Dissecting the Ca²⁺ entry pathways induced by rotavirus infection and NSP4-EGFP expression in Cos-7 cells.

Rotavirus infection modifies Ca(2+) homeostasis provoking an increase in Ca(2+) permeation, cytoplasmic Ca(2+) concentration (Ca(2+)), total Ca(2+) pools and, a decrease of Ca(2+) response to agonists. These effects are mediated by NSP4. The mechanism by which NSP4 deranges Ca(2+) homeostasis is not yet known. It has been proposed that the increase in Ca(2+) is the result of Ca(2+) release from intracellular stores, thereby activating store-operated Ca(2+) entry (SOCE). We studied the mechanisms involved in the changes of Ca(2+) permeability of the plasma membrane elicited by rotavirus infection and NSP4 expression in Cos-7 cells loaded with fura-2 or fluo-4, using inhibitors and activators of different pathways. Total depletion of ER Ca(2+) stores induced by thapsigargin or ATP was not able to elicit Ca(2+) entry in mock-infected cells to the level attained with infection or NSP4-EGFP expression. The pathway induced by NSP4-EGFP expression or infection shows properties shared by SOCE: it can be inactivated by high Ca(2+), is permeable to Mn(2+) and inhibited by La(3+) and the SOC inhibitor 2-aminoethoxydiphenyl borate (2-APB). Contribution of the agonist-operated channels (AOCs) to Ca(2+) entry is small and not modified by infection. The plasma membrane permeability to Ca(2+) in rotavirus infected or NSP4-EGFP expressing cells is also blocked by KB-R7943, an inhibitor of the plasma membrane Na(+)/Ca(2+) exchanger (NCX), operating in its reverse mode. In conclusion, the expression of NSP4 in infected Cos-7 cells appears to activate the NCX in reverse mode and the SOCE pathway to induce increased Ca(2+) entry.