The role of the integrated stress response in brown adipose tissue-mediated metabolic adaptations

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This project seeks to understand the role of the integrated stress response (ISR) in brown adipose tissue (BAT) for systemic metabolic homeostasis. The studies proposed herein have the potential of uncovering a novel pathway to amplify thermogenic activation of BAT and to induce the BAT secretome, that might be independent of β3-adrenergic activation, and might be leveraged to combat obesity and its comorbidities. BAT plays a critical role in maintaining core body temperature through adaptive thermogenesis, and can affect adiposity by regulating key pathways in energy homeostasis. Accordingly, strategies designed to activate BAT, as well as to promote browning of white adipose tissue (WAT), could be attractive for combating obesity and associated diseases, such as diabetes and cardiovascular disease (CVD). Recent studies demonstrated that the ISR is activated in BAT in response to acute cold exposure, which correlated with induction of the unfolded protein response (UPR) and secretion of batokines known to exert protective effects on systemic metabolism, such as fibroblast growth factor 21 (FGF21) and growth and differentiation factor 15 (GDF15). We have generated preliminary data confirming these findings. In addition, we observed similar inductions of the UPR and the ISR in mice with BAT-specific deletion of the mitochondrial protein Optic atrophy 1 (OPA1). Interestingly, these mice had improved metabolic fitness and were better able to adapt to cold. Although studies suggest that BAT-derived FGF21 plays a negligible role on the systemic metabolic adaptations to cold exposure, the contribution of BAT-derived GDF15 on adaptive thermogenesis and on systemic metabolic homeostasis is incompletely understood. Therefore, we hypothesized that cold exposure and mitochondrial stress lead to the induction of the ISR and its master regulator activating transcription factor 4 (ATF4) in BAT, via the activation of the UPR kinase protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) to induce BAT secretion of GDF15, thereby improving systemic metabolic homeostasis. Extensive examination of the ISR in BAT will shed light on BAT physiology and adaptation to stress and may uncover novel therapeutic targets for the treatment of obesity, diabetes and CVD. Aim 1 of this proposal will investigate the requirement of PERK for ISR and ATF4 activation in BAT in response to cold and mitochondrial stress. Aim 2 will determine whether ATF4 is required and sufficient for GDF15 induction and for proper adaptive thermogenesis. Aim 3 will determine whether GDF15 is required to mediate the systemic metabolic adaptations following cold exposure and mitochondrial stress in BAT, and whether it mediates its effects through central activation of its receptor glial-derived neurotrophic factor receptor alpha-like (GFRAL). Generation of novel genetic mouse models will reveal the molecular mechanisms underlying ISR activation and the physiological roles of ATF4 and GDF15 in BAT, which may inform novel therapeutic strategies to combat obesity and its comorbidities.