Abstract
Resistance and tolerance are two coexisting defense strategies for fighting infections. Resistance is mediated by signaling pathways that induce transcriptional activation of resistance factors that directly eliminate the pathogen. Tolerance refers to adaptations that limit the health impact of a given pathogen burden, without targeting the infectious agent. The key players governing immune tolerance are largely unknown. In Drosophila, the histone H3 lysine 9 (H3K9) methyltransferase G9a was shown to mediate tolerance to virus infection and oxidative stress (OS), suggesting that abiotic stresses like OS may also evoke tolerance mechanisms. In response to both virus and OS, stress resistance genes were overinduced in Drosophila G9a mutants, suggesting an intact but overactive stress response. We recently demonstrated that G9a promotes tolerance to OS by maintaining metabolic homeostasis and safeguarding energy availability, but it remained unclear if this mechanism also applies to viral infection, or is conserved in other species and stress responses. To address these questions, we analyzed publicly available datasets from Drosophila, mouse, and human in which global gene expression levels were measured in G9a-depleted conditions and controls at different time points upon stress exposure. In all investigated datasets, G9a attenuates the transcriptional stress responses that confer resistance against the encountered stressor. Comparative analysis of conserved G9a-dependent stress response genes suggests that G9a is an intimate part of the design principles of stress resistance, buffering the induction of promiscuous stress signaling pathways and stress-specific resistance factors. Importantly, we find stress-dependent downregulation of metabolic genes to also be dependent on G9a across all of the tested datasets. These results suggest that G9a sets the balance between activation of resistance genes and maintaining metabolic homeostasis, thereby ensuring optimal organismal performance during exposure to diverse types of stress across different species. We therefore propose G9a as a potentially conserved master regulator underlying the widely important, yet poorly understood, concept of stress tolerance.
