The multi-faceted role of NADPH in regulating inflammation in activated myeloid cells

Metabolic control of microglia in health and disease

Metabolic states within cells are tightly linked to functional outcomes and finely regulated by nutrient availability. A growing body of the literature supports the idea that various metabolites can influence cellular functions, such as cell differentiation, migration, and proliferation in different contexts, with ample evidence coming from the immune system. Additionally, certain functional programs can trigger significant metabolic changes within cells, which are crucial not only to meet high energy demands, but also to produce intermediate metabolites necessary to support specific tasks. Microglia, the resident innate immune cells of the central nervous system, are constantly active, surveying the brain parenchyma and providing support to neighboring cells in the brain. They exhibit high metabolic flexibility, capable of quickly undergoing metabolic reprogramming based on nutrient availability and functional requirements. In this chapter, we will discuss the major metabolic pathways within cells and provide examples of how relevant enzymes and metabolites can impact microglial function in physiologic and pathologic contexts.

Measuring the rate of NADPH consumption by glutathione reductase in the cytosol and mitochondria

BACKGROUND

NADPH is an essential co-factor supporting the function of enzymes that participate in both inflammatory and anti-inflammatory pathways in myeloid cells, particularly macrophages. Although individual NADPH-dependent pathways are well characterized, how these opposing pathways are co-regulated to orchestrate an optimized inflammatory response is not well understood. To investigate this, techniques to track the consumption of NADPH need to be applied. Deuterium tracing of NADPH remains the gold standard in the field, yet this setup of mass-spectrometry is technically challenging and not readily available to most research groups. Furthermore, NADPH pools are compartmentalized in various organelles with no known membrane transporters, suggesting that NADPH-dependent pathways are regulated in an organelle-specific manner. Conventional methods such as commercial kits are limited to quantifying NADPH in whole cells and not at the resolution of specific organelles. These limitations reflect the need for a novel assay that can readily measure the consumption rate of NADPH in different organelles.

METHODS

We devised an assay that measures the consumption rate of NADPH by glutathione-disulfide reductase (GSR) in the mitochondria and the cytosol of RAW264.7 macrophage cell lines. RAW264.7 cells were transfected with Apollo-NADP+ sensors targeted to the mitochondria or the cytosol, followed by the treatment of 2-deoxyglucose and diamide. Intravital imaging over time then determined GSR-dependent NADPH consumption in an organelle-specific manner.

DISCUSSION

In lipopolysaccharide (LPS)-stimulated RAW264.7 cells, cytosolic and mitochondrial NADPH was consumed by GSR in a time-dependent manner. This finding was cross validated with a commercially available NADPH kit that detects NADPH in whole cells. Loading of RAW264.7 cells with oxidized low-density lipoprotein followed by LPS stimulation elevated GSR expression, and this correlated with a more rapid drop in cytosolic and mitochondrial NADPH in our assay. The current limitation of our assay is applicability to transfectable cell lines, and higher expression of plasmid-encoded sensors relative to endogenous glucose-6-phosphate dehydrogenase.

Transcriptional Regulation and Function of Malic Enzyme 1 in Human Macrophage Activation

Macrophages represent primary players of the innate immune system. Macrophage activation triggers several signaling pathways and is tightly associated with metabolic changes, which drive different immune subsets. Recent studies unveil the role of various metabolic enzymes in macrophage activation. Here, we show that malic enzyme 1 (ME1) is overexpressed in LPS-induced macrophages. Through chromatin immunoprecipitation, we demonstrate that ME1 transcriptional regulation is under control of NF-κB. Furthermore, ME1 activity is also increased in activated human PBMC-derived macrophages. Notably, ME1 gene silencing decreases nitric oxide as well as reactive oxygen species and prostaglandin E2 inflammatory mediators. Therefore, modulating ME1 provides a potential approach for immunometabolic regulation and in turn macrophage function.

Immunomodulatory properties of chicken cathelicidin-2 investigated on an ileal explant culture

As the threat posed by antimicrobial resistance grows more crucial, the development of compounds that can replace antibiotics becomes increasingly vital. Chicken cathelicidin-2 (Cath-2) belongs to the group of Host Defense Peptides (HDPs), which could provide a feasible solution for the treatment of gastrointestinal infections in poultry. It is a small peptide produced by the heterophil granulocytes of chickens as part of the innate immune response, and its immunomodulatory activity has already been demonstrated in several cell types. In this study, the effects of Cath-2 on the intestinal immune response were examined using ileal explant cultures isolated from chicken. Regarding our results, Cath-2 displayed a potent anti-inflammatory effect as it alleviated the LTA-caused elevation of interleukin (IL)-6 and IL-2 concentrations, and that of the IFN-γ/IL-10 ratio, furthermore, it increased the concentration of IL-10, alleviating the LTA-evoked decreased level of the anti-inflammatory cytokine. Moreover, when applied alone, it elevated the concentrations of IL-6, CXCLi2, and IL-2, providing evidence of its complex immunomodulatory mechanisms. In summary, Cath-2 was able to modulate the immune response of the intestinal wall not only by reducing pro-inflammatory cytokine release, but also through immune stimulation, demonstrating that it has the ability to improve innate immunity via a complex mechanism that may make it a suitable candidate for the control of intestinal infections.

Revisiting the role of hypoxia-inducible factors and nuclear factor erythroid 2-related factor 2 in regulating macrophage inflammation and metabolism

The recent birth of the immunometabolism field has comprehensively demonstrated how the rewiring of intracellular metabolism is critical for supporting the effector functions of many immune cell types, such as myeloid cells. Among all, the transcriptional regulation mediated by Hypoxia-Inducible Factors (HIFs) and Nuclear factor erythroid 2-related factor 2 (NRF2) have been consistently shown to play critical roles in regulating the glycolytic metabolism, redox homeostasis and inflammatory responses of macrophages (Mφs). Although both of these transcription factors were first discovered back in the 1990s, new advances in understanding their function and regulations have been continuously made in the context of immunometabolism. Therefore, this review attempts to summarize the traditionally and newly identified functions of these transcription factors, including their roles in orchestrating the key events that take place during glycolytic reprogramming in activated myeloid cells, as well as their roles in mediating Mφ inflammatory responses in various bacterial infection models.

John Yudkin's hypothesis: sugar is a major dietary culprit in the development of cardiovascular disease

To date, the risk of developing atherosclerosis has extended beyond Western countries and now affecting individuals from various ethnic backgrounds and age groups. Traditional risk factors of atherosclerosis, such as hypercholesterolemia, has been better controlled than before due to highly effective and inexpensive therapies at lowering plasma cholesterol levels. However, the role of reducing dietary cholesterol intake, as a public healthy strategy, in preventing the occurrence of cardiovascular mortalities has been recently challenged. Indeed, despite our continuous decline of dietary cholesterol intake within the last 50 years, the incidence of cardiovascular mortalities has continued to rise, thus raising the possibility that other dietary factors, such as fructose-containing sugars, are the major culprit. In the 1970s, John Yudkin first proposed that sugar was the predominant dietary factor that underlies the majority of cardiovascular mortalities, yet his hypothesis was dismissed. However, over the last 25 years substantial scientific evidence has been accumulated to support Yudkin's hypothesis. The objectives of this review are to highlight Yudkin's significant contribution to nutritional science by reviewing his hypothesis and summarizing the recent advances in our understanding of fructose metabolism. The metabolic consequences of fructose metabolism, such as fructose-induced uricemia, insulin resistance, lipoprotein hyperproduction and chronic inflammation, and how they are linked to atherosclerosis as risk factors will be discussed. Finally, the review will explore areas that warrant future research and raise important considerations that we need to evaluate when designing future studies.

Cholesterol accumulation impairs HIF-1α-dependent immunometabolic reprogramming of LPS-stimulated macrophages by upregulating the NRF2 pathway

Lipid accumulation in macrophages (Mφs) is a hallmark of atherosclerosis. Yet, how lipid loading modulates Mφ inflammatory responses remains unclear. We endeavored to gain mechanistic insights into how pre-loading with free cholesterol modulates Mφ metabolism upon LPS-induced TLR4 signaling. We found that activities of prolyl hydroxylases (PHDs) and factor inhibiting HIF (FIH) are higher in cholesterol loaded Mφs post-LPS stimulation, resulting in impaired HIF-1α stability, transactivation capacity and glycolysis. In RAW264.7 cells expressing mutated HIF-1α proteins resistant to PHDs and FIH activities, cholesterol loading failed to suppress HIF-1α function. Cholesterol accumulation induced oxidative stress that enhanced NRF2 protein stability and triggered a NRF2-mediated antioxidative response prior to and in conjunction with LPS stimulation. LPS stimulation increased NRF2 mRNA and protein expression, but it did not enhance NRF2 protein stability further. NRF2 deficiency in Mφs alleviated the inhibitory effects of cholesterol loading on HIF-1α function. Mutated KEAP1 proteins defective in redox sensing expressed in RAW264.7 cells partially reversed the effects of cholesterol loading on NRF2 activation. Collectively, we showed that cholesterol accumulation in Mφs induces oxidative stress and NRF2 stabilization, which when combined with LPS-induced NRF2 expression leads to enhanced NRF2-mediated transcription that ultimately impairs HIF-1α-dependent glycolytic and inflammatory responses.

Fructose overconsumption-induced reprogramming of microglia metabolism and function

The overconsumption of dietary fructose has been proposed as a major culprit for the rise of many metabolic diseases in recent years, yet the relationship between a high fructose diet and neurological dysfunction remains to be explored. Although fructose metabolism mainly takes place in the liver and intestine, recent studies have shown that a hyperglycemic condition could induce fructose metabolism in the brain. Notably, microglia, which are tissue-resident macrophages (Mφs) that confer innate immunity in the brain, also express fructose transporters (GLUT5) and are capable of utilizing fructose as a carbon fuel. Together, these studies suggest the possibility that a high fructose diet can regulate the activation and inflammatory response of microglia by metabolic reprogramming, thereby altering the susceptibility of developing neurological dysfunction. In this review, the recent advances in the understanding of microglia metabolism and how it supports its functions will be summarized. The results from both in vivo and in vitro studies that have investigated the mechanistic link between fructose-induced metabolic reprogramming of microglia and its function will then be reviewed. Finally, areas of controversies and their associated implications, as well as directions that warrant future research will be highlighted.