events.
Next, we determined how these RV-induced Ca 2+ signals differ with respect to different infectious doses. We infected MA104-GCaMP5G cells with SA114F or with a recombinant SA11 cl. 3 expressing mRuby from the NSP3 gene (SA11-mRuby) 28 . Cells were infected at MOI of 10, 1, or 0.1, and we performed time-lapse Ca 2+ imaging and single-cell analysis of the resulting Ca 2+ signaling. Infection of cells with native SA114F at different MOIs showed the expected infectious dose-dependent increase in the number of RV-positive cells (Fig. 3A). Similarly, the SA11cl3-mRuby-infected cells exhibited an infectious dose-dependent increase in the number of RFP-positive cells at 7 hpi (Fig. 3B), as well as an increase in RFP intensity from 7 hpi to 10 hpi (Fig. 3C). Representative single-cell Ca 2+ traces for SA114F-infected cells show similar dynamic increases in cytosolic Ca 2+ spikes as before, but cells infected with lower MOIs of 1 and 0.1 exhibited a later onset of the Ca 2+ signaling and generally fewer and lower amplitude Ca 2+ spikes (Fig. 3D). The virus dose-dependent differences in the Ca 2+ signaling are clearly demonstrated by the time-lapse imaging at 6-7 hpi, which are superimposed onto the immunofluorescence images to detect RV-positive cells (Supplementary Video 2). In a more detailed examination of cytosolic Ca 2+ in RV-infected cells, we used a higher image acquisition frequency (1 image/1.5 sec) and again observed active and dynamic Ca 2+ signaling. While virtually every cell exhibited multiple Ca 2+ transients over the course of 10 mins, the Ca 2+ spike frequency and amplitude were variable from cell-to-cell (Supplemental Fig. 1A and Supplementary Video 3). We quantitated the number of Ca 2+ spikes throughout the infection and found a dose-dependent decrease in the number of spikes per cell ( Fig. 3F) for lower MOI infections. A similar phenotype was observed in cells infected with SA11-mRuby, but in this case the mRuby expression enabled us to measure both Ca 2+ signaling and RV protein expression ( Fig. 3E and Supplementary Video 4 online). We again observed that for lower MOI infections the onset of Ca 2+ signaling was later, and the Ca 2+ spike number and amplitude were generally lower. Further, onset of the Ca 2+ signaling corresponded to the detection of mRuby from the NSP3 gene. Quantitation of the number of Ca 2+ spikes per cell for SA11-mRuby infections also showed a dose-dependent decrease with infectious dose (Fig. 3G). Thus, the number and amplitude of these Ca 2+ signaling events are related to both the infectious dose of RV and onset of RV protein synthesis.
Next, we sought to characterize the Ca 2+ signaling phenotype of different RV strains that infect humans or other animals. The manifestation of dynamic Ca 2+ spikes induced by the rhesus RV strain RRV was similar to that of SA114F and SA11-Ruby (Supplemental Fig. 1B,C). We then compared the Ca 2+ signaling in MA104-GCaMP5G cells infected at MOI 1 with simian strain SA114F to that of human strain Ito. Immunofluorescence staining of Ito-infected cells (Fig. 4A) showed a similar number of infected cells as that for SA114F infected above (Fig. 3A). As above, we quantitated the number of Ca 2+ spikes per cell and found that both SA114F and Ito induced a significant increase in Ca 2+ spikes compared to mock-infected cells (Fig. 4B). These findings demonstrate that the dynamic Ca 2+ signaling phenotype is not exclusively a feature of animal RV strains. Further, we investigated the attenuated and virulent porcine OSU strains (OSUa and OSUv). OSUv is pathogenic in gnotobiotic piglets but the tissue-culture attenuated OSUa is non-pathogenic 40 . Mutations in the OSUa NSP4 protein are associated with reduced elevation in cytosolic Ca 2+ levels in recombinant NSP4-expressing Sf9 cells 40 , but the Ca 2+ signaling phenotype of these two viruses had not been studied in the context of an infection. Immunofluorescence of OSUaand OSUv-infected MA104 cells at MOI 1 show a similar number of infected cells (Fig. 4A). However, while both OSUa and OSUv significantly increase the number of Ca 2+ spikes, the number of Ca 2+ spikes from OSUa-infected cells is significantly less than that of those infected with OSUv ( Fig. 4B and Supplementary Video 5 online). To characterize this difference further, we examined single-cell traces for OSUa-and OSUv-infected cells (Fig. 4C). We found that OSUa-infected cells initially induced a low-amplitude monophasic increase in cytosolic Ca 2+ levels (Fig. 4C, black arrows), with the onset of the dynamic Ca 2+ spikes occurring several hours later (Fig. 4C, dark blue traces). In contrast, OSUv infection induced a much earlier onset of the dynamic Ca 2+ spiking, which explains the higher number of Ca 2+ spikes per cell (Fig. 4C, blue traces). Interestingly, OSUv may also induce an early, low-amplitude increase in cytosolic Ca 2+ in addition to the dynamic Ca 2+ spikes, but the high number of Ca 2+ spikes makes it difficult to clearly ascertain this in all but a few cells (Fig. 4C, red arrow). Together, these data demonstrate that the dynamic Ca 2+ signaling phenotype is a common feature among RV strains and potentially related to the role of NSP4 in dysregulating host Ca 2+ homeostasis and virus virulence.
Ca 2+ signaling dynamics are dependent on NSP4 expression. RV NSP4 is the primary mediator of elevated Ca 2+ levels during RV infection. The differences in Ca 2+ signaling by OSUa and OSUv observed in Fig. 4 suggest that NSP4 expression is important for the induction of the dynamic Ca 2+ signaling during infection 1 . To test the role of NSP4 in these Ca 2+ signals, we made two GCaMP6s cell lines, each stably expressing a different short-hairpin RNA that targets SA11 NSP4 (NSP4 shRNA1 and NSP4 shRNA2), and a third GCaMP6s cell line stably expressing a non-targeted scrambled shRNA. Cells were infected with SA114F (MOI 0.01), and we found that cells expressing NSP4-targeted shRNAs exhibited knockdown of NSP4 protein levels (Fig. 5A). We normalized NSP4 expression to GAPDH and found ~40% knockdown in cells expressing NSP4 shRNA1 and ~85% knockdown in cells expressing NSP4 shRNA2 (Fig. 5B), which correlated with reduced RV plaque size (Fig. 5C). To examine the Ca 2+ signaling phenotype, we then infected the cells with SA114F (MOI 0.1) and used live-cell imaging to measure Ca 2+ signaling from ~2-18 hpi. The scrambled shRNA-expressing cells exhibited a similar degree of dynamic Ca 2+ signaling as observed in parental MA104 cells (Fig. 5D, red traces), whereas knockdown www.nature.com/scientificreports www.nature.com/scientificreports/ of NSP4 substantially decreased the degree of Ca 2+ signaling observed (Fig. 5D, blue traces). Upon quantitation, we found the number of Ca 2+ spikes was significantly reduced in the NSP4 knockdown cells (Fig. 5E). Together, these data show that NSP4 is responsible for inducing these dynamic Ca 2+ signals during infection. 2+ spikes require extracellular and eR Ca 2+ pools. NSP4 elevates cytosolic Ca 2+ by activating both uptake of extracellular Ca 2+ and release of ER Ca 2+ pools. Thus, we next sought to characterize which pools of Ca 2+ were critical for supporting the RV-induced Ca 2+ spikes. First, we tested whether extracellular Ca 2+ influenced the RV-induced Ca 2+ spikes. We infected MA104-GCaMP5G cells with SA114F (MOI 1), and at 1 HPI replaced the media with either normal media containing 2 mM Ca 2+ , media without Ca 2+ (supplemented with 1.8 mM EDTA), or media supplemented with Ca 2+ for a 10 mM final concentration. The imaging data shows www.nature.com/scientificreports www.nature.com/scientificreports/ that decreasing extracellular Ca 2+ strongly reduced the number and duration of Ca 2+ signaling (Fig. 6A, light blue), whereas cells maintained in media with 2 mM or 10 mM extracellular Ca 2+ showed increased dynamic Ca 2+ signaling (Fig. 6A, red & purple). As before, mock-infected cells in each condition exhibited little to no induction of the Ca 2+ signaling (Fig. 6A, black lines). Quantitation of the number of Ca 2+ spikes showed that RV-infected cells in low extracellular Ca 2+ exhibited significantly fewer Ca 2+ spikes than that of cells in normal extracellular Ca 2+ , but this was still greater than that of mock-infected cells (Fig. 6B). Interestingly, there was no difference in the number of Ca 2+ spikes per cell when maintained in 2 mM versus 10 mM Ca 2+ media (Fig. 6B). However, the traces indicated that the magnitude of the Ca 2+ spikes were greater in high Ca 2+ media. Thus, we then determined the Ca 2+ spike amplitude for the top 50 Ca 2+ spikes, and while this was highly variable from cell-to-cell, this trended to be greater with higher extracellular Ca 2+ concentrations (Fig. 6C). Together, these data indicate that normal extracellular Ca 2+ levels are critical for the RV-induced Ca 2+ spikes, which could occur both through discrete Ca 2+ influx events through the plasma membrane, and by influx of extracellular Ca 2+ serving to maintain ER Ca 2+ stores to feed ER Ca 2+ release events.

RV-induced Ca
The ER is the major intracellular Ca 2+ store, and RV NSP4 has been shown to decrease ER Ca 2+ levels both during infection and by recombinant expression 6,7 . However, controversy remains about whether RV causes a sustained depletion in ER Ca 2+ 22 . Thus to directly characterize the change in ER Ca 2+ during RV infection, and determine how this relates to the dynamic cytosolic Ca 2+ spikes, we generated an MA104 cell line co-expressing R-GECO1.2 and GCEPIAer (MA104-RGECO1/GCEPIAer), in which R-GECO1.2 is a red fluorescent cytoplasmic GECI and GCEPIAer is a green fluorescent ER-targeted GECI 24,41 . As above, infection with SA114F (MOI 1) induced highly dynamic cytoplasmic Ca 2+ signaling by ~8 hpi, as illustrated in two representative single-cell traces (Fig. 7A,B, red traces; Supplementary Video 6). Concomitant with the onset of the cytoplasmic Ca 2+ signaling was an equally dynamic decrease of ER Ca 2+ that persisted throughout the rest of the infection (Fig. 7A,B, green traces). We examined the relationship between the cytoplasmic Ca 2+ spikes and ER Ca 2+ troughs more closely from 8-12 hpi (Fig. 7B), which was during the onset of these signaling events. First, we found that the onset of Ca 2+ signals in the cytoplasm coincided with the ER Ca 2+ release events (Fig. 7B, black arrowheads). The persistent decrease in ER Ca 2+ observed was driven primarily by this continuous signaling, such that the ER Ca 2+ level never recovered to the baseline level (Fig. 7A). Interestingly, a small number of ER Ca 2+ troughs were not associated with a concomitant cytoplasmic Ca 2+ spike (Fig. 7B, magenta arrowheads). Over the course of the long-term imaging experiment, mock-infected cells exhibited a 10% decrease in GCEPIAer fluorescence, but RV-infected cell had a 30% decrease (Fig. 7C), which occurred rapidly from 8-12 HPI (Fig. 7A). The decrease During these studies, we observed that the ER-localized GCEPIAer protein is also redistributed during the RV infection, which is illustrated for a single cell in Fig. 7D,E and Supplementary Video 7. At the beginning of the imaging run (3 hpi), the GCEPIAer signal was high and localized throughout the ER in a reticular pattern (Fig. 7D,E, left) but by 10.7 HPI, RV-induced Ca 2+ signaling had decreased GCEPIAer fluorescence to its nadir (Fig. 7D,E, middle), representing a substantial decrease in ER Ca 2+ . Approximately 2 hrs after the initial decrease in ER Ca 2+ , the GCEPIAer began accumulating into circular domains that are likely the ER-derived compartment surrounding viroplasms (Fig. 7D,E, right). These structures become more pronounced through the late stages of the infection, ~13 hpi (Fig. 7D, arrows). While the absolute onset of the ER Ca 2+ release events was variable, the formation of viroplasm-associated membranes subsequent to the decrease in ER Ca 2+ was a consistent pattern among RV-infected cells (Supplementary Video 7). Using immunostaining and deconvolution microscopy, we confirmed that the structures are viroplasms because they contain RV nonstructural protein 2 (NSP2), a major component of viroplasms (Fig. 7F). Interestingly, during late stages of infection when viroplasms are forming, we detected a modest recovery in ER Ca 2+ from its nadir (Fig. 7E), which may reflect the increased 45   www.nature.com/scientificreports www.nature.com/scientificreports/ soCe blockers reduce RV-induced Ca 2+ spikes. Since removing extracellular Ca 2+ diminishes the RV-induced Ca 2+ spikes (Fig. 6), cellular Ca 2+ influx pathways are critical for these Ca 2+ signals. Several host Ca 2+ channels have been implicated in mediating Ca 2+ entry into RV-infected cells, including SOCE channels, voltage-activated Ca 2+ channels (VACC), and the sodium-calcium exchanger (NCX) [10][11][12] . To determine which pathway(s) were important for the dynamic Ca 2+ spikes in RV infection, we used pharmacological blockers targeting each pathway (2-APB for SOCE; D600 for VACC; KB-R7943 for NCX). MA104-GCaMP5G cells were infected with SA114F (MOI 1), and then treated with different concentrations of the blockers at 1 hpi and imaged to measure GCaMP5G fluorescence. None of the blockers exhibited cytotoxic effects to uninfected cells (data not shown). Cells treated with DMSO as a vehicle control exhibited the dynamic Ca 2+ spikes as above (Fig. 8A, red trace). In contrast, cells treated with the SOCE blocker 2-APB exhibited a dose-dependent decrease in both the number and amplitude of the Ca 2+ signaling (Fig. 8A, green traces). Traces from cells treated with the NCX blocker KB-R7943 showed a modest decrease in Ca 2+ signaling (Fig. 8A, brown traces), whereas there was no difference in Ca 2+ signaling for cells treated with D600 (data not shown). We also noted that RV-infected cells treated with 10 μM KB-R7943 underwent cell death more frequently than any other treatment, which was marked by a rapid increase in cytosolic Ca 2+ and then lysis (Fig. 8A, arrowhead); however, cell death was not observed in uninfected cells treated with KB-R7943 (data not shown). We quantitated the number of Ca 2+ spikes per cell, which showed a significant, dose-dependent decrease in the number of Ca 2+ spikes for both 2-APB-treated (Fig. 8B, green) and, to a lesser extent, KB-R7943-treated (Fig. 8B, brown) cells, but no difference for D600-treated cells (Fig. 8B, blue). We further investigated the effects of 2-APB and KB-R7934 by examining the amplitude of the largest 50 Ca 2+ spikes of three representative cells shown in Fig. 8C,D. Treatment with 2-APB showed a dose-dependent decrease in the Ca 2+ spike amplitude (Fig. 8C), consistent with the single-cell traces, but treatment with KB-R7943 showed no difference in spike amplitude.
Since elevated cytosolic Ca 2+ is critical for RV replication, we examined RV protein levels by immunoblot to determine whether 2-APB or KB-R7943 reduced the Ca 2+ signaling by merely blocking RV or NSP4 protein synthesis or protein stability 42 . Immunoblot detection with an anti-RV antisera (Fig. 8E) or an anti-NSP4 specific antisera (Fig. 8F) show that none of the Ca 2+ channel blockers caused substantial decrease in RV or NSP4 protein levels. However, we observed that 2-APB treatment significantly increased the 20 kDa unglycosylated NSP4 band (Fig. 8F) by gel densitometry analysis (Fig. 8G).
Overall the SOCE blocker 2-APB was the most potent inhibitor of the RV-induced dynamic Ca 2+ signaling, so we examined the effect of other SOCE blockers that also target the Orai1 Ca 2+ channel. MA104 cells express the Orai1 Ca 2+ channel and the STIM1 and STIM2 ER Ca 2+ sensors, which are the core machinery for  www.nature.com/scientificreports www.nature.com/scientificreports/ the SOCE pathway (Fig. 9A). While MA104 cells also express the Orai3 Ca 2+ channel, this isoform is not activated by ER Ca 2+ store depletion but arachidonic acid and leukotrienes 43 . We tested four SOCE blockers (2-APB, BTP2, Synta66, and GSK7975A) for the ability to block thapsigargin-induced SOCE and found that all of them showed a similar inhibition of Ca 2+ entry after ER store depletion (Fig. 9B). Thus, we treated SA114F-infected MA104-GCaMP5G cells with each of these blockers at ~1 hpi and performed Ca 2+ imaging to measure the RV-induced Ca 2+ spikes (Fig. 9C,D). Representative single-cell traces illustrate that all the SOCE blockers inhibited the RV-induced dynamic Ca 2+ signaling (Fig. 9C) and significantly inhibited the number of Ca 2+ spikes per cell (Fig. 9D). The blockers displayed varying degrees of potency but 2-APB and BTP2 treatment caused the greatest decrease in RV-induced Ca 2+ signaling (Supplementary Video 8). Further, we found that treatment with the SOCE blockers significantly reduced RV yield from MA104 cells (Fig. 9E), which is consistent with the importance of elevated cytosolic Ca 2+ for RV replication 33 . As with the other Ca 2+ channel blockers, we examined whether the SOCE blockers affected viral protein levels by immunoblot. As above, we found that 2-APB treatment increased the abundance of the 20 kDa unglycosylated NSP4 band (NSP4-20), and Synta66 treatment also caused a modest increase in NSP4-20 (Fig. 9F). In contrast to the other SOCE blockers, BTP2 treatment caused an overall decrease in RV proteins, which correlates with the strong suppression of RV-induced Ca 2+ signaling during the infection (Fig. 9D,F). Together, these data support the previous observation that shRNA knockdown of the Ca 2+ sensor STIM1 reduces RV replication, and further show that Ca 2+ influx via SOCE channels is critical for RV-induced Ca 2+ signaling and replication 10 . Human intestinal enteroid characterization of RV-induced Ca 2+ signaling. Although MA104 cells provide a robust model for RV replication and form a single epithelial sheet ideal for microscopy studies, they are neither of human nor of intestinal cell origin. Human intestinal enteroids (HIEs) have been developed as a model in vitro system of the epithelial cells of the small intestine, and support RV infection and replication, particularly for human RV strains 26,44 . HIEs are grown in "mini-gut" three-dimensional (3D) cultures from human intestinal stem cells and are non-transformed cells, which make them a biologically relevant system to study the GI epithelium 45 . Thus, we sought to determine if the dynamic cytosolic Ca 2+ signaling observed in MA104 cells were also observed in HIEs with RV infection.
We next tested if 3D jHIE-GCaMP6s enteroids would exhibit similar Ca 2+ dynamics during RV infection as observed in MA104-GCaMP5G cells. jHIE-GCaMP6s enteroids were mock-or RV-infected with the human RV strain Ito, seeded into chamber slides in diluted Matrigel, and imaged every 2-3 minutes for phase contrast and GCaMP6s fluorescence throughout the RV infection (~16 hrs). At 24 hpi, the HIEs were fixed and immunostained for RV antigens to confirm successful infection, which is evident by both infected cells within the HIEs as well as strong positive staining of the dead cells sloughed from the HIEs (Fig. 10D). Examination of the Ca 2+ signaling showed little Ca 2+ signaling activity in the mock-infected jHIE-GCaMP6s enteroids, but RV-infected enteroids exhibited significantly increased Ca 2+ dynamics, as illustrated in representative traces from three mock-or RV-infected HIEs ( Fig. 10E and Supplementary Video 9). Similar to the Ca 2+ signaling observed in MA104 cells, initially there were no or only modest changes in cytosolic Ca 2+ , and the onset of strong and dynamic Ca 2+ signals occurred ~8-10 HPI. For HIEs it was not possible to accurately measure Ca 2+ signaling at the single-cell level. We were able to track and measure Ca 2+ signaling over the entire jHIE-GCaMP6s enteroid and quantify these changes as Ca 2+ spikes/enteroid. We found that RV significantly increased the number of Ca 2+ spikes/enteroid (Fig. 10F) and that the Ca 2+ spike amplitudes are also substantially greater in RV-infected than in mock-infected jHIE-GCaMP6s enteroids (Fig. 10G). Thus, the RV-induced Ca 2+ signaling in enteroids closely parallels that observed in MA104 cells and demonstrate that these dynamic Ca 2+ signals are a biologically relevant aspect of how RV disrupts host Ca 2+ homeostasis.
Since SOCE played a prominent role in the RV-induced dynamic Ca 2+ signaling in MA104 cells, we investigated whether it was also critical for the Ca 2+ signaling observed in HIEs. Similar to MA104 cells, jejunum-derived HIEs expressed the core SOCE proteins Orai1, STIM1, and STIM2, as well as the non-store operated Orai3 channel (Fig. 11A). The expression levels were not substantially altered by differentiating the jHIEs through removal of growth factors. To test whether SOCE is important for RV-induced Ca 2+ signaling, we first tested 2-APB treatment of 3D jHIE-GCaMP6s enteroids either mock-or RV-infected with strain Ito. While RV infection increased the number of Ca 2+ spikes per enteroid consistent with above (Figs. 11B), 2-APB treatment did not attenuate the Ca 2+ signaling (Fig. 11B,C). We speculated that the 3D format or the Matrigel used to support 3D HIEs might interfere with 2-APB blocking SOCE, so we repeated these studies using jHIE-GCaMP6s monolayers. First, we confirmed that 2-APB can block thapsigargin-induced SOCE in jHIE-GCaMP6s monolayers, which exhibited a 32% reduction in Ca 2+ re-entry after store depletion (Fig. 11D). Interestingly, 2-APB shows a much less potent block of SOCE in enteroids than in MA104 cells, which exhibited a >80% inhibition of Ca 2+ re-entry after store depletion (Fig. 9C). We also tested if VACC or NCX may contribute to RV-induced Ca 2+ signaling in enteroids, but treatment with D600 or KB-R7943 did not reduce Ca 2+ spikes in RV-infected jHIE-GCaMP6s monolayers (Fig. 11E). Nevertheless, 2-APB treatment of both mock-inoculated (Fig. 11F, black vs. grey traces) and RV-infected jHIE-GCaMP6s monolayers (Fig. 11F, red vs. blue traces) reduced the observed Ca 2+ signaling, as illustrated in the representative traces (see Supplementary Video 10). We quantitated the Ca 2+ signaling per FOV and confirmed that 2-APB treatment significantly reduced the number of Ca 2+ spikes for both mock and RV-infected enteroids (Fig. 11G), as well as substantially reducing the amplitude of the Ca 2+ signals (Fig. 11H). Thus, like the MA104 model, SOCE is critical for supporting the dynamic Ca 2+ signaling induced in RV-infected jHIEs.

Discussion
A hallmark of RV infection, and several other viruses, is an elevation in cytosolic Ca 2+ and decrease in ER Ca 2+ stores, which facilitates virus replication and contributes to pathogenesis through a variety of downstream pathways 1,46 . The importance of RV-induced dysregulation of Ca 2+ levels for many of these downstream pathways has been determined, but thus far characteristics of the Ca 2+ signaling itself have not been extensively investigated 1,5 . Thus, the primary goal of this study was to determine the nature of the RV-induced elevation in cytosolic Ca 2+ and characterize how the dysregulation of Ca 2+ signaling manifests during the infection. By leveraging GECI-expressing cell lines to perform long-term Ca 2+ imaging, we found that RV induces a vast increase in Ca 2+ signaling events that increased in frequency and magnitude over the course of the infection. These results are consistent with previous measurements of cytosolic Ca 2+ in RV-infected cells that show a monophasic increase www.nature.com/scientificreports www.nature.com/scientificreports/ over time, which is similar to our imaging data when it is averaged out across the whole FOV (i.e., a cell population). Yet, what is paradigm changing is that at the individual cell level, RV does not merely cause a steady increase in cytosolic Ca 2+ , but rather activates a cacophony of discrete Ca 2+ signaling events. Further, by generating GECI-expressing HIEs, this study is the first characterization of RV-mediated Ca 2+ signaling in normal, human small intestinal enterocytes. We found that the prominence of the Ca 2+ spikes in RV-infected HIEs is similar to that in MA104 cells, underlining that this is a biologically relevant phenomenon. Data are the mean ± SD of three independent infections. ****p < 0.0001; **p < 0.01 by one-way ANOVA. (F) Western blot analysis of MA104-GCaMP5G cells mock or RV-infected MOI 1 and treated with DMSO or the SOCE blockers. Control RV-infected lysates treated with Endoglycosadase H (+EndoH) or untreated (-EndoH) are also shown. Blots were detected with α-RV, α-NSP4(120-147), and α-GAPDH for the loading control. Fulllength blots are presented in Supplementary Fig. 2. www.nature.com/scientificreports www.nature.com/scientificreports/ The characterization of the RV-induced increase in cytosolic Ca 2+ as a series of discrete, transient Ca 2+ signals is an important new insight into the cellular pathophysiology of RV infection. Transient increases in cytosolic Ca 2+ serve as pro-survival signals by activating phosphoinositide 3-kinase (PI3K) and by calcineurin-dependent NFAT activation. Further, Ca 2+ oscillations stimulate mitochondrial Ca 2+ uptake that enhances ATP synthesis, and this increase in mitochondrial metabolism contributes to cell survival pathways. In contrast, strong sustained elevation of cytosolic Ca 2+ drives pro-apoptotic signaling through mitochondrial Ca 2+ overload 20 . Thus, even though the mean cytosolic Ca 2+ level is progressively increasing in the RV-infected cell, early activation of the intrinsic apoptotic cascade may be prevented because it occurs as hundreds of transient Ca 2+ signals over hours. This premise is consistent with studies showing that early activation of PI3K during RV infection delays apoptosis 47 . Concomitantly, the elevated Ca 2+ signaling activates cellular pathways, such as autophagy, that promote RV replication and assembly of progeny virus 33 . Whether the initial Ca 2+ dynamics enhance mitochondria bioenergetics or ATP synthesis early during RV infection has not been studied, but a loss of mitochondria membrane potential and decrease in ATP output occur at late stages of infection 47,48 . Ultimately the massive increase in Ca 2+ signaling damages the cell and triggers cell death, and cell lysis was observed in our time-lapse imaging, but this data cannot differentiate whether this was through apoptosis, necrosis, and/or pyroptosis 18,47,49 . Thus, RV exploitation of discrete Ca 2+ signals, rather than a sustained increase in cytosolic Ca 2+ , may function in concert with other RV anti-apoptotic proteins, such as NSP1, to forestall the onset of cell death and enable sustained viral replication.   www.nature.com/scientificreports www.nature.com/scientificreports/ and the serotonin and chloride secretion that causes RV diarrhea 26,47,50,51 . Tracking these dynamic relationships poses a challenge that may be addressed by further engineering GECI-expressing cell lines/HIEs to express other biosensors such that both processes can be measured throughout the infection.
Many studies show that RV infection (or NSP4 expression) reduces the ER Ca 2+ stores based on a blunted cytosolic Ca 2+ release in response to agonists (e.g., ATP) or thapsigargin treatment to prevent SERCA-mediated refilling 22,52 . However, other results show increased in radioactive 45 Ca 2+ loading into the ER in RV-infected cells, which is hypothesized to be due to an increase in Ca 2+ binding proteins (e.g., VP7 or ER chaperone proteins) 7,11 . Thus, controversy remains about whether RV causes a decrease in the ER Ca 2+ store. To address this question, we developed MA104-RGECO1.2/GCEPIAer cells to directly measure cytosolic and ER Ca 2+ together during the RV infection. RV induces a dynamic decrease in ER Ca 2+ levels that occurs in conjunction with the increase in cytosolic Ca 2+ signaling. In most instances, the cytosolic Ca 2+ spike correlated with a decrease in ER Ca 2+ , indicating release of ER Ca 2+ substantially contributes to the increased cytosolic Ca 2+ signaling. Further, despite the 30% reduction in steady-state ER Ca 2+ , the dynamic nature of the ER Ca 2+ signaling suggests that SERCA pumps continually work to refill the ER. The observed depletion of ER Ca 2+ levels is consistent with the blunted cytosolic response to Ca 2+ agonists like ATP, the NSP4 function as a Ca 2+ -conducting viroporin in the ER, and the activation of the ER Ca 2+ sensor STIM1 7,9,10 . In contrast, it is more difficult to reconcile the previously observed increase in 45 Ca 2+ loading into the ER with the 30% reduction in steady-state ER Ca 2+ levels detected by GCEPIAer imaging in this study. It has been hypothesized that increased 45 Ca 2+ loading may be due to increased ER Ca 2+ buffering capacity, caused by the high levels of RV VP7 and/or chaperones BiP and endoplasmin 11 . However, our data suggest this is unlikely to be the case because this would sequester Ca 2+ and render GCEPIAer unresponsive to changes in ER Ca 2+ 24 , yet this is not the case because GCEPIAer remains dynamic throughout the infection. Alternatively, the increase 45 Ca 2+ may reflect loading into the ER-derived autophagy-like microdomains that surround viroplasms, which we observed form after the initial depletion in ER Ca 2+ and during a partial recovery of ER stores 33,53 . These ER microdomains are the site VP7 assembly onto nascent RV particles, which requires high Ca 2+ , so Ca 2+ sequestration in these microdomains may occur independently of the rest of the ER. Future studies using GCEPIAer and viroplasm-targeted GECIs are needed to determine whether the ER and viroplasm-associated membranes are functionally distinct compartments.
The pleiotropic functions of NSP4 are responsible for the RV-mediated dysregulation of host Ca 2+ homeostasis through the ion channel function of iNSP4 and Ca 2+ agonist function of the secreted eNSP4 enterotoxin 5,9,10 . Our data show that NSP4 governs the dynamic Ca 2+ signaling induced by RV infection since NSP4 knockdown significantly abrogated the number and amplitude of the Ca 2+ spikes. Unfortunately, it is not possible to determine the relative roles of iNSP4 versus eNSP4 in the induction of the Ca 2+ spikes from these data because the shRNA decreased total NSP4 synthesis, and therefore both pathways would be attenuated. The importance of NSP4 for the Ca 2+ signaling is also demonstrated by the extremely different Ca 2+ signaling profiles of OSUa-and OSUv-infected cells. These differences correlate with the attenuated elevation of cytosolic Ca 2+ caused by recombinant OSUa NSP4 both when expressed in Sf9 cells (i.e., iNSP4) and exogenous treatment of cells (i.e., eNSP4) 40 . The attenuated NSP4 phenotype is the result of mutations in the NSP4 enterotoxin domain, indicating that this domain is critical for induction of the Ca 2+ spikes by OSU 25,40 . However, it is important to note that these two viruses are not isogenic so the genetic backgrounds of the OSUa and OSUv NSP4 are different, requiring further Ca 2+ imaging studies using recombinant RV bearing these attenuating NSP4 mutations to fully dissect the relative importance iNSP4-and eNSP4-mediated Ca 2+ signaling and whether there is a difference in Ca 2+ signaling between pathogenic and non-pathogenic RV strains.
NSP4 is the trigger of the dynamic Ca 2+ signaling, yet these signals are maintained through host Ca 2+ channels and signaling pathways both in the ER and PM. Removal of extracellular Ca 2+ significantly attenuated the Ca 2+ spikes, demonstrating that Ca 2+ influx is crucial for these signals. Three classes of Ca 2+ channels (SOCE, NCX, and VACC) have been implicated RV-induced Ca 2+ influx [10][11][12]23 . Our results using different pharmacological blockers indicate SOCE is the primary Ca 2+ influx pathway that supports the RV-induced dynamic Ca 2+ spikes, both in MA104 cells and in HIEs. Blocking SOCE significantly reduced the number and amplitude of the RV-induced Ca 2+ spikes. Since elevated Ca 2+ levels are critical for RV replication, the attenuated Ca 2+ signaling caused by the SOCE blockers significantly reduced RV yield 33 . Interestingly, blocking SOCE in HIEs significantly reduced the Ca 2+ spikes, but the effect was less pronounced than in MA104 cells, suggesting other pathway(s) may exist that support RV-induced Ca 2+ spikes in HIEs. Further, RV can also infect other cell types, including monocytes and macrophages, hepatocytes, cholangiocytes, and enteroendocrine cells, all of which express a different repertoire of Ca 2+ channels 51,54-56 . Thus, characterizing the nature of RV-induced Ca 2+ signaling in these cell types will help elucidate whether these signals are involved in diseases caused by RV.
One unresolved question is the relative contribution of ER Ca 2+ release and Ca 2+ entry through the PM to the Ca 2+ spikes observed in the long-term Ca 2+ imaging studies. Our results from dual imaging of both cytosolic Ca 2+ and ER Ca 2+ showed that many of the cytosolic Ca 2+ spikes coincide with ER Ca 2+ troughs, indicating that ER Ca 2+ release contributes to a large number of these events. However, some cytosolic Ca 2+ spikes were observed that did not correspond with a decrease in ER Ca 2+ , which would suggest these events involve Ca 2+ entry. ER Ca 2+ release could occur either through iNSP4 channel activity or activation of the IP 3 -Receptor Ca 2+ channel, and our studies with Orai1 blockers indicate that SOCE is responsible for Ca 2+ entry-mediated cytosolic spikes. Nevertheless, future studies with genetic knockout of IP 3 -Receptor or Orai channels, as well as engineered mutation of NSP4 using the new RV reverse genetics system, will be needed to dissect the contribution of iNSP4, IP 3 -Receptors, and SOCE to the cytosolic Ca 2+ signaling. Furthermore, these studies will help determine whether Ca 2+ signals generated from these different channels selectively regulate downstream pathways important for RV replication (e.g., autophagy).
In summary, RV dysregulates host Ca 2+ homeostasis by a massive and progressive increase in discrete Ca 2+ signaling events, mainly from ER Ca 2+ release. Many viruses elevate cytosolic Ca 2+ and alter ER Ca 2+ , leading us to question whether dynamic Ca 2+ spikes, as seen in RV infection, is a common manifestation for virus-induced Ca 2+ signaling. If so, the host channels that support these Ca 2+ signals, such as Orai1, may represent novel targets for broadly acting host-directed antiviral therapeutics.