Abstract
Toll-like receptors (TLRs) are the central players in innate immunity. In particular, TLR9 initiates inflammatory response by recognizing DNA, imported by infection or released from tissue damage. Inflammation is, however, harmful to terminally differentiated organs, such as the heart and brain, with poor regenerative capacity, yet the role of TLR9 in such nonimmune cells, including cardiomyocytes and neurons, is undefined. Here we uncover an unexpected role of TLR9 in energy metabolism and cellular protection in cardiomyocytes and neurons. TLR9 stimulation reduced energy substrates and increased the AMP/ATP ratio, subsequently activating AMP-activated kinase (AMPK), leading to increased stress tolerance against hypoxia in cardiomyocytes without inducing the canonical inflammatory response. Analysis of the expression profiles between cardiomyocytes and macrophages identified that unc93 homolog B1 (C. elegans) was a pivotal switch for the distinct TLR9 responses by regulating subcellular localization of TLR9. Furthermore, this alternative TLR9 signaling was also found to operate in differentiated neuronal cells. These data propose an intriguing model that the same ligand-receptor can concomitantly increase the stress tolerance in cardiomyocytes and neurons, whereas immune cells induce inflammation upon tissue injury.