Glucose-induced activation of Tas1R3 disrupts calcium signaling and macrophage functionality

Abstract

The incidence of <em>diabetes mellitus</em> has risen alarmingly in recent years, becoming one of the leading causes of death worldwide. The disease is characterized by hyperglycemia, increased blood glucose levels, and is often accompanied by secondary diseases, including cardiovascular diseases, atherosclerosis and kidney failure. Moreover, diabetes has been linked to metaflammation, a chronic low-grade inflammation. One of the important immune cell types implicated in the onset of metaflammation and also diabetes are macrophages. These cells serve as key immune sentinels, playing an essential role in maintaining tissue homeostasis and mediating responses to microenvironmental changes. <br/> This thesis investigated how hyperglycemia influences the activation and functionality of murine and human macrophages, with a particular focus on calcium (Ca²⁺) signaling. <br/> We demonstrated that elevated glucose concentrations promote the secretion of pro-inflammatory cytokines, such as TNF and IL-1β, which is mediated via mTOR and NF-κB signaling. Furthermore, we showed that hyperglycemia leads to the depletion of Ca²⁺ from intracellular stores, such as the endoplasmic reticulum (ER), and consequently impaired Ca²⁺ signaling. We identified that macrophages sense glucose via the taste receptor Tas1R3 and delineated the downstream signaling mechanism. Glucose-mediated activation of Tas1R3 induces the release of Ca²⁺ from the ER via activation of the inositol triphosphate (IP₃ ) receptor and inhibition of the sarco /endoplasmic reticulum Ca²⁺ ATPase (SERCA). We revealed that the Tas1R3 signaling cascade involves the phospholipase C-mediated generation of IP₃ and the modulation of phospholamban, a potent modulator of SERCA, via PLC/PKC/PP1 signaling. Accordingly, by simultaneously promoting Ca²⁺ release from the ER and inhibiting Ca²⁺ reuptake, glucose induced an initial increase in cytosolic Ca²⁺ concentrations, followed by a loss of Ca²⁺ and depletion of Ca²⁺ stores over time. <br/> The intensity of glucose-induced Ca²⁺ signaling correlated with Tas1R3 expression across different cell subsets of murine and human cohorts. Moreover, we observed a strong negative correlation between blood glucose concentrations and the Ca²⁺ signaling intensity in human CD14⁺ monocytes and murine peritoneal macrophages, further highlighting that elevated glucose concentrations substantially influence Ca²⁺ signaling in macrophages within a physiological context. <br/> Finally, we demonstrated that hyperglycemia-induced disruption of Ca²⁺ homeostasis impairs macrophage functionality. Specifically, glucose-mediated depletion of Ca²⁺ from its stores induced ER stress and inhibited MHC I-restricted antigen presentation. In addition, elevated glucose concentrations disrupted chemokine-mediated Ca²⁺ signaling, thereby impairing cellular migration. <br/>In summary, these data revealed that hyperglycemic macrophages are primed towards a pro-inflammatory cytokine profile and exhibit impaired Ca²⁺ homeostasis, which further compromises Ca²⁺ -dependent cellular functions. Strikingly, our findings reveal a novel role of Tas1R3 in macrophages and provide new insights into the understanding and molecular mechanisms underlying immune dysfunction in the context of diabetes.