Human resident liver myeloid cells protect against metabolic stress in obesity.

Although multiple populations of macrophages have been described in the human liver, their function and turnover in patients with obesity at high risk of developing non-alcoholic fatty liver disease (NAFLD) and cirrhosis are currently unknown. Herein, we identify a specific human population of resident liver myeloid cells that protects against the metabolic impairment associated with obesity. By studying the turnover of liver myeloid cells in individuals undergoing liver transplantation, we find that liver myeloid cell turnover differs between humans and mice. Using single-cell techniques and flow cytometry, we determine that the proportion of the protective resident liver myeloid cells, denoted liver myeloid cells 2 (LM2), decreases during obesity. Functional validation approaches using human 2D and 3D cultures reveal that the presence of LM2 ameliorates the oxidative stress associated with obese conditions. Our study indicates that resident myeloid cells could be a therapeutic target to decrease the oxidative stress associated with NAFLD.

SARS-CoV-2 Nsp13 encodes for an HLA-E-stabilizing peptide that abrogates inhibition of NKG2A-expressing NK cells.

Natural killer (NK) cells are innate immune cells that contribute to host defense against virus infections. NK cells respond to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in vitro and are activated in patients with acute coronavirus disease 2019 (COVID-19). However, by which mechanisms NK cells detect SARS-CoV-2-infected cells remains largely unknown. Here, we show that the Non-structural protein 13 of SARS-CoV-2 encodes for a peptide that is presented by human leukocyte antigen E (HLA-E). In contrast with self-peptides, the viral peptide prevents binding of HLA-E to the inhibitory receptor NKG2A, thereby rendering target cells susceptible to NK cell attack. In line with these observations, NKG2A-expressing NK cells are particularly activated in patients with COVID-19 and proficiently limit SARS-CoV-2 replication in infected lung epithelial cells in vitro. Thus, these data suggest that a viral peptide presented by HLA-E abrogates inhibition of NKG2A+ NK cells, resulting in missing self-recognition.

A subset of Kupffer cells regulates metabolism through the expression of CD36.

Tissue macrophages are immune cells whose phenotypes and functions are dictated by origin and niches. However, tissues are complex environments, and macrophage heterogeneity within the same organ has been overlooked so far. Here, we used high-dimensional approaches to characterize macrophage populations in the murine liver. We identified two distinct populations among embryonically derived Kupffer cells (KCs) sharing a core signature while differentially expressing numerous genes and proteins: a major CD206loESAM- population (KC1) and a minor CD206hiESAM+ population (KC2). KC2 expressed genes involved in metabolic processes, including fatty acid metabolism both in steady-state and in diet-induced obesity and hepatic steatosis. Functional characterization by depletion of KC2 or targeted silencing of the fatty acid transporter Cd36 highlighted a crucial contribution of KC2 in the liver oxidative stress associated with obesity. In summary, our study reveals that KCs are more heterogeneous than anticipated, notably describing a subpopulation wired with metabolic functions.

Hepatic miR-144 Drives Fumarase Activity Preventing NRF2 Activation During Obesity.

BACKGROUND AND AIMS: Oxidative stress plays a key role in the development of metabolic complications associated with obesity, including insulin resistance and the most common chronic liver disease worldwide, nonalcoholic fatty liver disease. We have recently discovered that the microRNA miR-144 regulates protein levels of the master mediator of the antioxidant response, nuclear factor erythroid 2-related factor 2 (NRF2). On miR-144 silencing, the expression of NRF2 target genes was significantly upregulated, suggesting that miR-144 controls NRF2 at the level of both protein expression and activity. Here we explored a mechanism whereby hepatic miR-144 inhibited NRF2 activity upon obesity via the regulation of the tricarboxylic acid (TCA) metabolite, fumarate, a potent activator of NRF2. METHODS: We performed transcriptomic analysis in liver macrophages (LMs) of obese mice and identified the immuno-responsive gene 1 (Irg1) as a target of miR-144. IRG1 catalyzes the production of a TCA derivative, itaconate, an inhibitor of succinate dehydrogenase (SDH). TCA enzyme activities and kinetics were analyzed after miR-144 silencing in obese mice and human liver organoids using single-cell activity assays in situ and molecular dynamic simulations. RESULTS: Increased levels of miR-144 in obesity were associated with reduced expression of Irg1, which was restored on miR-144 silencing in vitro and in vivo. Furthermore, miR-144 overexpression reduces Irg1 expression and the production of itaconate in vitro. In alignment with the reduction in IRG1 levels and itaconate production, we observed an upregulation of SDH activity during obesity. Surprisingly, however, fumarate hydratase (FH) activity was also upregulated in obese livers, leading to the depletion of its substrate fumarate. miR-144 silencing selectively reduced the activities of both SDH and FH resulting in the accumulation of their related substrates succinate and fumarate. Moreover, molecular dynamics analyses revealed the potential role of itaconate as a competitive inhibitor of not only SDH but also FH. Combined, these results demonstrate that silencing of miR-144 inhibits the activity of NRF2 through decreased fumarate production in obesity. CONCLUSIONS: Herein we unravel a novel mechanism whereby miR-144 inhibits NRF2 activity through the consumption of fumarate by activation of FH. Our study demonstrates that hepatic miR-144 triggers a hyperactive FH in the TCA cycle leading to an impaired antioxidant response in obesity.

Kupffer Cell Protein Isolation and Detection by Western Blot.

Kupffer cells are the majority of tissue macrophages present in the body and are actively involved in the regulation of liver homeostasis as well as in the progression of liver diseases. Here we describe a methodology to successfully isolate and detect proteins from Kupffer cells and avoid the contamination of other liver cell populations.

Liver macrophages inhibit the endogenous antioxidant response in obesity-associated insulin resistance.

Obesity and insulin resistance are risk factors for nonalcoholic fatty liver disease (NAFLD), the most common chronic liver disease worldwide. Because no approved medication nor an accurate and noninvasive diagnosis is currently available for NAFLD, there is a clear need to better understand the link between obesity and NAFLD. Lipid accumulation during obesity is known to be associated with oxidative stress and inflammatory activation of liver macrophages (LMs). However, we show that although LMs do not become proinflammatory during obesity, they display signs of oxidative stress. In livers of both humans and mice, antioxidant nuclear factor erythroid 2-related factor 2 (NRF2) was down-regulated with obesity and insulin resistance, yielding an impaired response to lipid accumulation. At the molecular level, a microRNA-targeting NRF2 protein, miR-144, was elevated in the livers of obese insulin-resistant humans and mice, and specific silencing of miR-144 in murine and human LMs was sufficient to restore NRF2 protein expression and the antioxidant response. These results highlight the pathological role of LMs and their therapeutic potential to restore the impaired endogenous antioxidant response in obesity-associated NAFLD.

Extracellular vesicles in metabolic disease.

Extracellular vesicles (EVs) are submicron-sized lipid envelopes that are produced and released from a parent cell and can be taken up by a recipient cell. EVs are capable of mediating cellular signalling by carrying nucleic acids, proteins, lipids and cellular metabolites between cells and organs. Metabolic dysfunction is associated with changes in plasma concentrations of EVs as well as alterations in their EV cargo. Since EVs can act as messengers between parent and recipient cells, they could be involved in cell-to-cell and organ-to-organ communication in metabolic diseases. Recent literature has shown that EVs are produced by cells within metabolic tissues, such as adipose tissue, pancreas, muscle and liver. These vesicles have therefore been proposed as a novel intercellular communication mode in systemic metabolic regulation. In this review, we will describe and discuss the current literature that investigates the role of adipose-derived EVs in the regulation of obesity-associated metabolic disease. We will particularly focus on the EV-dependent communication between adipocytes, the vasculature and immune cells in type 2 diabetes.

Liver macrophages regulate systemic metabolism through non-inflammatory factors.

Liver macrophages (LMs) have been proposed to contribute to metabolic disease through secretion of inflammatory cytokines. However, anti-inflammatory drugs lead to only modest improvements in systemic metabolism. Here we show that LMs do not undergo a proinflammatory phenotypic switch in obesity-induced insulin resistance in flies, mice and humans. Instead, we find that LMs produce non-inflammatory factors, such as insulin-like growth factor-binding protein 7 (IGFBP7), that directly regulate liver metabolism. IGFBP7 binds to the insulin receptor and induces lipogenesis and gluconeogenesis via activation of extracellular-signal-regulated kinase (ERK) signalling. We further show that IGFBP7 is subject to RNA editing at a higher frequency in insulin-resistant than in insulin-sensitive obese patients (90% versus 30%, respectively), resulting in an IGFBP7 isoform with potentially higher capacity to bind to the insulin receptor. Our study demonstrates that LMs can contribute to insulin resistance independently of their inflammatory status and indicates that non-inflammatory factors produced by macrophages might represent new drug targets for the treatment of metabolic diseases.

Author Correction: Liver macrophages regulate systemic metabolism through non-inflammatory factors.

In the version of this article initially published, author Volker M. Lauschke had affiliation number 13; the correct affiliation number is 12. The error has been corrected in the HTML and PDF versions of the article.

Stable Oxidative Cytosine Modifications Accumulate in Cardiac Mesenchymal Cells From Type2 Diabetes Patients: Rescue by α-Ketoglutarate and TET-TDG Functional Reactivation.

RATIONALE: Human cardiac mesenchymal cells (CMSCs) are a therapeutically relevant primary cell population. Diabetes mellitus compromises CMSC function as consequence of metabolic alterations and incorporation of stable epigenetic changes. OBJECTIVE: To investigate the role of α-ketoglutarate (αKG) in the epimetabolic control of DNA demethylation in CMSCs. METHODS AND RESULTS: Quantitative global analysis, methylated and hydroxymethylated DNA sequencing, and gene-specific GC methylation detection revealed an accumulation of 5-methylcytosine, 5-hydroxymethylcytosine, and 5-formylcytosine in the genomic DNA of human CMSCs isolated from diabetic donors. Whole heart genomic DNA analysis revealed iterative oxidative cytosine modification accumulation in mice exposed to high-fat diet (HFD), injected with streptozotocin, or both in combination (streptozotocin/HFD). In this context, untargeted and targeted metabolomics indicated an intracellular reduction of αKG synthesis in diabetic CMSCs and in the whole heart of HFD mice. This observation was paralleled by a compromised TDG (thymine DNA glycosylase) and TET1 (ten-eleven translocation protein 1) association and function with TET1 relocating out of the nucleus. Molecular dynamics and mutational analyses showed that αKG binds TDG on Arg275 providing an enzymatic allosteric activation. As a consequence, the enzyme significantly increased its capacity to remove G/T nucleotide mismatches or 5-formylcytosine. Accordingly, an exogenous source of αKG restored the DNA demethylation cycle by promoting TDG function, TET1 nuclear localization, and TET/TDG association. TDG inactivation by CRISPR/Cas9 knockout or TET/TDG siRNA knockdown induced 5-formylcytosine accumulation, thus partially mimicking the diabetic epigenetic landscape in cells of nondiabetic origin. The novel compound (S)-2-[(2,6-dichlorobenzoyl)amino]succinic acid (AA6), identified as an inhibitor of αKG dehydrogenase, increased the αKG level in diabetic CMSCs and in the heart of HFD and streptozotocin mice eliciting, in HFD, DNA demethylation, glucose uptake, and insulin response. CONCLUSIONS: Restoring the epimetabolic control of DNA demethylation cycle promises beneficial effects on cells compromised by environmental metabolic changes.

Age-dependent increase of oxidative stress regulates microRNA-29 family preserving cardiac health.

The short-lived turquoise killifish Nothobranchius furzeri (Nfu) is a valid model for aging studies. Here, we investigated its age-associated cardiac function. We observed oxidative stress accumulation and an engagement of microRNAs (miRNAs) in the aging heart. MiRNA-sequencing of 5 week (young), 12-21 week (adult) and 28-40 week (old) Nfu hearts revealed 23 up-regulated and 18 down-regulated miRNAs with age. MiR-29 family turned out as one of the most up-regulated miRNAs during aging. MiR-29 family increase induces a decrease of known targets like collagens and DNA methyl transferases (DNMTs) paralleled by 5´methyl-cytosine (5mC) level decrease. To further investigate miR-29 family role in the fish heart we generated a transgenic zebrafish model where miR-29 was knocked-down. In this model we found significant morphological and functional cardiac alterations and an impairment of oxygen dependent pathways by transcriptome analysis leading to hypoxic marker up-regulation. To get insights the possible hypoxic regulation of miR-29 family, we exposed human cardiac fibroblasts to 1% O2 levels. In hypoxic condition we found miR-29 down-modulation responsible for the accumulation of collagens and 5mC. Overall, our data suggest that miR-29 family up-regulation might represent an endogenous mechanism aimed at ameliorating the age-dependent cardiac damage leading to hypertrophy and fibrosis.

Genetic epidemiology of titin-truncating variants in the etiology of dilated cardiomyopathy.

Heart failure (HF) is a complex clinical syndrome defined by the inability of the heart to pump enough blood to meet the body's metabolic demands. Major causes of HF are cardiomyopathies (diseases of the myocardium associated with mechanical and/or electrical dysfunction), among which the most common form is dilated cardiomyopathy (DCM). DCM is defined by ventricular chamber enlargement and systolic dysfunction with normal left ventricular wall thickness, which leads to progressive HF. Over 60 genes are linked to the etiology of DCM. Titin (TTN) is the largest known protein in biology, spanning half the cardiac sarcomere and, as such, is a basic structural and functional unit of striated muscles. It is essential for heart development as well as mechanical and regulatory functions of the sarcomere. Next-generation sequencing (NGS) in clinical DCM cohorts implicated truncating variants in titin (TTNtv) as major disease alleles, accounting for more than 25% of familial DCM cases, but these variants have also been identified in 2-3% of the general population, where these TTNtv blur diagnostic and clinical utility. Taking into account the published TTNtv and their association to DCM, it becomes clear that TTNtv harm the heart with position-dependent occurrence, being more harmful when present in the A-band TTN, presumably with dominant negative/gain-of-function mechanisms. However, these insights are challenged by the depiction of position-independent toxicity of TTNtv acting via haploinsufficient alleles, which are sufficient to induce cardiac pathology upon stress. In the current review, we provide an overview of TTN and discuss studies investigating various TTN mutations. We also present an overview of different mechanisms postulated or experimentally validated in the pathogenicity of TTNtv. DCM-causing genes are also discussed with respect to non-truncating mutations in the etiology of DCM. One way of understanding pathogenic variants is probably to understand the context in which they may or may not affect protein-protein interactions, changes in cell signaling, and substrate specificity. In this regard, we also provide a brief overview of TTN interactions in situ. Quantitative models in the risk assessment of TTNtv are also discussed. In summary, we highlight the importance of gene-environment interactions in the etiology of DCM and further mechanistic studies used to delineate the pathways which could be targeted in the management of DCM.

Higher cardiogenic potential of iPSCs derived from cardiac versus skin stromal cells.

Prior studies have demonstrated that founder cell type could influence induced pluripotent stem cells (iPSCs) molecular and developmental properties at early passages after establishing their pluripotent state. Herein, we evaluated the persistence of a functional memory related to the tissue of origin in iPSCs from syngeneic cardiac (CStC) vs skin stromal cells (SStCs). We found that, at passages greater than 15, iPSCs from cardiac stromal cells (C-iPSCs) produced a higher number of beating embryoid bodies than iPSCs from skin stromal cells (S-iPSCs). Flow cytometry analysis revealed that dissected beating areas from C-iPSCs exhibited more Troponin-T positive cells compared to S-iPSCs. Beating areas derived from C-iPSCs displayed higher expression of cardiac markers, more hyperpolarized diastolic potentials, larger action potential amplitude and higher contractility than beaters from skin. Also, different microRNA subsets were differentially modulated in CStCs vs SStCs during the reprogramming process, potentially accounting for the higher cardiogenic potentials of C-iPSCs vs S-iPSCs. Therefore, the present work supports the existence of a founder organ memory in iPSCs obtained from the stromal component of the origin tissue.

Acetylation mediates Cx43 reduction caused by electrical stimulation.

Communication between cardiomyocytes depends upon gap junctions (GJ). Previous studies have demonstrated that electrical stimulation induces GJ remodeling and modifies histone acetylase (HAT) and deacetylase (HDAC) activities, although these two results have not been linked. The aim of this work was to establish whether electrical stimulation modulates GJ-mediated cardiac cell-cell communication by acetylation-dependent mechanisms. Field stimulation of HL-1 cardiomyocytes at 0.5 Hz for 24 h significantly reduced connexin43 (Cx43) expression and cell-cell communication. HDAC activity was down-regulated whereas HAT activity was not modified resulting in increased acetylation of Cx43. Consistent with a post-translational mechanism, we did not observe a reduction in Cx43 mRNA in electrically stimulated cells, while the proteasomal inhibitor MG132 maintained Cx43 expression. Further, the treatment of paced cells with the HAT inhibitor Anacardic Acid maintained both the levels of Cx43 and cell-cell communication. Finally, we observed increased acetylation of Cx43 in the left ventricles of dogs subjected to chronic tachypacing as a model of abnormal ventricular activation. In conclusion, our findings suggest that altered electrical activity can regulate cardiomyocyte communication by influencing the acetylation status of Cx43.

Syngeneic cardiac and bone marrow stromal cells display tissue-specific microRNA signatures and microRNA subsets restricted to diverse differentiation processes.

MicroRNAs are key modulators at molecular level in different biological processes, including determination of cell fate and differentiation. Herein, microRNA expression profiling experiments were performed on syngeneic cardiac (CStC) and bone marrow (BMStC) mesenchymal stromal cells cultured in standard growth medium and then in vitro exposed to adipogenic, osteogenic, cardiomyogenic and endothelial differentiation media. Analysis identified a tissue-specific microRNA signature composed of 16 microRNAs that univocally discriminated cell type of origin and that were completely unaffected by in vitro differentiation media: 4 microRNAs were over-expressed in cardiac stromal cells, and 12 were overexpressed or present only in bone marrow stromal cells. Further, results revealed microRNA subsets specifically modulated by each differentiation medium, irrespective of the cell type of origin, and a subset of 7 microRNAs that were down-regulated by all media with respect to growth medium. Finally, we identified 16 microRNAs that were differentially modulated by the media when comparing the two tissues of origin. The existence of a tissue-specific microRNA signature surviving to any differentiation stimuli, strongly support the role if microRNAs determining cell identity related to tissue origin. Moreover, we identified microRNA subsets modulated by different culture conditions in a tissue-specific manner, pointing out their importance during differentiation processes.