Like many viruses, rotavirus (RV) dysregulates calcium homeostasis by elevating cytosolic calcium ([Ca 2+ ]cyt) and decreasing endoplasmic reticulum (ER) stores. While an overall, monophasic increase in [Ca 2+ ]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 [Ca 2+ ]cyt signaling that manifest as hundreds of discrete [Ca 2+ ]cyt spikes, which increase during peak infection. Knockdown of nonstructural protein 4 (NSP4) attenuates the [Ca 2+ ]cyt spikes, consistent with its role in dysregulating calcium homeostasis. RV-induced [Ca 2+ ]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 [Ca 2+ ]cyt spikes that were attenuated by inhibiting SOCE, underlining the relevance of these [Ca 2+ ]cyt spikes to gastrointestinal physiology and role of SOCE in RV pathophysiology. Thus, our discovery that RV increases [Ca 2+ ]cyt by dynamic calcium signaling, establishes a new, paradigm-shifting understanding of the spatial and temporal complexity of virus-induced calcium signaling.

Calcium signaling is a ubiquitous and versatile process involved in nearly every cellular process, and exploitation of host calcium signals is a common strategy used by viruses to facilitate replication and cause disease. Small molecule fluorescent calcium dyes have been used by many to examine changes in host cell calcium signaling and calcium channel activation during virus infections, but disadvantages of these dyes, including poor loading and poor long-term retention, complicate analysis of calcium imaging in virus-infected cells due to changes in cell physiology and membrane integrity. The recent expansion of genetically-encoded calcium indicators (GECIs), including blue and red-shifted color variants and variants with calcium affinities appropriate for calcium storage organelles like the endoplasmic reticulum (ER), make the use of GECIs an attractive alternative for calcium imaging in the context of virus infections. Here we describe the development and testing of cell lines stably expressing both green cytoplasmic (GCaMP5G and GCaMP6s) and red ER-targeted (RCEPIAer) GECIs. Using three viruses (rotavirus, poliovirus and respiratory syncytial virus) previously shown to disrupt host calcium homeostasis, we show the GECI cell lines can be used to detect simultaneous cytoplasmic and ER calcium signals. Further, we demonstrate the GECI expression has sufficient stability to enable long-term confocal imaging of both cytoplasmic and ER calcium during the course of virus infections.

28 Like many viruses, rotavirus (RV) dysregulates calcium homeostasis by elevating 29 cytosolic calcium ([Ca 2+ ]cyt) and decreasing endoplasmic reticulum (ER) stores. While 30 an overall, monophasic increase in [Ca 2+ ]cyt during RV infection has been shown, the 31 nature of the RV-induced aberrant calcium signals and how they manifest over time at 32 the single-cell level have not been characterized. Thus, we generated cell lines and 33 human intestinal enteroids (HIEs) stably expressing cytosolic and/or ER-targeted 34 genetically-encoded calcium indicators to characterize calcium signaling throughout RV 35 infection by time-lapse imaging. We found that RV induces highly dynamic [Ca 2+ ]cyt 36 signaling that manifest as hundreds of discrete [Ca 2+ ]cyt spikes, which increase during 37 peak infection. Knockdown of nonstructural protein 4 (NSP4) attenuates the [Ca 2+ ]cyt 38 spikes, consistent with its role in dysregulating calcium homeostasis. RV-induced 39 [Ca 2+ ]cyt spikes were primarily from ER calcium release and were attenuated by 40 inhibiting the store-operated calcium entry (SOCE) channel Orai1. RV-infected HIEs 41 also exhibited prominent [Ca 2+ ]cyt spikes that were attenuated by inhibiting SOCE, 42 underlining the relevance of these [Ca 2+ ]cyt spikes to gastrointestinal physiology and 43 role of SOCE in RV pathophysiology. Thus, our discovery that RV increases [Ca 2+ ]cyt 44 by dynamic Ca 2+ signaling, establishes a new, paradigm-shifting understanding of the 45 spatial and temporal complexity of virus-induced Ca 2+ signaling. 46 47 48 Eukaryotic signal transduction pathways employ a variety of signaling molecules 49 to regulate cellular processes. Calcium (Ca 2+ ) is one of the most ubiquitous secondary 50 messengers in the cell, which tightly regulates Ca 2+ movement through the coordinated 51 function of Ca 2+ channels, transporters, and pumps. Since Ca 2+ signaling modulates a 52 wide array of cellular processes, it is not surprising that many different viruses exploit 53 Ca 2+ signaling to facilitate their replication, and the resulting dysregulation of Ca 2+ 54 signaling causes pathogenesis. Rotavirus (RV), a member of the Reoviridae family, is 55 one of the first viruses shown to elevate cellular Ca 2+ levels and has become a widely-56 used model system to characterize mechanisms by which viruses dysregulate host Ca 2+ 57 homeostasis 1 . RV is a clinically important enteric virus that causes severe diarrhea and 58 vomiting in children, resulting in over approximately 258 million diarrhea episodes and 59 198,000 deaths in 2016 2 . Hyperactivation of cyclic nucleotide (e.g., cAMP/cGMP) and 60 Ca 2+ signaling pathways is a common strategy among enteric pathogens 3 . Thus, 61 understanding how RV exploits Ca 2+ signaling is key to understanding and combating 62 RV-induced diarrhea. 63 RV was first reported to elevate cytosolic [Ca 2+ ] by Michelangeli et al. (1991), 64 which stimulated subsequent research into how RV alters cellular Ca 2+ levels 4 . RV 65 causes a 2-fold steady-state increase in ...