# Activation of store-operated calcium entry and mitochondrial respiration by enterovirus 71 is essential for efficient virus replication

## TL;DR

Enterovirus 71 (EV71) uses calcium signaling to boost its replication and spread by activating specific host cell proteins and increasing mitochondrial activity.

## Contribution

EV71 induces store-operated calcium entry and mitochondrial respiration via STIM1 and Orai1, which are critical for efficient replication and viral release.

## Key findings

- EV71 activates store-operated calcium entry through STIM1 and Orai1, increasing cytosolic calcium levels.
- Mitochondrial calcium influx enhances electron transport chain activity, supporting viral replication and inducing apoptosis for viral release.
- SOCE-dependent EV replication was confirmed in mouse intestinal organoids, showing physiological relevance.

## Abstract

Enterovirus (EV) infections disrupt cellular calcium (Ca2+) homeostasis. The EV protein 2B is localized to the endoplasmic reticulum (ER) and causes depletion of ER Ca2+ stores. This depletion coincides with a substantial increase in cytosolic Ca2+ levels driven by extracellular Ca2+ influx. However, the precise mechanism underlying this influx remains elusive. In the present study, we demonstrated that EV71 infections induce store-operated Ca2+ entry (SOCE) by activating the Ca2+ sensor stromal interaction molecule 1 (STIM1), which subsequently interacts with Orai1, a plasma membrane (PM) Ca2+ channel. This finding was supported by confocal imaging, which revealed that STIM1, typically localized in the ER, becomes active and colocalizes with Orai1 at the PM in EV71-infected cells. Pharmacological inhibition of the STIM1–Orai1 interaction and knockdown of either STIM1 or Orai1 significantly reduced virus-induced cytosolic Ca2+ levels and viral replication. Global transcriptome analysis revealed that differentially expressed genes are primarily associated with the mitochondrial electron transport chain (ETC) upon SOCE activation, contributing to enhanced ATP generation and oxygen consumption. This increase in mitochondrial Ca2+ levels is correlated with the mid-stage of virus infection. Furthermore, we demonstrated that high levels of mitochondrial Ca2+ influx led to apoptotic cell death, favoring viral release at the late stage of virus infection. Finally, SOCE-dependent EV replication was observed in a mouse intestinal organoid culture, a more physiologically relevant cell system. Our results provide valuable insights into the mechanism through which EV infections induce SOCE-mediated spatial and temporal control of Ca2+ signaling, substantially affecting the virus life cycle.

Host cell Ca2+ signals play crucial roles in various steps of virus life cycles, including entry, replication, and exit. EV requires increased cytosolic Ca2+ levels for efficient replication, but the precise mechanisms underlying the association between Ca2+ levels and EV replication remain elusive. Using EV71 as a model virus, we demonstrated that EV71 infection elevated cytosolic Ca2+ levels through store-operated Ca2+ entry activation and progressive Ca2+ mobilization to mitochondria. This led to the upregulation of electron transport chain activity, which is essential for efficient virus replication and apoptotic cell death, facilitating viral release during the mid and late stages of the infectious cycle, respectively. These findings substantially enhance the understanding of how EVs co-opt host cell mechanisms to promote their life cycle. STIM1 and Orai1 may be novel targets for broad-spectrum host-directed therapeutics against EVs and other viruses that employ similar replication mechanisms.