Distinct lysosome phenotypes influence inflammatory function in peritoneal and bone marrow-derived macrophages

Macrophage variants in laboratory research: most are well done, but some are RAW

Macrophages play a pivotal role in the innate immune response. While their most characteristic function is phagocytosis, it is important not to solely characterize macrophages by this activity. Their crucial roles in body development, homeostasis, repair, and immune responses against pathogens necessitate a broader understanding. Macrophages exhibit remarkable plasticity, allowing them to modify their functional characteristics in response to the tissue microenvironment (tissue type, presence of pathogens or inflammation, and specific signals from neighboring cells) swiftly. While there is no single defined "macrophage" entity, there is a diverse array of macrophage types because macrophage ontogeny involves the differentiation of progenitor cells into tissue-resident macrophages, as well as the recruitment and differentiation of circulating monocytes in response to tissue-specific cues. In addition, macrophages continuously sense and respond to environmental cues and tissue conditions, adjusting their functional and metabolic states accordingly. Consequently, it is of paramount importance to comprehend the heterogeneous origins and functions of macrophages employed in in vitro studies, as each available in vitro macrophage model is associated with specific sets of strengths and limitations. This review centers its attention on a comprehensive comparison between immortalized mouse macrophage cell lines and primary mouse macrophages. It provides a detailed analysis of the strengths and weaknesses inherent in these in vitro models. Finally, it explores the subtle distinctions between diverse macrophage cell lines, offering insights into numerous factors beyond the model type that can profoundly influence macrophage function.

Macrophage-Derived 25-Hydroxycholesterol Promotes Vascular Inflammation, Atherogenesis, and Lesion Remodeling

BACKGROUND

Cross-talk between sterol metabolism and inflammatory pathways has been demonstrated to significantly affect the development of atherosclerosis. Cholesterol biosynthetic intermediates and derivatives are increasingly recognized as key immune regulators of macrophages in response to innate immune activation and lipid overloading. 25-Hydroxycholesterol (25-HC) is produced as an oxidation product of cholesterol by the enzyme cholesterol 25-hydroxylase (CH25H) and belongs to a family of bioactive cholesterol derivatives produced by cells in response to fluctuating cholesterol levels and immune activation. Despite the major role of 25-HC as a mediator of innate and adaptive immune responses, its contribution during the progression of atherosclerosis remains unclear.

METHODS

The levels of 25-HC were analyzed by liquid chromatography-mass spectrometry, and the expression of CH25H in different macrophage populations of human or mouse atherosclerotic plaques, respectively. The effect of CH25H on atherosclerosis progression was analyzed by bone marrow adoptive transfer of cells from wild-type or Ch25h-/- mice to lethally irradiated Ldlr-/- mice, followed by a Western diet feeding for 12 weeks. Lipidomic, transcriptomic analysis and effects on macrophage function and signaling were analyzed in vitro from lipid-loaded macrophage isolated from Ldlr-/- or Ch25h-/-;Ldlr-/- mice. The contribution of secreted 25-HC to fibrous cap formation was analyzed using a smooth muscle cell lineage-tracing mouse model, Myh11ERT2CREmT/mG;Ldlr-/-, adoptively transferred with wild-type or Ch25h-/- mice bone marrow followed by 12 weeks of Western diet feeding.

RESULTS

We found that 25-HC accumulated in human coronary atherosclerotic lesions and that macrophage-derived 25-HC accelerated atherosclerosis progression, promoting plaque instability through autocrine and paracrine actions. 25-HC amplified the inflammatory response of lipid-loaded macrophages and inhibited the migration of smooth muscle cells within the plaque. 25-HC intensified inflammatory responses of lipid-laden macrophages by modifying the pool of accessible cholesterol in the plasma membrane, which altered Toll-like receptor 4 signaling, promoted nuclear factor-κB-mediated proinflammatory gene expression, and increased apoptosis susceptibility. These effects were independent of 25-HC-mediated modulation of liver X receptor or SREBP (sterol regulatory element-binding protein) transcriptional activity.

CONCLUSIONS

Production of 25-HC by activated macrophages amplifies their inflammatory phenotype, thus promoting atherogenesis.

Dissolution Rate of Nanomaterials Determined by Ions and Particle Size under Lysosomal Conditions: Contributions to Standardization of Simulant Fluids and Analytical Methods

Dissolution of inhaled engineered nanomaterials (ENM) under physiological conditions is essential to predict the clearance of the ENM from the lungs and to assess their biodurability and the potential effects of released ions. Alveolar macrophage (AM) lysosomes contain a pH 4.5 saline brine with enzymes and other components. Different types of artificial phagolysosomal simulant fluids (PSFs) have been developed for dissolution testing, but the consequence of using different media is not known. In this study, we tested to which extent six fundamentally different PSFs affected the ENM dissolution kinetics and particle size as determined by a validated transmission electron microscopy (TEM) image analysis. Three lysosomal simulant media were consistent with each other and with in vivo clearance. These media predict the quick dissolution of ZnO, the partial dissolution of SiO2, and the very slow dissolution of TiO2. The valid media use either a mix of organic acids (with the total concentration below 0.5 g/L, thereof citric acid below 0.15 g/L) or another organic acid (KH phthalate). For several ENM, including ZnO, BaSO4, and CeO2, all these differences induce only minor modulation of the dissolution rates. Only for TiO2 and SiO2, the interaction with specific organic acids is highly sensitive, probably due to sequestration of the ions, and can lead to wrong predictions when compared to the in vivo behavior. The media that fail on TiO2 and SiO2 dissolution use citric acid at concentrations above 5 g/L (up to 28 g/L). In the present selection of ENM, fluids, and methods, the different lysosomal simulant fluids did not induce changes of particle morphology, except for small changes in SiO2 and BaSO4 particles most likely due to ion dissolution, reprecipitation, and coalescence between neighboring particles. Based on the current evidence, the particle size by TEM analysis is not a sufficiently sensitive analytical method to deduce the rate of ENM dissolution in physiological media. In summary, we recommend the standardization of ENM dissolution testing by one of the three valid lysosomal simulant fluids with determination of the dissolution rate and halftime by the quantification of ions. This recommendation was established for a continuous flow system but may be relevant as well for static (batch) solubility testing.

Evolution of Cellular Immunity Effector Cells; Perspective on Cytotoxic and Phagocytic Cellular Lineages

The immune system has evolved to protect organisms from infections caused by bacteria, viruses, and parasitic pathogens. In addition, it provides regenerative capacities, tissue maintenance, and self/non-self recognition of foreign tissues. Phagocytosis and cytotoxicity are two prominent cellular immune activities positioned at the base of immune effector function in mammals. Although these immune mechanisms have diversified into a wide heterogeneous repertoire of effector cells, it appears that they share some common cellular and molecular features in all animals, but also some interesting convergent mechanisms. In this review, we will explore the current knowledge about the evolution of phagocytic and cytotoxic immune lineages against pathogens, in the clearance of damaged cells, for regeneration, for histocompatibility recognition, and in killing virally infected cells. To this end, we give different immune examples of multicellular organism models, ranging from the roots of bilateral organisms to chordate invertebrates, comparing to vertebrates' lineages. In this review, we compare cellular lineage homologies at the cellular and molecular levels. We aim to highlight and discuss the diverse function plasticity within the evolved immune effector cells, and even suggest the costs and benefits that it may imply for organisms with the meaning of greater defense against pathogens but less ability to regenerate damaged tissues and organs.

Cellular and physiological functions of C9ORF72 and implications for ALS/FTD

The hexanucleotide repeat expansion (HRE) in the C9ORF72 gene is the main cause of two tightly linked neurodegenerative diseases, amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). HRE leads to not only a gain of toxicity from RNA repeats and dipeptide repeats but also reduced levels of C9ORF72 protein. However, the cellular and physiological functions of C9ORF72 were unknown until recently. Through proteomic analysis, Smith-Magenis chromosome regions 8 (SMCR8) and WD repeat-containing protein (WDR41) were identified as binding partners of C9ORF72. These three proteins have been shown to form a tight complex, but the exact functions of this complex remain to be characterized. Both C9ORF72 and SMCR8 contain a DENN domain, which has been shown to regulate the activities of small GTPases. The C9ORF72 complex has been implicated in many cellular processes, including vesicle trafficking, lysosome homeostasis, mTORC1 signaling , and autophagy. C9ORF72 deficiency in mice results in exaggerated inflammatory responses and human patients with C9ORF72 mutations have neuroinflammation phenotype. Recent studies indicate that C9ORF72 regulates trafficking and lysosomal degradation of inflammatory mediators, including toll-like receptors (TLRs) and STING, to affect inflammatory outputs. Further exploration of cellular and physiological functions of C9ORF72 will help dissect the pathological mechanism of ALS/FTD caused by C9ORF72 mutations.

Bone Marrow-Derived and Elicited Peritoneal Macrophages Are Not Created Equal: The Questions Asked Dictate the Cell Type Used

Macrophages are a heterogeneous and plastic population of cells whose phenotype changes in response to their environment. Macrophage biologists utilize peritoneal (pMAC) and bone marrow-derived macrophages (BMDM) for in vitro studies. Given that pMACs mature in vivo while BMDM are ex vivo differentiated from stem cells, it is likely that their responses differ under experimental conditions. Surprisingly little is known about how BMDM and pMACs responses compare under the same experimental conditionals. While morphologically similar with respect to forward and side scatter by flow cytometry, reports in the literature suggest that pMACs are more mature than their BMDM counterparts. Given the dearth of information comparing BMDM and pMACs, this work was undertaken to test the hypothesis that elicited pMACs are more responsive to defined conditions, including phagocytosis, respiratory burst, polarization, and cytokine and chemokine release. In all cases, our hypothesis was disproved. At steady state, BMDM are more phagocytic (both rate and extent) than elicited pMACs. In response to polarization, they upregulate chemokine and cytokine gene expression and release more cytokines. The results demonstrate that BMDM are generally more responsive and poised to respond to their environment, while pMAC responses are, in comparison, less pronounced. BMDM responses are a function of intrinsic differences, while pMAC responses reflect their differentiation in the context of the whole animal. This distinction may be important in knockout animals, where the pMAC phenotype may be influenced by the absence of the gene of interest.

C9orf72 and smcr8 mutant mice reveal MTORC1 activation due to impaired lysosomal degradation and exocytosis

How lysosome and MTORC1 signaling interact remains elusive in terminally differentiated cells. A G4C2 repeat expansion in C9orf72 is the most common cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) (C9ALS-FTD). We previously identified a C9orf72-SMCR8-containing complex. Here we found that c9orf72 and smcr8 double-knockout (dKO) mice exhibit similar but more severe immune defects than the individual knockouts. In c9orf72 or smcr8 mutant macrophages, lysosomal degradation and exocytosis were impaired due to the disruption of autolysosome acidification. As a result of impaired lysosomal degradation, MTOR protein was aberrantly increased, resulting in MTORC1 signaling overactivation. Inhibition of hyperactive MTORC1 partially rescued macrophage dysfunction, splenomegaly and lymphadenopathy in c9orf72 or smcr8 mutant mice. Pharmacological inhibition of lysosomal degradation upregulated MTOR protein and MTORC1 signaling in differentiated wild-type macrophages, which resemble phenotypes in KO mice. In contrast, C9orf72 or Smcr8 depletion in proliferating macrophages decreased MTORC1 signaling. Our studies causatively link C9orf72-SMCR8's cellular functions in lysosomal degradation, exocytosis, and MTORC1 signaling with their organism-level immune regulation, suggesting cell state (proliferation vs. differentiation)-dependent regulation of MTOR signaling via lysosomes.Abbreviations: ALS: amyotrophic lateral sclerosis; ATG13: autophagy related 13; BMDMs: bone marrow-derived macrophages; BafA₁: bafilomycin A₁; C9orf72: C9orf72, member of C9orf72-SMCR8 complex; CD68: CD68 antigen; ConA: concanamycin A; dKO: double knockout; DENN: differentially expressed in normal and neoplastic cells; FTD: frontotemporal dementia; GEF: guanine nucleotide exchange factor; IFNB1: interferon beta 1, fibroblast; IFNG: interferon gamma; IL1B/IL-1β: interleukin 1 beta; IL6: interleukin 6; iPSCs: induced pluripotent stem cells; LAMP1: lysosomal-associated membrane protein 1; LPOs: LAMP1-positive organelles; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; LPS: lipopolysaccharide; MTORC1: mechanistic target of rapamycin kinase complex 1; MEFs: mouse embryonic fibroblasts; MNs: motor neurons; NOS2/iNOS: nitric oxide synthase 2, inducible; RAN: repeat-associated non-AUG; RB1CC1/FIP200: RB1-inducible coiled-coil 1; RPS6/S6: ribosomal protein S6; RPS6KB1/S6K1: ribosomal protein S6 kinase, polypeptide 1; SMCR8: Smith-Magenis syndrome chromosome region, candidate 8; SQSTM1/p62: sequestosome 1; TFEB: transcription factor EB; TNF: tumor necrosis factor; TSC1: TSC complex subunit 1; ULK1: unc-51 like kinase 1; v-ATPase: vacuolar-type H⁺-translocating ATPase.

A novel genetic marker of decreased inflammation and improved survival after acute myocardial infarction

The CHRNA5 gene encodes a neurotransmitter receptor subunit involved in multiple processes, including cholinergic autonomic nerve activity and inflammation. Common variants in CHRNA5 have been linked with atherosclerotic cardiovascular disease. Association of variation in CHRNA5 and specific haplotypes with cardiovascular outcomes has not been described. The aim of this study was to examine the association of CHRNA5 haplotypes with gene expression and mortality among patients with acute myocardial infarction (AMI) and explore potential mechanisms of this association. Patients (N = 2054) hospitalized with AMI were genotyped for two common variants in CHRNA5. Proportional hazard models were used to estimate independent association of CHRNA5 haplotype with 1-year mortality. Both individual variants were associated with mortality (p = 0.0096 and 0.0004, respectively) and were in tight LD (D' = 0.99). One haplotype, HAP3, was associated with decreased mortality one year after AMI (adjusted HR = 0.42, 95% CI 0.26, 0.68; p = 0.0004). This association was validated in an independent cohort (N = 637) of post-MI patients (adjusted HR = 0.23, 95% CI 0.07, 0.79; p = 0.019). Differences in CHRNA5 expression by haplotype were investigated in human heart samples (n = 28). Compared with non-carriers, HAP3 carriers had threefold lower cardiac CHRNA5 mRNA expression (p = 0.023). Circulating levels of the inflammatory marker hsCRP were significantly lower in HAP3 carriers versus non-carriers (3.43 ± 4.2 versus 3.91 ± 5.1; p = 0.0379). Activation of the inflammasome, an important inflammatory complex involved in cardiovascular disease that is necessary for release of the pro-inflammatory cytokine IL-1 β, was assessed in bone marrow-derived macrophages (BMDM) from CHRNA5 knockout mice and wild-type controls. In BMDM from CHRNA5 knockout mice, IL-1β secretion was reduced by 50% compared to wild-type controls (p = 0.004). Therefore, a common haplotype of CHRNA5 that results in reduced cardiac expression of CHRNA5 and attenuated macrophage inflammasome activation is associated with lower mortality after AMI. These results implicate CHRNA5 and the cholinergic anti-inflammatory pathway in survival following AMI.

Sub-cellular In-situ Characterization of Ferritin(iron) in a Rodent Model of Spinal Cord Injury

Iron (Fe) is an essential metal involved in a wide spectrum of physiological functions. Sub-cellular characterization of the size, composition, and distribution of ferritin(iron) can provide valuable information on iron storage and transport in health and disease. In this study we employ magnetic force microscopy (MFM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS) to characterize differences in ferritin(iron) distribution and composition across injured and non-injured tissues by employing a rodent model of spinal cord injury (SCI). Our biophysical and ultrastructural analyses provide novel insights into iron distribution which are not obtained by routine biochemical stains. In particular, ferritin(iron) rich lysosomes revealed increased heterogeneity in MFM signal from tissues of SCI animals. Ultrastructural analysis using TEM elucidated that both cytosolic and lysosomal ferritin(iron) density was increased in the injured (spinal cord) and non-injured (spleen) tissues of SCI as compared to naïve animals. In-situ EELs analysis revealed that ferritin(iron) was primarily in Fe³⁺ oxidation state in both naïve and SCI animal tissues. The insights provided by this study and the approaches utilized here can be applied broadly to other systemic problems involving iron regulation or to understand the fate of exogenously delivered iron-oxide nanoparticles.

Inhibition or Stimulation of Autophagy Affects Early Formation of Lipofuscin-Like Autofluorescence in the Retinal Pigment Epithelium Cell

The accumulation of lipofuscin in the retinal pigment epithelium (RPE) is dependent on the effectiveness of photoreceptor outer segment material degradation. This study explored the role of autophagy in the fate of RPE lipofuscin degradation. After seven days of feeding with either native or modified rod outer segments, ARPE-19 cells were treated with enhancers or inhibitors of autophagy and the autofluorescence was detected by fluorescence-activated cell sorting. Supplementation with different types of rod outer segments increased lipofuscin-like autofluorescence (LLAF) after the inhibition of autophagy, while the induction of autophagy (e.g., application of rapamycin) decreased LLAF. The effects of autophagy induction were further confirmed by Western blotting, which showed the conversion of LC3-I to LC3-II, and by immunofluorescence microscopy, which detected the lysosomal activity of the autophagy inducers. We also monitored LLAF after the application of several autophagy inhibitors by RNA-interference and confocal microscopy. The results showed that, in general, the inhibition of the autophagy-related proteins resulted in an increase in LLAF when cells were fed with rod outer segments, which further confirms the effect of autophagy in the fate of RPE lipofuscin degradation. These results emphasize the complex role of autophagy in modulating RPE autofluorescence and confirm the possibility of the pharmacological clearance of RPE lipofuscin by small molecules.

Integrating immunometabolism and macrophage diversity

Macrophages are heterogeneous cells that play a key role in inflammatory and tissue reparative responses. Over the past decade it has become clear that shifts in cellular metabolism are important determinants of macrophage function and phenotype. At the same time, our appreciation of macrophage diversity in vivo has also been increasing. Factors such as cell origin and tissue localization are now recognized as important variables that influence macrophage biology. Whether different macrophage populations also have unique metabolic phenotypes has not been extensively explored. In this article, we will discuss the importance of understanding how macrophage origin can modulate metabolic programming and influence inflammatory responses.

Macrophagic control of the response to uropathogenic E. coli infection by regulation of iron retention in an IL-6-dependent manner

Introduction

Uropathogenic Escherichia coli (UPEC), the causative agent of over 85% of urinary tract infections (UTIs), elaborate a number of siderophores to chelate iron from the host. On the other hand, the host immune imperative is to limit the availability of iron to the bacteria. Little is known regarding the mechanisms underlying this host‐iron‐UPEC interaction. Our objective was to determine whether macrophages, in response to UPEC infection, retain extracellular siderophore‐bound and free iron, thus limiting the ability of UPEC to access iron.

Methods

Quantitative PCR, immunoblotting analysis, and gene expression analysis of wild type and IL‐6‐deficient macrophages was performed.

Results

We found that (1) macrophages upon UPEC infection increased expression of lipocalin 2, a siderophore‐binding molecule, of Dmt1, a molecule that facilitates macrophage uptake of free iron, and of the intracellular iron cargo molecule ferritin, and decreased expression of the iron exporter ferroportin; (2) bladder macrophages regulate expression of genes involved in iron retention upon UPEC infection; (3) IL‐6, a cytokine known to play an important role in regulating host iron homeostasis as well as host defense to UPEC, regulates this process, in part by promoting production of lipocalin 2; and finally, (4) inhibition of IL‐6 signaling genetically and by neutralizing antibodies against the IL‐6 receptor, promoted intra‐macrophagic UPEC growth in the presence of excess iron.

Conclusions

Together, our study suggests that macrophages retain siderophore‐bound and free iron in response to UPEC and IL‐6 signaling is necessary for macrophages to limit the growth of UPEC in the presence of excess iron. IL‐6 signaling and iron regulation is one mechanism by which macrophages may mediate UPEC clearance.

Downregulation of lysosomal and further gene expression characterization in lung cancer patients with bone metastasis

Molecular and functional mechanisms of bone metastases were poorly understood. This study was to screen out differentially expressed genes (DEGs) and functional proteins in bone metastases from lung for better understanding of the molecular and functional mechanisms. Our results suggested CTSS, CTSD, MX1, NKX2-1 might play a decisive role in bone metastasis. Collectively, these results demonstrated that bone metastasis from lung cancer would lead to changes in lysosome function, which may affect the decomposition and elimination of old bone matrix, thus affecting bone turnover. In addition, our findings provided new insights into the prediction and treatment of bone metastases.

Inhibition of mTOR reduces lipotoxic cell death in primary macrophages through an autophagy-independent mechanism

Macrophage dysfunction in obesity and diabetes is associated with persistent inflammation and poor wound healing responses. Relevant to these phenotypes, we have previously shown that macrophage activation in a high-fat environment results in cell death via a mechanism that involves lysosome damage. While searching for signaling pathways that were required for this response, we discovered that mTOR inhibitors, torin and rapamycin, were protective against lipotoxic cell death in primary peritoneal macrophages. The protective effect of mTOR inhibition was also confirmed by using genetic loss-of-function approaches. Given the importance of mTOR in regulation of autophagy we hypothesized that this pathway would be important in protection from cell death. We first demonstrated that autophagy was disrupted in response to palmitate and LPS as a consequence of impaired lysosome function. Conversely, the mTOR inhibitor, torin, increased macrophage autophagy and protected against lysosome damage; however, the beneficial effects of torin persisted in autophagy-deficient cells. Inhibition of mTOR also triggered nuclear localization of TFEB, a transcription factor that regulates lysosome biogenesis and function, but the rescue phenotype did not require the presence of TFEB. Instead, we demonstrated that mTOR inhibition reduces mitochondrial oxidative metabolism and attenuates the negative effects of palmitate on LPS-induced mitochondrial respiration. These results suggest that inhibition of mTOR is protective against lipotoxicity via an autophagy-independent mechanism that involves relieving mitochondrial substrate overload. On the basis of these findings, we suggest that therapies to reduce macrophage mTOR activation may protect against dysfunctional inflammation in states of overnutrition, such as diabetes.