DBA/2J Haplotype on Distal Chromosome 2 Reduces Mertk Expression, Restricts Efferocytosis, and Increases Susceptibility to Atherosclerosis

Off-target pharmacological activity at various kinases: Potential functional and pathological side effects

In drug discovery, during the lead optimization and candidate characterization stages, novel small molecules are frequently evaluated in a battery of in vitro pharmacology assays to identify potential unintended, off-target interactions with various receptors, transporters, ion channels, and enzymes, including kinases. Furthermore, these screening panels may also provide utility at later stages of development to provide a mechanistic understanding of unexpected safety findings. Here, we present a compendium of the most likely functional and pathological outcomes associated with interaction(s) to a panel of 95 kinases based on an extensive curation of the scientific literature. This panel of kinases was designed by AbbVie based on safety-related data extracted from the literature, as well as from over 20 years of institutional knowledge generated from discovery efforts. For each kinase, the scientific literature was reviewed using online databases and the most often reported functional and pathological effects were summarized. This work should serve as a practical guide for small molecule drug discovery scientists and clinical investigators to predict and/or interpret adverse effects related to pharmacological interactions with these kinases.

Microbial Metabolite 3-Indolepropionic Acid Mediates Immunosuppression

The microbial-derived metabolite, 3-indolepropionic acid (3-IPA), has been intensely studied since its origins were discovered in 2009; however, 3-IPA's role in immunosuppression has had limited attention. Untargeted metabolomic analyses of T-cell exhaustion and immunosuppression, represented by dysfunctional under-responsive CD8⁺ T cells, reveal a potential role of 3-IPA in these responses. T-cell exhaustion was examined via infection of two genetically related mouse strains, DBA/1J and DBA/2J, with lymphocytic choriomeningitis virus (LCMV) Clone 13 (Cl13). The different mouse strains produced disparate outcomes driven by their T-cell responses. Infected DBA/2J presented with exhausted T cells and persistent infection, and DBA/1J mice died one week after infection from cytotoxic T lymphocytes (CTLs)-mediated pulmonary failure. Metabolomics revealed over 70 metabolites were altered between the DBA/1J and DBA/2J models over the course of the infection, most of them in mice with a fatal outcome. Cognitive-driven prioritization combined with statistical significance and fold change were used to prioritize the metabolites. 3-IPA, a tryptophan-derived metabolite, was identified as a high-priority candidate for testing. To test its activity 3-IPA was added to the drinking water of the mouse models during LCMV Cl13 infection, with the results showing that 3-IPA allowed the mice to survive longer. This negative immune-modulation effect might be of interest for the modulation of CTL responses in events such as autoimmune diseases, type I diabetes or even COVID-19. Moreover, 3-IPA's bacterial origin raises the possibility of targeting the microbiome to enhance CTL responses in diseases such as cancer and chronic infection.

PM2.5 induce the defective efferocytosis and promote atherosclerosis via HIF-1α activation in macrophage

Epidemiological studies demonstrate that fine particulate matter (PM_2.5) promotes the development of atherosclerosis. However, the mechanism insight of PM_2.5-induced atherosclerosis is still lacking. The aim of this study was to explore the biological effects of hypoxia-inducible factor 1α (HIF-1α) on PM_2.5-triggered atherosclerosis. The vascular stiffness, carotid intima-media thickness (CIMT), lipid and atherosclerotic lesion were increased when von Hippel-Lindau (VHL)-null mice were exposed to PM_2.5. Yet, knockout of HIF-1α markedly decreased the PM_2.5-triggered atherosclerotic lesion. We firstly performed microarray analysis in PM_2.5-treated bone morrow-derived macrophages (BMDMs), which showed that PM_2.5 significantly changed the genes expression patterns and affected biological processes such as phagocytosis, apoptotic cell clearance, cellular response to hypoxia, apoptotic process and inflammatory response. Moreover, the data showed knockout of HIF-1α remarkably relieved PM_2.5-induced defective efferocytosis. Mechanistically, PM_2.5 inhibited the level of genes and proteins of efferocytosis receptor c-Mer tyrosine kinase (MerTK), especially in VHL-null BMDMs. In addition, PM_2.5 increased the genes and proteins of a disintegrin and metallopeptidase domain 17 (ADAM17), which caused the MerTK cleavage to form soluble MerTK (sMer) in plasma and cellular supernatant. The sMer was significantly up-regulated in plasma of VHL-null PM_2.5-exposed mice. Moreover, PM_2.5 could induce defective efferocytosis and activate inflammatory response through MerTK/IFNAR1/STAT1 signaling pathway in macrophages. Our results demonstrate that PM_2.5 could induce defective efferocytosis and inflammation by activating HIF-1α in macrophages, ultimately resulting in accelerating atherosclerotic lesion formation and development. Our data suggest HIF-1α in macrophages might be a potential target for PM_2.5-related atherosclerosis.

Reduction of Stabilin-2 Contributes to a Protection Against Atherosclerosis

We have previously identified a novel atherosclerosis quantitative trait locus (QTL), Arch atherosclerosis 5 (Aath5), on mouse chromosome 10 by three-way QTL analyses between Apoe−/− mice on a DBA/2J, 129S6 and C57BL/6J background. The DBA/2J haplotype at the Aath5 locus was associated with smaller plaque size. One of the candidate genes underlying Aath5 was Stabilin-2 (Stab2), which encodes a clearance receptor for hyaluronan (HA) predominantly expressed in liver sinusoidal endothelial cells (LSECs). However, the role of Stab2 in atherosclerosis is unknown. A congenic line of Apoe−/− mice carrying Aath5 covering the Stab2DBA allele on a background of 129S6 confirmed the small reductions of atherosclerotic plaque development. To further determine whether Stab2 is an underlying gene for Aath5, we generated Stab2−/−Apoe−/− mice on a C57BL/6J background. When fed with a Western diet for 8 weeks, Stab2−/−Apoe−/− males developed approximately 30% smaller plaques than Stab2+/+Apoe−/− mice. HA was accumulated in circulation but not in major organs in the Stab2 deficient mice. STAB2-binding molecules that are involved in atherosclerosis, including acLDL, apoptotic cells, heparin and vWF were not likely the direct cause of the protection in the Stab2−/−Apoe−/− males. These data indicate that reduction of Stab2 is protective against atherosclerotic plaque development, and that Stab2 is a contributing gene underlying Aath5, although its effect is small. To test whether non-synonymous amino acid changes unique to DBA/2J affect the function of STAB2 protein, we made HEK293 cell lines expressing STAB2129 or STAB2DBA proteins, as well as STAB2129 proteins carrying each of five DBA-unique replacements that have been predicted to be deleterious. These mutant cells were capable of internalizing 125I -HA and DiI-acLDL similarly to the control cells. These results indicate that the amino acid changes unique to DBA/2J are not affecting the function of STAB2 protein, and support our previous observation that the reduced transcription of Stab2 in the liver sinusoid as a consequence of the insertion of a viral-derived sequence, intracisternal A particle, is the primary contributor to the athero-protection conferred by the DBA/2J allele.

Fine Mapping of the Mouse Ath28 Locus Yields Three Atherosclerosis Modifying Sub-Regions

A mouse strain intercross between Apoe-/- AKR/J and DBA/2J mice identified three replicated atherosclerosis quantitative trait loci (QTLs). Our objective was to fine map mouse atherosclerosis modifier genes within a genomic region known to affect lesion development in apoE-deficient (Apoe-/-) mice. We dissected the Ath28 QTL on the distal end of chromosome 2 by breeding a panel of congenic strains and measuring aortic root lesion area in 16-week-old male and female mice fed regular laboratory diets. The parental congenic strain contained ~9.65 Mb of AKR/J DNA from chromosome 2 on the DBA/2J genetic background, which had lesions 55% and 47% smaller than female and male DBA/2J mice, respectively (p < 0.001). Seven additional congenic lines identified three separate regions associated with the lesion area, named Ath28.1, Ath28.2, and Ath28.3, where the AKR/J alleles were atherosclerosis-protective for two regions and atherosclerosis-promoting for the other region. These results were replicated in both sexes, and in combined analysis after adjusting for sex. The congenic lines did not greatly impact total and HDL cholesterol levels or body weight. Bioinformatic analyses identified all coding and non-coding genes in the Ath28.1 sub-region, as well as strain sequence differences that may be impactful. Even within a <10 Mb region of the mouse genome, evidence supports the presence of at least three atherosclerosis modifier genes that differ between the AKR/J and DBA/2J mouse strains, supporting the polygenic nature of atherosclerosis susceptibility.

Two-faced Janus: The dual role of macrophages in atherosclerotic calcification

Calcification is an independent predictor of atherosclerosis-related cardiovascular events. Microcalcification is linked to inflamed, unstable lesions, in comparison to the fibrotic stable plaque phenotype generally associated with advanced calcification. This paradox relates to recognition that calcification presents in a wide spectrum of manifestations that differentially impact plaque’s fate. Macrophages, the main inflammatory cells in atherosclerotic plaque, have a multifaceted role in disease progression. They crucially control the mineralization process, from microcalcification to the osteoid metaplasia of bone-like tissue. It is a bilateral interaction that weighs heavily on the overall plaque fate but remains rather unexplored. This review highlights current knowledge about macrophage phenotypic changes in relation to and interaction with the calcifying environment. On the one hand, macrophage-led inflammation kickstarts microcalcification through a multitude of interlinked mechanisms, which in turn stimulates phenotypic changes in vascular cell types to drive microcalcification. Macrophages may also modulate the expression/activity of calcification inhibitors and inducers, or eliminate hydroxyapatite nucleation points. Contrarily, direct exposure of macrophages to an early calcifying milieu impacts macrophage phenotype, with repercussions for plaque progression and/or stability. Macrophages surrounding macrocalcification deposits show a more reparative phenotype, modulating extracellular matrix, and expressing osteoclast genes. This phenotypic shift favours gradual displacement of the pro-inflammatory hubs; the lipid necrotic core, by macrocalcification. Parallels to bone metabolism may explain many of these changes to macrophage phenotype, with advanced calcification able to show homeostatic osteoid metaplasia. As the targeted treatment of vascular calcification developing in atherosclerosis is thus far severely lacking, it is crucial to better understand its mechanisms of development.

Defective Efferocytosis in a Murine Model of Sjögren's Syndrome Is Mediated by Dysfunctional Mer Tyrosine Kinase Receptor

Sjögren's syndrome (SjS) is a chronic autoimmune disease primarily involving the exocrine glands in which the involvement of the innate immune system is largely uncharacterized. Mer signaling has been found to be protective in several autoimmune diseases but remains unstudied in SjS. Here, we investigated the role of Mer signaling in SjS. Mer knockout (MerKO) mice were examined for SjS disease criteria. SjS-susceptible (SjS^S) C57BL/6.NOD-Aec1Aec2 mice were assessed for defective Mer signaling outcomes, soluble Mer (sMer) levels, A disintegrin and metalloprotease 17 (ADAM17) activity, and Rac1 activation. In addition, SjS patient plasma samples were evaluated for sMer levels via ELISA, and sMer levels were correlated to disease manifestations. MerKO mice developed submandibular gland (SMG) lymphocytic infiltrates, SMG apoptotic cells, anti-nuclear autoantibodies (ANA), and reduced saliva flow. Mer signaling outcomes were observed to be diminished in SjS^S mice, as evidenced by reduced Rac1 activation in SjS^S mice macrophages in response to apoptotic cells and impaired efferocytosis. Increased sMer was also detected in SjS^S mouse sera, coinciding with higher ADAM17 activity, the enzyme responsible for cleavage and inactivation of Mer. sMer levels were elevated in patient plasma and positively correlated with focus scores, ocular staining scores, rheumatoid factors, and anti-Ro60 levels. Our data indicate that Mer plays a protective role in SjS, similar to other autoimmune diseases. Furthermore, we suggest a series of events where enhanced ADAM17 activity increases Mer inactivation and depresses Mer signaling, thus removing protection against the loss of self-tolerance and the onset of autoimmune disease in SjS^S mice.

Atherosclerosis in Different Vascular Locations Unbiasedly Approached with Mouse Genetics

Atherosclerosis in different vascular locations leads to distinct clinical consequences, such as ischemic stroke and myocardial infarction. Genome-wide association studies in humans revealed that genetic loci responsible for carotid plaque and coronary artery disease were not overlapping, suggesting that distinct genetic pathways might be involved for each location. While elevated plasma cholesterol is a common risk factor, plaque development in different vascular beds is influenced by hemodynamics and intrinsic vascular integrity. Despite the limitation of species differences, mouse models provide platforms for unbiased genetic approaches. Mouse strain differences also indicate that susceptibility to atherosclerosis varies, depending on vascular locations, and that the location specificity is genetically controlled. Quantitative trait loci analyses in mice suggested candidate genes, including Mertk and Stab2, although how each gene affects the location-specific atherosclerosis needs further elucidation. Another unbiased approach of single-cell transcriptome analyses revealed the presence of a small subpopulation of vascular smooth muscle cells (VSMCs), which are “hyper-responsive” to inflammatory stimuli. These cells are likely the previously-reported Sca1⁺ progenitor cells, which can differentiate into multiple lineages in plaques. Further spatiotemporal analyses of the progenitor cells are necessary, since their distribution pattern might be associated with the location-dependent plaque development.

Plaque Calcification

Vascular calcification (VC) is strongly associated with all-cause mortality and is an independent predictor of cardiovascular events. Resulting from its complex, multifaceted nature, targeted treatments for VC have not yet been developed. Lipoproteins are well characterized in the pathogenesis of atherosclerotic plaques, leading to the development of plaque regressing therapeutics. Although their roles in plaque progression are well documented, their roles in VC, and calcification of a plaque, are not well understood. In this review, early in vitro data and clinical correlations suggest an inhibitory role for HDL (high-density lipoproteins) in VC, a stimulatory role for LDL (low-density lipoprotein) and VLDL (very low-density lipoprotein) and a potentially causal role for Lp(a) (lipoprotein [a]). Additionally, after treatment with a statin or PCSK9 (proprotein convertase subtilisin/kexin type 9) inhibitor, plaque calcification is observed to increase. With the notion that differing morphologies of plaque calcification associate with either a more stable or unstable plaque phenotype, uncovering the mechanisms of lipoprotein-artery wall interactions could produce targeted therapeutic options for VC.

New Insights into the Roles of Monocytes/Macrophages in Cardiovascular Calcification Associated with Chronic Kidney Disease

Cardiovascular disease (CVD) is an important cause of death in patients with chronic kidney disease (CKD), and cardiovascular calcification (CVC) is one of the strongest predictors of CVD in this population. Cardiovascular calcification results from complex cellular interactions involving the endothelium, vascular/valvular cells (i.e., vascular smooth muscle cells, valvular interstitial cells and resident fibroblasts), and monocyte-derived macrophages. Indeed, the production of pro-inflammatory cytokines and oxidative stress by monocyte-derived macrophages is responsible for the osteogenic transformation and mineralization of vascular/valvular cells. However, monocytes/macrophages show the ability to modify their phenotype, and consequently their functions, when facing environmental modifications. This plasticity complicates efforts to understand the pathogenesis of CVC-particularly in a CKD setting, where both uraemic toxins and CKD treatment may affect monocyte/macrophage functions and thereby influence CVC. Here, we review (i) the mechanisms by which each monocyte/macrophage subset either promotes or prevents CVC, and (ii) how both uraemic toxins and CKD therapies might affect these monocyte/macrophage functions.

Quantitative Trait Locus Mapping of Macrophage Cholesterol Metabolism and CRISPR/Cas9 Editing Implicate an ACAT1 Truncation as a Causal Modifier Variant

OBJECTIVE

Cholesterol metabolism is a dynamic process involving intracellular trafficking, cholesterol esterification, and cholesterol ester hydrolysis. Our objective was to identify genes that regulate macrophage cholesterol metabolism.

APPROACHES AND RESULTS

We performed quantitative trait loci mapping of free and esterified cholesterol levels and the ratio of esterified to free cholesterol in acetylated low-density lipoprotein-loaded bone marrow-derived macrophages from an AKR×DBA/2 strain intercross. Ten distinct cholesterol modifier loci were identified, and bioinformatics was used to prioritize candidate genes. The strongest locus was located on distal chromosome 1, which we named Mcmm1 (macrophage cholesterol metabolism modifier 1). This locus harbors the Soat1 (sterol O-acyltransferase 1) gene, encoding Acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1), which esterifies free cholesterol. The parental AKR strain has an exon 2 deletion in Soat1, which leads to a 33 amino acid N-terminal truncation in ACAT1. CRISPR/Cas9 editing of DBA/2 embryonic stem cells was performed to replicate the AKR strain Soat1 exon 2 deletion, while leaving the remainder of the genome unaltered. DBA/2 stem cells and stem cells heterozygous and homozygous for the Soat1 exon 2 deletion were differentiated into macrophages and loaded with acetylated low-density lipoprotein. DBA/2 stem cell-derived macrophages accumulated less free cholesterol and more esterified cholesterol relative to cells heterozygous and homozygous for the Soat1 exon 2 deletion.

CONCLUSIONS

A Soat1 deletion present in AKR mice, and resultant N-terminal ACAT1 truncation, was confirmed to be a significant modifier of macrophage cholesterol metabolism. Other Mcmm loci candidate genes were prioritized via bioinformatics.