*5.3. IP3 Receptors*

The engagemen<sup>t</sup> of receptors or agonist binding to the cell surface receptors activate phospholipase C (PLC), which hydrolyses phosphatidylinositol-4,5-biphosphate (PIP2) to produce IP3. The activation of IP3R triggers Ca2+ release from stores and further increases IP3R's sensitivity to Ca2+. Targeting IP3Rs is more universal among di fferent virus types. In turn, this process generates a more profound effect on host cell physiology, including metabolic stress, neurotoxicity, and enterotoxicity. Modifying IP3 and directly interfering with IP3R are the main ways in which viruses a ffect calcium release through IP3R. For example, human cytomegalovirus (HCMV) UL37x1 protein interacts with the host P2Y2 purinergic receptor to increase intracellular Ca2+ levels via the PLC–IP3 pathway, and this activity is important to viral replication [61]. HIV-1 Gp120 and Tat upregulate intracellular IP3 [37,62], while HIV Nef [63,64] and p12(I) human T-cell lymphotropic virus type 1 (HTLV-1) [65] activate IP3R directly as an agonist. Glycoprotein E of HSV redistributes the density of IP3Rs within infected cells [66]. Some virus proteins activate IP3R through both ways. Enterotoxin NSP4 of RV and HCMV UL37x1-encoded protein increases the basal permeability of the ER as IP3 di ffuses and binds to IP3Rs to stimulate Ca2+ release. These virus proteins deplete ER Ca2+ storage during early stages of viral infection to increase the replication ability of viruses. On the other hand, the depletion of ER calcium storage triggers the calcium influx through SOCs (STIM1/ORAI1).

### **6. Calcium Pumps**

The calcium pumps (Ca<sup>2</sup>+ ATPase) and the Na<sup>+</sup>–Ca2<sup>+</sup> exchanger are the main regulators of intracellular Ca2+ concentrations [67]. Three Ca2+ ATPase types (pumps) have been described in animal cells located in the membranes of the sarcoplasmic reticulum (SERCA pump), in those of the Golgi network (Secretory Pathway Ca2+ ATPase, SPCA pump), and in the plasma membrane (PMCA pump) [68]. At the end of a stimulus, these pumps participate in returning the cell to its resting state by the decrease in the cytosolic Ca2+ concentration. For example, SERCA transports Ca2+ from the cytoplasm into the lumen of the ER. PMCA pumps function to remove Ca2+ from the cell. They serve as the main regulators of intracellular and extracellular Ca2+ concentrations. All three calcium pumps are reported to be involved in di fferent viral lifecycles.

Maturation is the last assembly step of a virus particle. In this step, the virus acquires the infectivity, which is essential for its lifecycle. The results obtained from a genome-wide knockout screen indicated

that SPCA1 in the Golgi network is critical for human respiratory syncytial virus (RSV) infection [69–71]. Further study demonstrated that similar to RSV, other viruses from the Paramyxoviridae, Flaviviridae, and Togaviridae families also failed to spread in SPCA1-deficient cells. Studies on this mechanism revealed that Ca2+ pumped into Golgi by SPCA1 is the trigger to produce normal functional viral glycoproteins that are essential for virus spread. Therefore, SPCA1 may be a promising target for therapeutic intervention against a diverse set of viruses (Figure 2B). However, in this study, herpes simplex 1 (HSV-1), vesicular stomatitis (VSV), bunyamwera (BUNV), and LCMV were unaffected by the loss of SPCA1. That is probably because the maturation of these viral glycoproteins requires different triggers.

Cui et al. [72] screened the natural product library and found that cyclopiazonic acid, an inhibitor of SERCA, was shown to have activity in the low micromolar range against RSV strains at the step of virus genome replication. Further study found that another SERCA inhibitor BHQ had a similar effect [72]. It is probable that SERCA inhibitors prevent cytosolic calcium returning to the ER from the cytosol via SERCA, resulting in an increase in intracellular calcium concentration. A continuous higher concentration of intracellular calcium may impair viral genome replication and/or transcription, thereby reducing virus yield.
