Toll-like receptor 9 protects non-immune cells from stress by modulating mitochondrial ATP synthesis through the inhibition of SERCA2

Toll‐like receptor 9 (TLR9) has a key role in the recognition of pathogen DNA in the context of infection and cellular DNA that is released from damaged cells. Pro‐inflammatory TLR9 signalling pathways in immune cells have been well investigated, but we have recently discovered an alternative pathway in which TLR9 temporarily reduces energy substrates to induce cellular protection from stress in cardiomyocytes and neurons. However, the mechanism by which TLR9 stimulation reduces energy substrates remained unknown. Here, we identify the calcium‐transporting ATPase, SERCA2 (also known as Atp2a2), as a key molecule for the alternative TLR9 signalling pathway. TLR9 stimulation reduces SERCA2 activity, modulating Ca 2+ handling between the SR/ER and mitochondria, which leads to a decrease in mitochondrial ATP levels and the activation of cellular protective machinery. These findings reveal how distinct innate responses can be elicited in immune and non‐immune cells—including cardiomyocytes—using the same ligand‐receptor system. Synopsis TLR9 signalling has recently been shown to protect cardiomyocytes and neurons from stress by modulating energy metabolism. This study shows that it does so by inhibiting SERCA2 activity and thus the Ca 2+ transfer between SR/ER and mitochondria. SERCA2 is a key adaptor for the alternative TLR9 signalling pathway. SERCA2 inhibition by TLR9 modulates Ca 2+ handling between the SR/ER and mitochondria. A decrease in mitochondrial Ca 2+ and subsequent ATP synthesis activates cellular protective machinery. Graphical Abstract TLR9 signalling has recently been shown to protect cardiomyocytes and neurons from stress by modulating energy metabolism. This study shows that it does so by inhibiting SERCA2 activity and thus the Ca 2+ transfer between SR/ER and mitochondria.