Salt Transiently Inhibits Mitochondrial Energetics in Mononuclear Phagocytes

Abstract

Background: Dietary high salt (HS) is a leading risk factor for mortality and morbidity. Serum sodium transiently increases postprandially, but can also accumulate at sites of inflammation affecting differentiation and function of innate and adaptive immune cells. Here, we focus on how changes in extracellular sodium, mimicking alterations in the circulation and tissues, affect the early metabolic, transcriptional and functional adaption of human and murine mononuclear phagocytes (MNP). Methods: Using Seahorse technology, pulsed stable isotope-resolved metabolomics and enzyme activity assays we characterize the central carbon metabolism and mitochondrial function of human and murine MNP under HS in vitro . HS as well as pharmacologic uncoupling of the electron transport chain (ETC) under normal salt (NS) is used to analyze mitochondrial function on immune cell activation and function (as determined by E.coli killing and CD4 + T cell migration capacity). In two independent clinical studies we analyze the impact of a HS diet over two weeks (NCT02509962) and short-term salt challenge by a single meal (NCT04175249) on mitochondrial function of human monocytes in vivo . Results: Extracellular sodium was taken up into the intracellular compartment followed by the inhibition of mitochondrial respiration in murine and human macrophages (MΦ). Mechanistically, HS reduces mitochondrial membrane potential, ETC complex II activity, oxygen consumption, and ATP production independently of the polarization status of MΦ. Subsequently, cell activation is altered with improved bactericidal function in HS-treated M1-like MΦ and diminished CD4+ T cell migration in HS-treated M2-like MΦ. Pharmacologic uncoupling of the ETC under NS phenocopies HS-induced transcriptional changes and bactericidal function of human and murine MNP. Clinically, also in vivo rise in plasma sodium concentration within the physiological range reversibly reduces mitochondrial function in human monocytes. In both, a 14-day and single meal HS challenge, healthy volunteers displayed a plasma sodium increase of ̃x = 2 mM and ̃x = 2.3 mM , respectively, that correlated with decreased monocytic mitochondrial oxygen consumption. Conclusions: Our data identify the disturbance of mitochondrial respiration as the initial step by which HS mechanistically influences immune cell function. While these functional changes might help to resolve bacterial infections, a shift towards pro-inflammation could accelerate inflammatory CVD.

High Na+ Environments Impair Phagocyte Oxidase-Dependent Antibacterial Activity of Neutrophils

“…As demonstrated earlier (8,12,13,65), addition of 40 mM NaCl to standard cell culture media (= high salt, HS) increased the antimicrobial activity of murine BMDM against E. coli (Figure 1A). In contrast, however, HS conditions impaired the antibacterial activity of bone marrow-derived murine neutrophils (BMN) (Figure 1B).…”

Mitochondria directly sense osmotic stress to trigger rapid metabolic remodeling via PDH regulation

“…Surprisingly, prior inhibition of glycolysis by 2-deoxy-glucose (2-DG) strongly abrogated the decrease in OCR upon hyperosmotic stress, suggesting that the alteration in glycolysis is the main reason for the acute decrease in OCR (Fig 3A , B). Considering that OCR was almost maintained even under hyperosmotic stress in 2-DG conditions, in contrast to a previous report regarding mononuclear phagocytes (Geisberger et al, 2021), a direct defect in the electron transport chain is not the main reason for the acute OCR decrease upon hyperosmotic stress in the present context. In contrast, prior inhibition of β-oxidation by etomoxir treatment scarcely abolished the decrease in OCR induced by hyperosmotic stress (Fig 3C , D).…”

“…In recent years, it has been suggested that in addition to the kidney, a high-sodium environment is observed in various sites of our body and regulates the immune system, implying that a hyperosmotic environment regulates immunity (Schatz et al , 2017; Müller et al , 2019). It has been reported that hyperosmotic conditions promote the differentiation of monocytes into proinflammatory M1 macrophages (Wilck et al , 2019; Geisberger et al , 2021). Furthermore, previous reports suggested in mouse models that a high-salt diet leads to the accumulation of sodium in the tumor and enhances antitumor immunity (Willebrand et al , 2019; He et al , 2020).…”

High-Salt Diet Induces Depletion of Lactic Acid-Producing Bacteria in Murine Gut

“…In mice, HSD was shown to be correlated to inflammatory response from small intestine and colonic mucosal immunity, leading to exacerbation of the DSS-induced colitis [ 19 ]. HSD-promoted inflammatory effects could be explained by an increased induction or activation of pro-inflammatory immune cells like T helper 17 cells (Th17) [ 20 , 21 ], IL-17-producing innate lymphoid type 3 cells (ILC3) [ 17 ], dendritic cells (DCs) [ 22 ] and M1-like macrophages [ 23 ], and a parallel inhibition of immune suppressive cell types like regulatory T cells (Tregs) or M2-like macrophages [ 24 , 25 ].…”