Fowl adenovirus serotype 4 infection upregulates calcium levels via endoplasmic reticulum stress to promote viral replication in LMH cells.

Hepatitis-hydropericardium syndrome (HHS), caused by fowl adenovirus serotype 4 (FAdV-4), is an acute and highly fatal disease that poses a major threat to the global poultry industry. Calcium ions serve as critical second messengers that regulate multiple stages of the viral life cycle, including entry, replication, and assembly. However, whether FAdV-4 exploits intracellular calcium signaling to facilitate its own replication remains largely unexplored. Here, we demonstrate that FAdV-4 infection significantly elevates intracellular calcium levels in LMH cells in a time- and dose-dependent manner. Mechanistically, FAdV-4 induces endoplasmic reticulum (ER) stress, which in turn upregulates the expression of STIM1 and ORAI1 and promotes the translocation of STIM1 to the plasma membrane, thereby activating the store-operated calcium entry (SOCE) channel and driving sustained intracellular calcium elevation. Functionally, elevation of intracellular calcium enhances viral protein expression and increases viral titers, whereas calcium depletion markedly suppresses FAdV-4 replication. Collectively, our findings uncover a previously unrecognized mechanism by which FAdV-4 hijacks intracellular calcium homeostasis to promote viral replication, offering new potential targets for antiviral therapy.