Characteristics of plaque lipid-associated macrophages and their possible roles in the pathogenesis of atherosclerosis

A Novel Mutation in LXRα Uncovers a Role for Cholesterol Sensing in Limiting Metabolic Dysfunction-Associated Steatohepatitis (MASH)

Liver x receptor alpha (LXRα, Nr1h3) functions as an important intracellular cholesterol sensor that regulates fat and cholesterol metabolism at the transcriptional level in response to the direct binding of cholesterol derivatives. We have generated mice with a mutation in LXRα that reduces activity in response to endogenous cholesterol derived LXR ligands while still allowing transcriptional activation by synthetic agonists. The mutant LXRα functions as a dominant negative that shuts down cholesterol sensing. When fed a high fat, high cholesterol diet LXRα mutant mice rapidly develop pathologies associated with Metabolic Dysfunction-Associated Steatohepatitis (MASH) including ballooning hepatocytes, liver inflammation, and fibrosis. Strikingly LXRα mutant mice have decreased liver triglycerides but increased liver cholesterol. Therefore, MASH-like phenotypes can arise in the absence of large increases in triglycerides. Reengaging LXR signaling by treatment with synthetic agonist reverses MASH suggesting that LXRα normally functions to impede the development of liver disease.

Interaction of high-fat diet and brain trauma alters adipose tissue macrophages and brain microglia associated with exacerbated cognitive dysfunction

Obesity increases the morbidity and mortality of traumatic brain injury (TBI). Detailed analyses of transcriptomic changes in the brain and adipose tissue were performed to elucidate the interactive effects between high-fat diet-induced obesity (DIO) and TBI. Adult male mice were fed a high-fat diet (HFD) for 12 weeks prior to experimental TBI and continuing after injury. High-throughput transcriptomic analysis using Nanostring panels of the total visceral adipose tissue (VAT) and cellular components in the brain, followed by unsupervised clustering, principal component analysis, and IPA pathway analysis were used to determine shifts in gene expression patterns and molecular pathway activity. Cellular populations in the cortex and hippocampus, as well as in VAT, during the chronic phase after combined TBI-HFD showed amplification of central and peripheral microglia/macrophage responses, including superadditive changes in selected gene expression signatures and pathways. Furthermore, combined TBI and HFD caused additive dysfunction in Y-Maze, Novel Object Recognition (NOR), and Morris water maze (MWM) cognitive function tests. These novel data suggest that HFD-induced obesity and TBI can independently prime and support the development of altered states in brain microglia and VAT, including the disease-associated microglia/macrophage (DAM) phenotype observed in neurodegenerative disorders. The interaction between HFD and TBI promotes a shift toward chronic reactive microglia/macrophage transcriptomic signatures and associated pro-inflammatory disease-altered states that may, in part, underlie the exacerbation of cognitive deficits. Thus, targeting of HFD-induced reactive cellular phenotypes, including in peripheral adipose tissue immune cell populations, may serve to reduce microglial maladaptive states after TBI, attenuating post-traumatic neurodegeneration and neurological dysfunction.

Unveiling macrophage diversity in myocardial ischemia-reperfusion injury: identification of a distinct lipid-associated macrophage subset

Background and objective

Macrophages play a crucial and dichotomous role cardiac repair following myocardial ischemia-reperfusion, as they can both facilitate tissue healing and contribute to injury. This duality is intricately linked to environmental factors, and the identification of macrophage subtypes within the context of myocardial ischemia-reperfusion injury (MIRI) may offer insights for the development of more precise intervention strategies.

Methods

Specific marker genes were used to identify macrophage subtypes in GSE227088 (mouse single-cell RNA sequencing dataset). Genome Set Enrichment Analysis (GSEA) was further employed to validate the identified LAM subtypes. Trajectory analysis and single-cell regulatory network inference were executed using the R packages Monocle2 and SCENIC, respectively. The conservation of LAM was verified using human ischemic cardiomyopathy heart failure samples from the GSE145154 (human single-cell RNA sequencing dataset). Fluorescent homologous double-labeling experiments were performed to determine the spatial localization of LAM-tagged gene expression in the MIRI mouse model.

Results

In this study, single-cell RNA sequencing (scRNA-seq) was employed to investigate the cellular landscape in ischemia-reperfusion injury (IRI). Macrophage subtypes, including a novel Lipid-Associated Macrophage (LAM) subtype characterized by high expression of Spp1, Trem2, and other genes, were identified. Enrichment and Progeny pathway analyses highlighted the distinctive functional role of the SPP1+ LAM subtype, particularly in lipid metabolism and the regulation of the MAPK pathway. Pseudotime analysis revealed the dynamic differentiation of macrophage subtypes during IRI, with the activation of pro-inflammatory pathways in specific clusters. Transcription factor analysis using SCENIC identified key regulators associated with macrophage differentiation. Furthermore, validation in human samples confirmed the presence of SPP1+ LAM. Co-staining experiments provided definitive evidence of LAM marker expression in the infarct zone. These findings shed light on the role of LAM in IRI and its potential as a therapeutic target.

Conclusion

In conclusion, the study identifies SPP1+ LAM macrophages in ischemia-reperfusion injury and highlights their potential in cardiac remodeling.

Defining the activity of pro-reparative extracellular vesicles in wound healing based on miRNA payloads and cell type-specific lineage mapping

Small extracellular vesicles (EVs) are released by cells and deliver biologically active payloads to coordinate the response of multiple cell types in cutaneous wound healing. Here we used a cutaneous injury model as a donor of pro-reparative EVs to treat recipient diabetic obese mice, a model of impaired wound healing. We established a functional screen for microRNAs (miRNAs) that increased the pro-reparative activity of EVs and identified a down-regulation of miR-425-5p in EVs in vivo and in vitro associated with the regulation of adiponectin. We tested a cell type-specific reporter of a tetraspanin CD9 fusion with GFP to lineage map the release of EVs from macrophages in the wound bed, based on the expression of miR-425-5p in macrophage-derived EVs and the abundance of macrophages in EV donor sites. Analysis of different promoters demonstrated that EV release under the control of a macrophage-specific promoter was most abundant and that these EVs were internalized by dermal fibroblasts. These findings suggested that pro-reparative EVs deliver miRNAs, such as miR-425-5p, that stimulate the expression of adiponectin that has insulin-sensitizing properties. We propose that EVs promote intercellular signaling between cell layers in the skin to resolve inflammation, induce proliferation of basal keratinocytes, and accelerate wound closure.

Defining and targeting macrophage heterogeneity in the mammary gland and breast cancer

INTRODUCTION

Macrophages are innate immune cells that are associated with extensive phenotypic and functional plasticity and contribute to normal development, tissue homeostasis, and diseases such as cancer. In this review, we discuss the heterogeneity of tissue resident macrophages in the normal mammary gland and tumor-associated macrophages in breast cancer. Tissue resident macrophages are required for mammary gland development, where they have been implicated in promoting extracellular matrix remodeling, apoptotic clearance, and cellular crosstalk. In the context of cancer, tumor-associated macrophages are key drivers of growth and metastasis via their ability to promote matrix remodeling, angiogenesis, lymphangiogenesis, and immunosuppression.

METHOD

We identified and summarized studies in Pubmed that describe the phenotypic and functional heterogeneity of macrophages and the implications of targeting individual subsets, specifically in the context of mammary gland development and breast cancer. We also identified and summarized recent studies using single-cell RNA sequencing to identify and describe macrophage subsets in human breast cancer samples.

RESULTS

Advances in single-cell RNA sequencing technologies have yielded nuances in macrophage heterogeneity, with numerous macrophage subsets identified in both the normal mammary gland and breast cancer tissue. Macrophage subsets contribute to mammary gland development and breast cancer progression in differing ways, and emerging studies highlight a role for spatial localization in modulating their phenotype and function.

CONCLUSION

Understanding macrophage heterogeneity and the unique functions of each subset in both normal mammary gland development and breast cancer progression may lead to more promising targets for the treatment of breast cancer.

Emerging applications of single-cell profiling in precision medicine of atherosclerosis

Atherosclerosis is a chronic, progressive, inflammatory disease that occurs in the arterial wall. Despite recent advancements in treatment aimed at improving efficacy and prolonging survival, atherosclerosis remains largely incurable. In this review, we discuss emerging single-cell sequencing techniques and their novel insights into atherosclerosis. We provide examples of single-cell profiling studies that reveal phenotypic characteristics of atherosclerosis plaques, blood, liver, and the intestinal tract. Additionally, we highlight the potential clinical applications of single-cell analysis and propose that combining this approach with other techniques can facilitate early diagnosis and treatment, leading to more accurate medical interventions.

A lipid-associated macrophage lineage rewires the spatial landscape of adipose tissue in early obesity

Adipose tissue macrophage (ATM) infiltration is associated with adipose tissue dysfunction and insulin resistance in mice and humans. Recent single-cell data highlight increased ATM heterogeneity in obesity but do not provide a spatial context for ATM phenotype dynamics. We integrated single-cell RNA-Seq, spatial transcriptomics, and imaging of murine adipose tissue in a time course study of diet-induced obesity. Overall, proinflammatory immune cells were predominant in early obesity, whereas nonresident antiinflammatory ATMs predominated in chronic obesity. A subset of these antiinflammatory ATMs were transcriptomically intermediate between monocytes and mature lipid-associated macrophages (LAMs) and were consistent with a LAM precursor (pre-LAM). Pre-LAMs were spatially associated with early obesity crown-like structures (CLSs), which indicate adipose tissue dysfunction. Spatial data showed colocalization of ligand-receptor transcripts related to lipid signaling among monocytes, pre-LAMs, and LAMs, including Apoe, Lrp1, Lpl, and App. Pre-LAM expression of these ligands in early obesity suggested signaling to LAMs in the CLS microenvironment. Our results refine understanding of ATM diversity and provide insight into the dynamics of the LAM lineage during development of metabolic disease.

Environmental and genetic predictors of human cardiovascular ageing

Cardiovascular ageing is a process that begins early in life and leads to a progressive change in structure and decline in function due to accumulated damage across diverse cell types, tissues and organs contributing to multi-morbidity. Damaging biophysical, metabolic and immunological factors exceed endogenous repair mechanisms resulting in a pro-fibrotic state, cellular senescence and end-organ damage, however the genetic architecture of cardiovascular ageing is not known. Here we use machine learning approaches to quantify cardiovascular age from image-derived traits of vascular function, cardiac motion and myocardial fibrosis, as well as conduction traits from electrocardiograms, in 39,559 participants of UK Biobank. Cardiovascular ageing is found to be significantly associated with common or rare variants in genes regulating sarcomere homeostasis, myocardial immunomodulation, and tissue responses to biophysical stress. Ageing is accelerated by cardiometabolic risk factors and we also identify prescribed medications that are potential modifiers of ageing. Through large-scale modelling of ageing across multiple traits our results reveal insights into the mechanisms driving premature cardiovascular ageing and reveal potential molecular targets to attenuate age-related processes.

Serum soluble TREM2 is an independent biomarker associated with coronary heart disease

BACKGROUND

Triggering receptor expressed on myeloid cells 2 (TREM2) is a unique receptor expressed by macrophages in atherosclerotic plaque and is involved in the progression of atherosclerosis. Whether serum soluble TREM2 (sTREM2) levels has a relationship with coronary heart disease (CHD) remains unclear.

METHODS

The cross-sectional study included 86 patients with CHD and 86 controls matched with age and sex. Demographic information, medication history, and laboratory data were collected. sTREM2 concentrations were detected by enzyme-linked immunosorbent assay. We compared the sTREM2 levels in two groups and constructed stepwise linear regression analysis for factors related to the sTREM2 level in patients with CHD; we further used the logistic regression model to evaluate the relationship between sTREM2 and CHD. The diagnostic value of sTREM2 and other biomarkers in CHD was evaluated by the receiver operating characteristic curve (ROC).

RESULTS

The serum level of sTREM2 in CHD patients is higher than that in controls. In CHD patients, the stepwise linear regression analysis found that sTREM2 levels were correlated with triglyceride (TG), high-density lipoprotein cholesterols (HDL-C), apolipoprotein B (ApoB) and smoking status. Logistic regression models showed that sTREM2 was associated independently with CHD after adjusted confounders. The ROC curve showed a sensitivity of 59.3% and specificity of 81.4% with an area under the curve of 0.781 (95% CI: 0.711-0.852) for the diagnosis of CHD with serum sTREM2 at a cut-off value of > 1104.894 pg/ml, indicating a higher diagnostic value than high sensitivity C reaction protein (hs-CRP) and apolipoprotein B (ApoB).

CONCLUSION

In this study, we provide evidence that sTREM2 levels are elevated in CHD patients and are associated with various cardiovascular risk factors. Additionally, sTREM2 demonstrates better diagnostic performance compared to traditional indicators in identifying CHD. These findings suggest that sTREM2 may serve as a potential biomarker for coronary heart disease.

Interaction of high-fat diet and brain trauma alters adipose tissue macrophages and brain microglia associated with exacerbated cognitive dysfunction

Obesity increases the morbidity and mortality of traumatic brain injury (TBI). We performed a detailed analysis of transcriptomic changes in the brain and adipose tissue to examine the interactive effects between high-fat diet-induced obesity (DIO) and TBI in relation to central and peripheral inflammatory pathways, as well as neurological function. Adult male mice were fed a high-fat diet (HFD) for 12 weeks prior to experimental TBI and continuing after injury. Combined TBI and HFD resulted in additive dysfunction in the Y-Maze, novel object recognition (NOR), and Morris water maze (MWM) cognitive function tests. We also performed high-throughput transcriptomic analysis using Nanostring panels of cellular compartments in the brain and total visceral adipose tissue (VAT), followed by unsupervised clustering, principal component analysis, and IPA pathway analysis to determine shifts in gene expression programs and molecular pathway activity. Analysis of cellular populations in the cortex and hippocampus as well as in visceral adipose tissue during the chronic phase after combined TBI-HFD showed amplification of central and peripheral microglia/macrophage responses, including superadditive changes in select gene expression signatures and pathways. These data suggest that HFD-induced obesity and TBI can independently prime and support the development of altered states in brain microglia and visceral adipose tissue macrophages, including the disease-associated microglia/macrophage (DAM) phenotype observed in neurodegenerative disorders. The interaction between HFD and TBI promotes a shift toward chronic reactive microglia/macrophage transcriptomic signatures and associated pro-inflammatory disease-altered states that may, in part, underlie the exacerbation of cognitive deficits. Targeting of HFD-induced reactive cellular phenotypes, including in peripheral adipose tissue macrophages, may serve to reduce microglial maladaptive states after TBI, attenuating post-traumatic neurodegeneration and neurological dysfunction.

Regulatory mechanisms of macrophage polarization in adipose tissue

In adipose tissue, macrophages are the most abundant immune cells with high heterogeneity and plasticity. Depending on environmental cues and molecular mediators, adipose tissue macrophages (ATMs) can be polarized into pro- or anti-inflammatory cells. In the state of obesity, ATMs switch from the M2 polarized state to the M1 state, which contributes to chronic inflammation, thereby promoting the pathogenic progression of obesity and other metabolic diseases. Recent studies show that multiple ATM subpopulations cluster separately from the M1 or M2 polarized state. Various factors are related to ATM polarization, including cytokines, hormones, metabolites and transcription factors. Here, we discuss our current understanding of the potential regulatory mechanisms underlying ATM polarization induced by autocrine and paracrine factors. A better understanding of how ATMs polarize may provide new therapeutic strategies for obesity-related diseases.

Targeting mitochondrial function in macrophages: A novel treatment strategy for atherosclerotic cardiovascular disease?

Atherosclerotic cardiovascular disease is a major cause of morbidity and mortality due to chronic arterial injury caused by hyperlipidemia, hypertension, inflammation and oxidative stress. Recent studies have shown that the progression of this disease is associated with mitochondrial dysfunction and with the accumulation of mitochondrial alterations within macrophages of atherosclerotic plaques. These alterations contribute to processes of inflammation and oxidative stress. Among the many players involved, macrophages play a pivotal role in atherogenesis as they can exert both beneficial and deleterious effects due to their anti- and pro-inflammatory properties. Their atheroprotective functions, such as cholesterol efflux and efferocytosis, as well as the maintenance of their polarization towards an anti-inflammatory state, are particularly dependent on mitochondrial metabolism. Moreover, in vitro studies have demonstrated deleterious effects of oxidized LDL on macrophage mitochondrial function, resulting in a switch to a pro-inflammatory state and to a potential loss of atheroprotective capacity. Therefore, preservation of mitochondrial function is now considered a legitimate therapeutic strategy. This review focuses on the potential therapeutic strategies that could improve the mitochondrial function of macrophages, enabling them to maintain their atheroprotective capacity. These emerging therapies could play a valuable role in counteracting the progression of atherosclerotic lesions and possibly inducing their regression.

Myeloid-restricted CD68 deficiency attenuates atherosclerosis via inhibition of ROS-MAPK-apoptosis axis

In atherosclerosis, macrophages derived from blood monocytes contribute to non-resolving inflammation, which subsequently primes necrotic core formation, and ultimately triggers acute thrombotic vascular disease. Nevertheless, little is known about how inflammatory cells, especially the macrophages fuel atherosclerosis. CD68, a unique class D scavenger receptor (SRD) family member, is specifically expressed in monocytes/macrophages and remarkably up-regulated upon oxidized low-density lipoprotein (ox-LDL) stimulation. Nonetheless, whether and how myeloid-specific CD68 affects atherosclerosis remains to be defined. To determine the essential in vivo role and mechanism linking CD68 to atherosclerosis, we engineered global and myeloid-specific CD68-deficient mice on an ApoE background. On Western diet, both the mice with global and the myeloid-restricted deletion of CD68 on ApoE background attenuated atherosclerosis, accompanied by diminished immune/inflammatory cell burden and necrotic core content, but increased smooth muscle cell content in atherosclerotic plaques. In vitro experiments revealed that CD68 deficiency in macrophages resulted in attenuated ox-LDL-induced macrophage apoptosis. Additionally, CD68 deficiency suppressed ROS production, while removal of ROS can markedly reversed this effect. We further showed that CD68 deficiency affected apoptosis through inactivation of the mitogen-activated protein kinase (MAPK) pathway. Our findings establish CD68 as a macrophage lineage-specific regulator of "ROS-MAPK-apoptosis" axis, thus providing a previously unknown mechanism for the prominence of CD68 as a risk factor for coronary artery disease. Its therapeutic inhibition may provide a potent lever to alleviate the cardiovascular disease.

Heterogeneity of macrophages in atherosclerosis revealed by single-cell RNA sequencing

Technology at the single-cell level has advanced dramatically in characterizing molecular heterogeneity. These technologies have enabled cell subtype diversity to be seen in all tissues, including atherosclerotic plaques. Critical in atherosclerosis pathogenesis and progression are macrophages. Previous studies have only determined macrophage phenotypes within the plaque, mainly by bulk analysis. However, recent progress in single-cell technologies now enables the comprehensive mapping of macrophage subsets and phenotypes present in plaques. In this review, we have updated and discussed the definition and classification of macrophage subsets in mice and humans using single-cell RNA sequencing. We summarized the different classification methods and perspectives: traditional classification with an updated scoring system, inflammatory macrophages, foamy macrophages, and atherosclerotic-resident macrophages. In addition, some special types of macrophages were identified by specific markers, including IFN-inducible and cavity macrophages. Furthermore, we discussed macrophage subset-specific markers and their functions. In the future, these novel insights into the characteristics and phenotypes of these macrophage subsets within atherosclerotic plaques can provide additional therapeutic targets for cardiovascular diseases.

Natural compounds from botanical drugs targeting mTOR signaling pathway as promising therapeutics for atherosclerosis: A review

Atherosclerosis (AS) is a chronic inflammatory disease that is a major cause of cardiovascular diseases (CVDs), including coronary artery disease, hypertension, myocardial infarction, and heart failure. Hence, the mechanisms of AS are still being explored. A growing compendium of evidence supports that the activity of the mechanistic/mammalian target of rapamycin (mTOR) is highly correlated with the risk of AS. The mTOR signaling pathway contributes to AS progression by regulating autophagy, cell senescence, immune response, and lipid metabolism. Various botanical drugs and their functional compounds have been found to exert anti- AS effects by modulating the activity of the mTOR signaling pathway. In this review, we summarize the pathogenesis of AS based on the mTOR signaling pathway from the aspects of immune response, autophagy, cell senescence, and lipid metabolism, and comb the recent advances in natural compounds from botanical drugs to inhibit the mTOR signaling pathway and delay AS development. This review will provide a new perspective on the mechanisms and precision treatments of AS.

The intracellular signaling pathways governing macrophage activation and function in human atherosclerosis

Atherosclerosis is a chronic inflammatory disease characterized by lipid accumulation and plaque formation in arterial vessel walls. Atherosclerotic plaques narrow the arterial lumen to increase the risk of heart attacks, ischemic stroke and peripheral vascular disease, which are major and worldwide health and economic burdens. Macrophage accumulation within plaques is characteristic of all stages of atherosclerosis and their presence is a potential marker of disease activity and plaque stability. Macrophages engulf lipids and modified lipoproteins to form foam cells that express pro-inflammatory and chemotactic effector molecules, stress inducing factors and reactive oxygen species. They control plaque stability and rupture through secretion of metalloproteinases and extracellular matrix degradation. Although macrophages can worsen disease by propagating inflammation, they can stabilize atherosclerotic plaques through tissue remodeling, promoting the formation of a fibrous cap, clearing apoptotic cells to prevent necrotic core formation and through vascular repair. In atherosclerosis, macrophages respond to dyslipidaemia, cytokines, dying cells, metabolic factors, lipids, physical stimuli and epigenetic factors and exhibit heterogeneity in their activation depending on the stimuli they receive. Understanding these signals and the pathways driving macrophage function within developing and established plaques and how they can be pharmacologically modulated, represents a strategy for the prevention and treatment of atherosclerosis. This review focusses on the current understanding of factors controlling macrophage heterogeneity and function in atherosclerosis. Particular attention is given to the macrophage intracellular signaling pathways and transcription factors activated by biochemical and biophysical stimuli within plaques, and how they are integrated to regulate plaque formation and stability.