A growth factor-expressing macrophage subpopulation orchestrates regenerative inflammation via GDF-15

Macrophages as a Source and Target of GDF-15

Growth differentiation factor 15 (GDF-15) is a multifunctional cytokine that belongs to the transforming growth factor-beta (TGF-β) superfamily. GDF-15 is involved in immune tolerance and is elevated in several acute and chronic stress conditions, often correlating with disease severity and patient prognosis in cancer172 and metabolic and cardiovascular disorders. Despite these clinical associations, the molecular mechanisms orchestrating its effects remain to be elucidated. The effects of GDF-15 are pleiotropic but cell-specific and dependent on the microenvironment. While GDF-15 expression can be stimulated by inflammatory mediators, its predominant effects were reported as anti-inflammatory and pro-fibrotic. The role of GDF-15 in the macrophage system has been increasingly investigated in recent years. Macrophages produce high levels of GDF-15 during oxidative and lysosomal stress, which can lead to fibrogenesis and angiogenesis at the tissue level. At the same time, macrophages can respond to GDF-15 by switching their phenotype to a tolerogenic one. Several GDF-15-based therapies are under development, including GDF-15 analogs/mimetics and GDF-15-targeting monoclonal antibodies. In this review, we summarize the major physiological and pathological contexts in which GDF-15 interacts with macrophages. We also discuss the major challenges and future perspectives in the therapeutic translation of GDF-15.

Skeletal muscle: molecular structure, myogenesis, biological functions, and diseases

Skeletal muscle is an important motor organ with multinucleated myofibers as its smallest cellular units. Myofibers are formed after undergoing cell differentiation, cell-cell fusion, myonuclei migration, and myofibril crosslinking among other processes and undergo morphological and functional changes or lesions after being stimulated by internal or external factors. The above processes are collectively referred to as myogenesis. After myofibers mature, the function and behavior of skeletal muscle are closely related to the voluntary movement of the body. In this review, we systematically and comprehensively discuss the physiological and pathological processes associated with skeletal muscles from five perspectives: molecule basis, myogenesis, biological function, adaptive changes, and myopathy. In the molecular structure and myogenesis sections, we gave a brief overview, focusing on skeletal muscle-specific fusogens and nuclei-related behaviors including cell-cell fusion and myonuclei localization. Subsequently, we discussed the three biological functions of skeletal muscle (muscle contraction, thermogenesis, and myokines secretion) and its response to stimulation (atrophy, hypertrophy, and regeneration), and finally settled on myopathy. In general, the integration of these contents provides a holistic perspective, which helps to further elucidate the structure, characteristics, and functions of skeletal muscle.

Immunoengineering Biomaterials for Musculoskeletal Tissue Repair across Lifespan

Musculoskeletal diseases and injuries are among the leading causes of pain and morbidity worldwide. Broad efforts have focused on developing pro-regenerative biomaterials to treat musculoskeletal conditions; however, these approaches have yet to make a significant clinical impact. Recent studies have demonstrated that the immune system is central in orchestrating tissue repair and that targeting pro-regenerative immune responses can improve biomaterial therapeutic outcomes. However, aging is a critical factor negatively affecting musculoskeletal tissue repair and immune function. Hence, understanding how age affects the response to biomaterials is essential for improving musculoskeletal biomaterial therapies. This review focuses on the intersection of the immune system and aging in response to biomaterials for musculoskeletal tissue repair. The article introduces the general impacts of aging on tissue physiology, the immune system, and the response to biomaterials. Then, it explains how the adaptive immune system guides the response to injury and biomaterial implants in cartilage, muscle, and bone and discusses how aging impacts these processes in each tissue type. The review concludes by highlighting future directions for the development and translation of personalized immunomodulatory biomaterials for musculoskeletal tissue repair.

Molecular evidence sterile tissue damage during pathogenesis of pododermatitis aseptica hemorrhagica circumscripta is associated with disturbed epidermal-dermal homeostasis

Podermatitis aseptica hemorrhagica circumscripta is associated with metalloproteinase 2 weakening of distal phalangeal suspensory structures and sinkage of the distal phalanx in the claw capsule. Pressure from the tuberculum flexorium on the sole epidermis and dermis produces hemorrhagic tissue injury and defective horn production appearing as yellow-red, softened claw horn in region 4 of the sole. A model of the MAPK/ERK signal cascade orchestrating epidermal-dermal homeostasis was employed to determine if sterile inflammatory responses are linked to disturbed signal transduction for epidermal homeostasis in sole epidermis and dermis. The objective was to assess shifts in target genes of inflammation, up- and downstream MAPK/ERK signal elements, and targeted genes supporting epidermal proliferation and differentiation. Sole epidermis and dermis was removed from lateral claws bearing lesions of podermatitis aseptica hemorrhagica circumscripta, medial claws from the same limb and lateral claws from completely normal limbs of multiparous, lactating Holstein cows. The abundance levels of targeted transcripts were evaluated by real-time QPCR. Lesion effects were assessed by ANOVA, and mean comparisons were performed with t-tests to assess variations between mean expression in ulcer-bearing or medial claw dermis and epidermis and completely normal lateral claw dermis and epidermis or between ulcer-bearing dermis and epidermis and medial claw dermis and epidermis. The lesions were sterile and showed losses across multiple growth factors, their receptors, several downstream AP1 transcription components, CMYC, multiple cell cycle and terminal differentiation elements conducted by MAPK/ERK signals and β 4, α 6 and collagen 17A hemidesmosome components. These losses coincided with increased cytokeratin 6, β 1 integrin, proinflammatory metalloproteinases 2 and 9, IL1B and physiologic inhibitors of IL1B, the decoy receptor and receptor antagonist. Medial claw epidermis and dermis from limbs with lateral claws bearing podermatitis aseptica hemorrhagica circumscripta showed reductions in upstream MAPK/ERK signal elements and downstream targets that paralleled those in hemorrhagic lesions. Inhibitors of IL1B increased in the absence of real increases in inflammatory targets in the medial claw dermis and epidermis. Losses across multiple signal path elements and downstream targets were associated with negative effects on targeted transcripts supporting claw horn production and wound repair across lesion-bearing lateral claws and lesion-free medial claw dermis and epidermis. It was unclear if the sterile inflammation was causative or a consequence of these perturbations.

Histone lactylation in macrophages is predictive for gene expression changes during ischemia induced-muscle regeneration

OBJECTIVES

We have previously shown that lactate is an essential metabolite for macrophage polarisation during ischemia-induced muscle regeneration. Recent in vitro work has implicated histone lactylation, a direct derivative of lactate, in macrophage polarisation. Here, we explore the in vivo relevance of histone lactylation for macrophage polarisation after muscle injury.

METHODS

To evaluate macrophage dynamics during muscle regeneration, we subjected mice to ischemia-induced muscle damage by ligating the femoral artery. Muscle samples were harvested at 1, 2, 4, and 7 days post injury (dpi). CD45+CD11b+F4/80+CD64+ macrophages were isolated and processed for RNA sequencing, Western Blotting, and CUT&Tag-sequencing to investigate gene expression, histone lactylation levels, and histone lactylation genomic localisation and enrichment, respectively.

RESULTS

We show that, over time, macrophages in the injured muscle undergo extensive gene expression changes, which are similar in nature and in timing to those seen after other types of muscle-injuries. We find that the macrophage histone lactylome is modified between 2 and 4 dpi, which is a crucial window for macrophage polarisation. Absolute histone lactylation levels increase, and, although subtly, the genomic enrichment of H3K18la changes. Overall, we find that histone lactylation is important at both promoter and enhancer elements. Lastly, H3K18la genomic profile changes from 2 to 4 dpi were predictive for gene expression changes later in time, rather than being a reflection of prior gene expression changes.

CONCLUSIONS

Our results suggest that histone lactylation dynamics are functionally important for the function of macrophages during muscle regeneration.

Growth and differentiation factor-15: A link between inflammaging and cardiovascular disease

Age-related disorders are closely linked to the accumulation of senescent cells. The senescence-associated secretory phenotype (SASP) sustains and progresses chronic inflammation, which is involved in cellular and tissue dysfunction. SASP-related growth and differentiation factor-15 (GDF-15) is an immunoregulatory cytokine that is coupled to aging and thus may have a regulatory role in the development and maintenance of atherosclerosis, a major cause of cardiovascular disease (CVD). Although the effects of GDF-15 are tissue-specific and dependent on microenvironmental changes such as inflammation, available data suggest that GDF-15 has a significant role in CVD. Thus, GDF-15 is a promising biomarker and potential therapeutic target for atherosclerotic CVD.

Growth Differentiation Factor-15 Is Considered a Predictive Biomarker of Long COVID in Non-hospitalized Patients

Mitochondrial dysfunction is associated with various diseases. Mitochondria plays a regulatory role during infection. The association between mitokines and subsequent COVID progression has not been previously studied. The retrospective cohort study aimed to investigate the potential of serum mitokines as long COVID biomarkers in non-hospitalized patients. Patients with confirmed SARS-CoV-2 infection and blood test reports between January 2021 and April 2023 were included. Patients were categorized into two groups, the recovered and long COVID groups, based on fatigue, decline in focus, and pain. Serum levels of growth differentiation factor 15 (GDF-15) and fibroblast growth factor-21 (FGF-21), which are affected by mitochondrial function, along with inflammatory and vascular endothelium markers, were measured using enzyme-linked immunosorbent assays (ELISA). A receiver operating characteristic curve was used to screen the biomarkers. The threshold value of GDF-15 in the acute phase was 965 pg/mL (sensitivity: 71.4%, specificity: 83.3%), indicating that GDF-15 may be associated with the presence of symptoms three months post onset. No association with inflammatory markers and vascular structures was observed. Therefore, elevated GDF-15 levels in the acute phase may act as a predictive biomarker of long COVID.

Regulation of muscle hypertrophy through granulin: Relayed communication among mesenchymal progenitors, macrophages, and satellite cells

Skeletal muscles exert remarkable regenerative or adaptive capacities in response to injuries or mechanical loads. However, the cellular networks underlying muscle adaptation are poorly understood compared to those underlying muscle regeneration. We employed single-cell RNA sequencing to investigate the gene expression patterns and cellular networks activated in overloaded muscles and compared these results with those observed in regenerating muscles. The cellular composition of the 4-day overloaded muscle, when macrophage infiltration peaked, closely resembled that of the 10-day regenerating muscle. In addition to the mesenchymal progenitor-muscle satellite cell (MuSC) axis, interactome analyses or targeted depletion experiments revealed communications between mesenchymal progenitors-macrophages and macrophages-MuSCs. Furthermore, granulin, a macrophage-derived factor, inhibited MuSC differentiation, and Granulin-knockout mice exhibited blunted muscle hypertrophy due to the premature differentiation of overloaded MuSCs. These findings reveal the critical role of granulin through the relayed communications of mesenchymal progenitors, macrophages, and MuSCs in facilitating efficient muscle hypertrophy.

Identification of a distinct cluster of GDF15high macrophages induced by in vitro differentiation exhibiting anti-inflammatory activities

Introduction

Macrophage-mediated inflammatory response may have crucial roles in the pathogenesis of a variety of human diseases. Growth differentiation factor 15 (GDF15) is a cytokine of the transforming growth factor-β superfamily, with potential anti-inflammatory activities. Previous studies observed in human lungs some macrophages which expressed a high level of GDF15.

Methods

In the present study, we employed multiple techniques, including immunofluorescence, flow cytometry, and single-cell RNA sequencing, in order to further clarify the identity of such GDF15high macrophages.

Results

We demonstrated that macrophages derived from human peripheral blood mononuclear cells and rat bone marrow mononuclear cells by in vitro differentiation with granulocyte-macrophage colony stimulating factor contained a minor population (~1%) of GDF15high cells. GDF15high macrophages did not exhibit a typical M1 or M2 phenotype, but had a unique molecular signature as revealed by single-cell RNA sequencing. Functionally, the in vitro derived GDF15high macrophages were associated with reduced responsiveness to pro-inflammatory activation; furthermore, these GDF15high macrophages could inhibit the pro-inflammatory functions of other macrophages via a paracrine mechanism. We further confirmed that GDF15 per se was a key mediator of the anti-inflammatory effects of GDF15high macrophage. Also, we provided evidence showing that GDF15high macrophages were present in other macrophage-residing human tissues in addition to the lungs. Further scRNA-seq analysis in rat lung macrophages confirmed the presence of a GDF15high sub-population. However, these data indicated that GDF15high macrophages in the body were not a uniform population based on their molecular signatures. More importantly, as compared to the in vitro derived GDF15high macrophage, whether the tissue resident GDF15high counterpart is also associated with anti-inflammatory functions remains to be determined. We cannot exclude the possibility that the in vitro priming/induction protocol used in our study has a determinant role in inducing the anti-inflammatory phenotype in the resulting GDF15high macrophage cells.

Conclusion

In summary, our results suggest that the GDF15high macrophage cells obtained by in vitro induction may represent a distinct cluster with intrinsic anti-inflammatory functions. The (patho)physiological importance of these cells in vivo warrants further investigation.

Temporal Single-Cell Sequencing Analysis Reveals That GPNMB-Expressing Macrophages Potentiate Muscle Regeneration

Macrophages play a crucial role in coordinating the skeletal muscle repair response, but their phenotypic diversity and the transition of specialized subsets to resolution-phase macrophages remain poorly understood. To address this issue, we induced injury and performed single-cell RNA sequencing on individual cells in skeletal muscle at different time points. Our analysis revealed a distinct macrophage subset that expressed high levels of Gpnmb and that coexpressed critical factors involved in macrophage-mediated muscle regeneration, including Igf1, Mertk, and Nr1h3. Gpnmb gene knockout inhibited macrophage-mediated efferocytosis and impaired skeletal muscle regeneration. Functional studies demonstrated that GPNMB acts directly on muscle cells in vitro and improves muscle regeneration in vivo. These findings provide a comprehensive transcriptomic atlas of macrophages during muscle injury, highlighting the key role of the GPNMB macrophage subset in regenerative processes. Targeting GPNMB signaling in macrophages could have therapeutic potential for restoring skeletal muscle integrity and homeostasis.

Spatially-resolved transcriptomics reveal macrophage heterogeneity and prognostic significance in diffuse large B-cell lymphoma

Macrophages are abundant immune cells in the microenvironment of diffuse large B-cell lymphoma (DLBCL). Macrophage estimation by immunohistochemistry shows varying prognostic significance across studies in DLBCL, and does not provide a comprehensive analysis of macrophage subtypes. Here, using digital spatial profiling with whole transcriptome analysis of CD68+ cells, we characterize macrophages in distinct spatial niches of reactive lymphoid tissues (RLTs) and DLBCL. We reveal transcriptomic differences between macrophages within RLTs (light zone /dark zone, germinal center/ interfollicular), and between disease states (RLTs/ DLBCL), which we then use to generate six spatially-derived macrophage signatures (MacroSigs). We proceed to interrogate these MacroSigs in macrophage and DLBCL single-cell RNA-sequencing datasets, and in gene-expression data from multiple DLBCL cohorts. We show that specific MacroSigs are associated with cell-of-origin subtypes and overall survival in DLBCL. This study provides a spatially-resolved whole-transcriptome atlas of macrophages in reactive and malignant lymphoid tissues, showing biological and clinical significance.

Monocyte-derived Kupffer cells dominate in the Kupffer cell pool during liver injury

Healthy Kupffer cell (KC) pool is dominated by embryonic KCs (EmKCs), preserving liver homeostasis. How the KC pool varies upon injury remains unclear. Using chimeric mice with bone marrow (BM) cells labeled with enhanced green fluorescent protein, we identify that BM monocyte-derived KCs (MoKCs) become dominant in cholestatic- or toxic-injured livers via immunofluorescence and mass cytometry. Single-cell RNA sequencing (scRNA-seq) unveils the enhanced proliferative, anti-apoptotic properties and repair potential of MoKCs compared with EmKCs, which are confirmed in vivo and ex vivo through flow cytometry, qPCR, Cell Counting Kit-8, and immunofluorescence. Furthermore, compared with EmKC-dominated livers, MoKC-dominated livers exhibit less functional damage, necrosis, and fibrosis under damage, as tested by serum alanine aminotransferase activity detection, H&E and Sirius red staining, qPCR, and western blot. Collectively, we highlight that MoKCs dominate the KC pool in injured livers and show enhanced proliferative and anti-apoptotic properties while also promoting repair and attenuating fibrosis.

Loss of adenosine A3 receptors accelerates skeletal muscle regeneration in mice following cardiotoxin-induced injury

Skeletal muscle regeneration is a complex process orchestrated by multiple interacting steps. An increasing number of reports indicate that inflammatory responses play a central role in linking initial muscle injury responses to timely muscle regeneration following injury. The nucleoside adenosine has been known for a long time as an endogenously produced anti-inflammatory molecule that is generated in high amounts during tissue injury. It mediates its physiological effects via four types of adenosine receptors. From these, adenosine A3 receptors (A3Rs) are not expressed by the skeletal muscle but are present on the surface of various inflammatory cells. In the present paper, the effect of the loss of A3Rs was investigated on the regeneration of the tibialis anterior (TA) muscle in mice following cardiotoxin-induced injury. Here we report that regeneration of the skeletal muscle from A3R-/- mice is characterized by a stronger initial inflammatory response resulting in a larger number of transmigrating inflammatory cells to the injury site, faster clearance of cell debris, enhanced proliferation and faster differentiation of the satellite cells (the muscle stem cells), and increased fusion of the generated myoblasts. This leads to accelerated skeletal muscle tissue repair and the formation of larger myofibers. Though the infiltrating immune cells expressed A3Rs and showed an increased inflammatory profile in the injured A3R-/- muscles, bone marrow transplantation experiments revealed that the increased response of the tissue-resident cells to tissue injury is responsible for the observed phenomenon. Altogether our data indicate that A3Rs are negative regulators of injury-related regenerative inflammation and consequently also that of the muscle fiber growth in the TA muscle. Thus, inhibiting A3Rs might have a therapeutic value during skeletal muscle regeneration following injury.

Traumatic muscle injury

Traumatic muscle injury represents a collection of skeletal muscle pathologies caused by trauma to the muscle tissue and is defined as damage to the muscle tissue that can result in a functional deficit. Traumatic muscle injury can affect people across the lifespan and can result from high stresses and strains to skeletal muscle tissue, often due to muscle activation while the muscle is lengthening, resulting in indirect and non-contact muscle injuries (strains or ruptures), or from external impact, resulting in direct muscle injuries (contusion or laceration). At a microscopic level, muscle fibres can repair focal damage but must be completely regenerated after full myofibre necrosis. The diagnosis of muscle injury is based on patient history and physical examination. Imaging may be indicated to eliminate differential diagnoses. The management of muscle injury has changed within the past 5 years from initial rest, immobilization and (over)protection to early activation and progressive loading using an active approach. One challenge of muscle injury management is that numerous medical treatment options, such as medications and injections, are often used or proposed to try to accelerate muscle recovery despite very limited efficacy evidence. Another challenge is the prevention of muscle injury owing to the multifactorial and complex nature of this injury.

Loss of growth differentiation factor 15 exacerbates lung injury in neonatal mice

Growth differentiation factor 15 (GDF15) is a divergent member of the transforming growth factor-β (TGF-β) superfamily, and its expression increases under various stress conditions, including inflammation, hyperoxia, and senescence. GDF15 expression is increased in neonatal murine bronchopulmonary dysplasia (BPD) models, and GDF15 loss exacerbates oxidative stress and decreases cellular viability in vitro. Our overall hypothesis is that the loss of GDF15 will exacerbate hyperoxic lung injury in the neonatal lung in vivo. We exposed neonatal Gdf15-/- mice and wild-type (WT) controls on a similar background to room air or hyperoxia (95% [Formula: see text]) for 5 days after birth. The mice were euthanized on postnatal day 21 (PND 21). Gdf15-/- mice had higher mortality and lower body weight than WT mice after exposure to hyperoxia. Hyperoxia exposure adversely impacted alveolarization and lung vascular development, with a greater impact in Gdf15-/- mice. Interestingly, Gdf15-/- mice showed lower macrophage count in the lungs compared with WT mice both under room air and after exposure to hyperoxia. Analysis of the lung transcriptome revealed marked divergence in gene expression and enriched biological pathways in WT and Gdf15-/- mice and differed markedly by biological sex. Notably, pathways related to macrophage activation and myeloid cell homeostasis were negatively enriched in Gdf15-/- mice. Loss of Gdf15 exacerbates mortality, lung injury, and the phenotype of the arrest of alveolarization in the developing lung with loss of female-sex advantage in Gdf15-/- mice.NEW & NOTEWORTHY We show for the first time that loss of Gdf15 exacerbates mortality, lung injury, and the phenotype of the arrest of alveolarization in the developing lung with loss of female-sex advantage in Gdf15-/- mice. We also highlight the distinct pulmonary transcriptomic response in the Gdf15-/- lung including pathways related to macrophage recruitment and activation.

From time-series transcriptomics to gene regulatory networks: A review on inference methods

Inference of gene regulatory networks has been an active area of research for around 20 years, leading to the development of sophisticated inference algorithms based on a variety of assumptions and approaches. With the ever increasing demand for more accurate and powerful models, the inference problem remains of broad scientific interest. The abstract representation of biological systems through gene regulatory networks represents a powerful method to study such systems, encoding different amounts and types of information. In this review, we summarize the different types of inference algorithms specifically based on time-series transcriptomics, giving an overview of the main applications of gene regulatory networks in computational biology. This review is intended to give an updated reference of regulatory networks inference tools to biologists and researchers new to the topic and guide them in selecting the appropriate inference method that best fits their questions, aims, and experimental data.

Single-cell and spatial transcriptomics identify a macrophage population associated with skeletal muscle fibrosis

Macrophages are essential for skeletal muscle homeostasis, but how their dysregulation contributes to the development of fibrosis in muscle disease remains unclear. Here, we used single-cell transcriptomics to determine the molecular attributes of dystrophic and healthy muscle macrophages. We identified six clusters and unexpectedly found that none corresponded to traditional definitions of M1 or M2 macrophages. Rather, the predominant macrophage signature in dystrophic muscle was characterized by high expression of fibrotic factors, galectin-3 (gal-3) and osteopontin (Spp1). Spatial transcriptomics, computational inferences of intercellular communication, and in vitro assays indicated that macrophage-derived Spp1 regulates stromal progenitor differentiation. Gal-3+ macrophages were chronically activated in dystrophic muscle, and adoptive transfer assays showed that the gal-3+ phenotype was the dominant molecular program induced within the dystrophic milieu. Gal-3+ macrophages were also elevated in multiple human myopathies. These studies advance our understanding of macrophages in muscular dystrophy by defining their transcriptional programs and reveal Spp1 as a major regulator of macrophage and stromal progenitor interactions.

Single-cell and spatial transcriptomics identify a macrophage population associated with skeletal muscle fibrosis

The monocytic/macrophage system is essential for skeletal muscle homeostasis, but its dysregulation contributes to the pathogenesis of muscle degenerative disorders. Despite our increasing knowledge of the role of macrophages in degenerative disease, it still remains unclear how macrophages contribute to muscle fibrosis. Here, we used single-cell transcriptomics to determine the molecular attributes of dystrophic and healthy muscle macrophages. We identified six novel clusters. Unexpectedly, none corresponded to traditional definitions of M1 or M2 macrophage activation. Rather, the predominant macrophage signature in dystrophic muscle was characterized by high expression of fibrotic factors, galectin-3 and spp1. Spatial transcriptomics and computational inferences of intercellular communication indicated that spp1 regulates stromal progenitor and macrophage interactions during muscular dystrophy. Galectin-3 + macrophages were chronically activated in dystrophic muscle and adoptive transfer assays showed that the galectin-3 + phenotype was the dominant molecular program induced within the dystrophic milieu. Histological examination of human muscle biopsies revealed that galectin-3 + macrophages were also elevated in multiple myopathies. These studies advance our understanding of macrophages in muscular dystrophy by defining the transcriptional programs induced in muscle macrophages, and reveal spp1 as a major regulator of macrophage and stromal progenitor interactions.

Identification of Scoparone from Chinese Olive Fruit as a Modulator of Macrophage Polarization

Chinese olive (Canarium album L.) has been highlighted for its remarkable health benefits. We previously showed that the ethyl acetate fraction of Chinese olive (COE) is an effective anti-inflammatory agent. In this study, we used a luciferase-based RAW 264.7 cell platform to detect the transcriptional activity of NF-κB, a key mediator of inflammation, and the promoter activity of its downstream target, COX-2. Through functional-oriented screening using these platforms, we further divided COE into several subfractions. Subsequently, we used silica gel column chromatography for purification, and the active compounds were separated and isolated by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). The structure of the resulting compound with high anti-inflammatory activity was then identified as scoparone. Our results showed that scoparone not only inhibited lipopolysaccharide (LPS)-induced secretion of nitric oxide and suppressed M1 macrophage markers (iNOS, Il-6, Ccl2, and Tnf-α) but also markedly decreased the production of pro-inflammatory cytokines (IL-6, CCL2, and TNF-α). Treatment with scoparone significantly reduced the protein level of TNF-α in LPS-treated bone-marrow-derived macrophages (BMDMs). In addition, scoparone promoted macrophages toward an M2 anti-inflammatory phenotype, as determined by the significantly increased gene expression of M2 macrophage markers (Arg1, Ym1, Mrc1, Il-10, and Cd206) and the protein level of Arg1. This study indicates that COE fruit has high therapeutic potential for various inflammatory diseases as a result of switching the macrophage phenotype from pro-inflammatory M1 to anti-inflammatory M2.

Brain and blood single-cell transcriptomics in acute and subacute phases after experimental stroke

Cerebral ischemia triggers a powerful inflammatory reaction involving both peripheral leukocytes and brain resident cells. Recent evidence indicates that their differentiation into a variety of functional phenotypes contributes to both tissue injury and repair. However, the temporal dynamics and diversity of post-stroke immune cell subsets remain poorly understood. To address these limitations, we performed a longitudinal single-cell transcriptomic study of both brain and mouse blood to obtain a composite picture of brain-infiltrating leukocytes, circulating leukocytes, microglia and endothelium diversity over the ischemic/reperfusion time. Brain cells and blood leukocytes isolated from mice 2 or 14 days after transient middle cerebral artery occlusion or sham surgery were purified by FACS sorting and processed for droplet-based single-cell transcriptomics. The analysis revealed a strong divergence of post-ischemic microglia, macrophages, and neutrophils over time, while such diversity was less evident in dendritic cells, B, T and NK cells. Conversely, brain endothelial cells and brain associated-macrophages showed altered transcriptomic signatures at 2 days post-stroke, but low divergence from sham at day 14. Pseudotime trajectory inference predicted the in-situ longitudinal progression of monocyte-derived macrophages from their blood precursors into day 2 and day 14 phenotypes, while microglia phenotypes at these two time points were not connected. In contrast to monocyte-derived macrophages, neutrophils were predicted to be continuously de-novo recruited from the blood. Brain single-cell transcriptomics from both female and male aged mice did not show major changes in respect to young mice, but aged and young brains differed in their immune cell composition. Furthermore, blood leukocyte analysis also revealed altered transcriptomes after stroke. However, brain-infiltrating leukocytes displayed higher transcriptomic divergence than their circulating counterparts, indicating that phenotypic diversification into cellular subsets occurs within the brain in the early and the recovery phase of ischemic stroke. In addition, this resource report contains a searchable database https://anratherlab.shinyapps.io/strokevis/ to allow user-friendly access to our data. The StrokeVis tool constitutes a comprehensive gene expression atlas that can be interrogated at the gene and cell type level to explore the transcriptional changes of endothelial and immune cell subsets from mouse brain and blood after stroke.

Association of Growth Differentiation Factor 15 with Arterial Stiffness and Endothelial Function in Subpopulations of Patients with Coronary Artery Disease: A Proof-of-Concept Study

BACKGROUND

Growth-differentiation factor-15 (GDF-15) is a biomarker belonging to the transforming growth factor-beta cytokine superfamily, which is linked to many pathological conditions, including inflammation and myocardial injury. Pulse wave velocity (cfPWV) and augmentation index (AIx) are indices of arterial stiffness, which are associated with the severity of coronary artery disease (CAD). Flow-mediated dilatation (FMD) is a well-studied surrogate marker of endothelial-dependent dysfunction and systemic inflammation.

OBJECTIVE

In this proof-of-concept study, we aimed to investigate the relationship between circulating GDF-15, endothelial dysfunction, and indices of arterial stiffness in different settings of coronary artery disease and myocardial injury.

METHODS

In this cross-sectional single-center study, we enrolled patients (n = 22) after interventional treatment for acute myocardial infarction (AMI), patients (n = 11) admitted with chest pain and elevated cardiac enzymes but without evidence of obstructing CAD (MI-NOCAD) in percutaneous coronary angiography (CAG), and patients (n = 20) who underwent CAG according to indications without evident obstructive CAD in CAG (NOCAD). FMD was assessed at the brachial artery. AIx of the central aortic pressure and cfPWV were estimated by applanation tonometry at the radial and carotid-femoral site, respectively, with a validated acquisition system (Sphygmo- Cor, AtCor Medical, Sydney (NSW), Australia). ELISA was used to determine circulating GDF- 15 serum levels (R&D Systems, Minneapolis, MN). Clinical and demographic data and values of routine biochemical biomarkers were obtained. The highest high-sensitive cardiac Troponin I (hsTpnI) value during hospitalization was also recorded. Left ventricular ejection fraction (LVEF) was assessed with a transthoracic echocardiogram.

RESULTS

Patients with AMI were older, had worse LVEF, higher values of hsTpnI and increased circulating GDF-15 levels. Importantly, AMI patients had increased cfPWV values, deteriorated AIx values, blunted FMD and worse serum creatinine levels compared to MI-NOCAD and NOCAD patients, respectively, whereas MI-NOCAD and NOCAD did not differ from each other significantly on these biomarkers. Both AMI and MI-NOCAD patients presented a higher but comparable white blood cell count than NOCAD patients. A strong linear correlation between GDF-15 and cfPWV, hsTpnI, AIx, white blood cell count and creatinine but not with FMD was demonstrated in the general study population.

CONCLUSION

This proof-of-concept study showed that higher circulating levels of GDF-15, an inflammatory biomarker, were associated significantly with increased arterial stiffness only in AMI patients, whereas elevated GDF-15 demonstrated a linear relationship with the severity of the myocardial injury.

Nur77 and PPARγ regulate transcription and polarization in distinct subsets of M2-like reparative macrophages during regenerative inflammation

Macrophage polarization is a process whereby macrophages develop a specific phenotype and functional response to different pathophysiological stimuli and tissue environments. In general, two main macrophage phenotypes have been identified: inflammatory (M1) and alternatively activated (M2) macrophages characterized specifically by IL-1β and IL-10 production, respectively. In the cardiotoxin-induced skeletal muscle injury model bone marrow-derived macrophages (BMDMs) play the central role in regulating tissue repair. Bone marrow-derived monocytes arriving at the site of injury differentiate first to M1 BMDMs that clear cell debris and trigger proliferation and differentiation of the muscle stem cells, while during the process of efferocytosis they change their phenotype to M2 to drive resolution of inflammation and tissue repair. The M2 population is formed from at least three distinct subsets: antigen presenting, resolution-related and growth factor producing macrophages, the latest ones expressing the transcription factor PPARγ. Nuclear receptor subfamily 4 group A member 1 (NR4A1; also termed Nur77) transcription factor is expressed as an early response gene, and has been shown to suppress the expression of pro-inflammatory genes during efferocytosis. Here we demonstrate that (1) Nur77 null BMDMs are characterized by elevated expression of PPARγ resulting in enhanced efferocytosis capacity; (2) Nur77 and PPARγ regulate transcription in different subsets of M2 skeletal muscle macrophages during muscle repair; (3) the loss of Nur77 prolongs M1 polarization characterized by increased and prolonged production of IL-1β by the resolution-related macrophages normally expressing Nur77; whereas, in contrast, (4) it promotes M2 polarization detected via the increased number of IL-10 producing CD206⁺ macrophages generated from the PPARγ-expressing subset.

The Cytokine Growth Differentiation Factor-15 and Skeletal Muscle Health: Portrait of an Emerging Widely Applicable Disease Biomarker

Growth differentiation factor 15 (GDF-15) is a stress-induced transforming growth factor-β superfamily cytokine with versatile functions in human health. Elevated GDF-15 blood levels associate with multiple pathological conditions, and are currently extensively explored for diagnosis, and as a means to monitor disease progression and evaluate therapeutic responses. This review analyzes GDF-15 in human conditions specifically focusing on its association with muscle manifestations of sarcopenia, mitochondrial myopathy, and autoimmune and viral myositis. The use of GDF-15 as a widely applicable health biomarker to monitor muscle disease is discussed, and its potential as a therapeutic target is explored.

Role of macrophages during skeletal muscle regeneration and hypertrophy-Implications for immunomodulatory strategies

Skeletal muscle is a plastic tissue that regenerates ad integrum after injury and adapts to raise mechanical loading/contractile activity by increasing its mass and/or myofiber size, a phenomenon commonly refers to as skeletal muscle hypertrophy. Both muscle regeneration and hypertrophy rely on the interactions between muscle stem cells and their neighborhood, which include inflammatory cells, and particularly macrophages. This review first summarizes the role of macrophages in muscle regeneration in various animal models of injury and in response to exercise-induced muscle damage in humans. Then, the potential contribution of macrophages to skeletal muscle hypertrophy is discussed on the basis of both animal and human experiments. We also present a brief comparative analysis of the role of macrophages during muscle regeneration versus hypertrophy. Finally, we summarize the current knowledge on the impact of different immunomodulatory strategies, such as heat therapy, cooling, massage, nonsteroidal anti-inflammatory drugs and resolvins, on skeletal muscle regeneration and their potential impact on muscle hypertrophy.

Single nuclei transcriptomics of muscle reveals intra-muscular cell dynamics linked to dystrophin loss and rescue

In Duchenne muscular dystrophy, dystrophin loss leads to chronic muscle damage, dysregulation of repair, fibro-fatty replacement, and weakness. We develop methodology to efficiently isolate individual nuclei from minute quantities of frozen skeletal muscle, allowing single nuclei sequencing of irreplaceable archival samples and from very small samples. We apply this method to identify cell and gene expression dynamics within human DMD and mdx mouse muscle, characterizing effects of dystrophin rescue by exon skipping therapy at single nuclei resolution. DMD exon 23 skipping events are directly observed and increased in myonuclei from treated mice. We describe partial rescue of type IIa and IIx myofibers, expansion of an MDSC-like myeloid population, recovery of repair/remodeling M2-macrophage, and repression of inflammatory POSTN1 + fibroblasts in response to exon skipping and partial dystrophin restoration. Use of this method enables exploration of cellular and transcriptomic mechanisms of dystrophin loss and repair within an intact muscle environment. Our initial findings will scaffold our future work to more directly examine muscular dystrophies and putative recovery pathways.

Retinoids Promote Mouse Bone Marrow-Derived Macrophage Differentiation and Efferocytosis via Upregulating Bone Morphogenetic Protein-2 and Smad3

Clearance of apoptotic cells by bone marrow-derived macrophages differentiated from monocytes plays a central role in the resolution of inflammation, as the conversion of pro-inflammatory M1 macrophages to M2 macrophages that mediate the resolution process occurs during efferocytosis. Thus, proper efferocytosis is a prerequisite for proper resolution of inflammation, and failure in efferocytosis is associated with the development of chronic inflammatory diseases. Previous studies from our laboratory have shown that (13R)-all-trans-13,14-dihydroretinol (DHR), the product of retinol saturase, acting from day 4 of monocyte differentiation enhances the efferocytosis capacity of the resulted macrophages. Loss of retinol saturase in mice leads to impaired efferocytosis, and to development of autoimmunity. In the present paper, we report that in differentiating monocytes DHR, retinol, and all-trans retinoic acid all act directly on retinoic acid receptors and enhance the clearance of apoptotic cells by upregulating the expression of several efferocytosis-related genes. The effect of retinoids seems to be mediated by bone morphogenetic protein (BMP)-2, and the Smad3 transcription factor. In addition, retinoids also upregulate the expression of the vitamin D receptor and that of vascular endothelial growth factor A, indicating that altogether retinoids promote the generation of a pro-reparative M2 macrophage population during monocyte differentiation.

Involvement of phosphatidylserine receptors in the skeletal muscle regeneration: therapeutic implications

Sarcopenia is a progressive loss of muscle mass and strength with a risk of adverse outcomes such as disability, poor quality of life, and death. Increasing evidence indicates that diminished ability of the muscle to activate satellite cell-dependent regeneration is one of the factors that might contribute to its development. Skeletal muscle regeneration following myogenic cell death results from the proliferation and differentiation of myogenic stem cells, called satellite cells, located beneath the basal lamina of the muscle fibres. Satellite cell differentiation is not a satellite cell-autonomous process but depends on signals provided by the surrounding cells. Infiltrating macrophages play a key role in the process partly by clearing the necrotic cell debris, partly by producing cytokines and growth factors that guide myogenesis. At the beginning of the muscle regeneration process, macrophages are pro-inflammatory, and the cytokines produced by them trigger the proliferation and differentiation of satellite cells. Following the uptake of dead cells, however, a transcriptionally regulated phenotypic change (macrophage polarization) is induced in them resulting in their transformation into healing macrophages that guide resolution of inflammation, completion of myoblast differentiation, myoblast fusion and growth, and return to homeostasis. Impaired efferocytosis results in delayed cell death clearance, delayed macrophage polarization, prolonged inflammation, and impaired muscle regeneration. Thus, proper efferocytosis by macrophages is a determining factor during muscle repair. Here we review that both efferocytosis and myogenesis are dependent on the cell surface phosphatidylserine (PS), and surprisingly, these two processes share a number of common PS receptors and signalling pathways. Based on these findings, we propose that stimulating the function of PS receptors for facilitating muscle repair following injury could be a successful approach, as it would enhance efferocytosis and myogenesis simultaneously. Because increasing evidence indicates a pathophysiological role of impaired efferocytosis in the development of chronic inflammatory conditions, as well as in impaired muscle regeneration both contributing to the development of sarcopenia, improving efferocytosis should be considered also in its management. Again applying or combining those treatments that target PS receptors would be expected to be the most effective, because they would also promote myogenesis. A potential PS receptor-triggering candidate molecule is milk fat globule-EGF-factor 8 (MFG-E8), which not only stimulates PS-dependent efferocytosis and myoblast fusion but also promotes extracellular signal-regulated kinase (ERK) and Akt activation-mediated cell proliferation and cell cycle progression in myoblasts.

DCs unchained: Whetting dendritic-cell appetite boosts wound healing

Apoptotic-cell uptake (efferocytosis) by dendritic cells (DCs) has been mainly linked to their antigen presentation property. In a recent issue of Nature, Maschalidi et al. identified a break to efferocytosis in DCs, the inhibition of which improves skin debris cleansing after a wound, accelerating healing.

Differences in muscle satellite cell dynamics during muscle hypertrophy and regeneration

Skeletal muscle homeostasis and function are ensured by orchestrated cellular interactions among several types of cells. A noticeable aspect of skeletal muscle biology is the drastic cell–cell communication changes that occur in multiple scenarios. The process of recovering from an injury, which is known as regeneration, has been relatively well investigated. However, the cellular interplay that occurs in response to mechanical loading, such as during resistance training, is poorly understood compared to regeneration. During muscle regeneration, muscle satellite cells (MuSCs) rebuild multinuclear myofibers through a stepwise process of proliferation, differentiation, fusion, and maturation, whereas during mechanical loading-dependent muscle hypertrophy, MuSCs do not undergo such stepwise processes (except in rare injuries) because the nuclei of MuSCs become directly incorporated into the mature myonuclei. In this review, six specific examples of such differences in MuSC dynamics between regeneration and hypertrophy processes are discussed.

CD36, a signaling receptor and fatty acid transporter that regulates immune cell metabolism and fate

CD36 is a type 2 cell surface scavenger receptor widely expressed in many immune and non-immune cells. It functions as both a signaling receptor responding to DAMPs and PAMPs, as well as a long chain free fatty acid transporter. Recent studies have indicated that CD36 can integrate cell signaling and metabolic pathways through its dual functions and thereby influence immune cell differentiation and activation, and ultimately help determine cell fate. Its expression along with its dual functions in both innate and adaptive immune cells contribute to pathogenesis of common diseases, including atherosclerosis and tumor progression, which makes CD36 and its downstream effectors potential therapeutic targets. This review comprehensively examines the dual functions of CD36 in a variety of immune cells, especially macrophages and T cells. We also briefly discuss CD36 function in non-immune cells, such as adipocytes and platelets, which impact the immune system via intercellular communication. Finally, outstanding questions in this field are provided for potential directions of future studies.

Targeting SLC7A11 improves efferocytosis by dendritic cells and wound healing in diabetes

Chronic non-healing wounds are a major complication of diabetes, which affects 1 in 10 people worldwide. Dying cells in the wound perpetuate the inflammation and contribute to dysregulated tissue repair^1-3. Here we reveal that the membrane transporter SLC7A11 acts as a molecular brake on efferocytosis, the process by which dying cells are removed, and that inhibiting SLC7A11 function can accelerate wound healing. Transcriptomics of efferocytic dendritic cells in mouse identified upregulation of several SLC7 gene family members. In further analyses, pharmacological inhibition of SLC7A11, or deletion or knockdown of Slc7a11 using small interfering RNA enhanced efferocytosis in dendritic cells. Slc7a11 was highly expressed in dendritic cells in skin, and single-cell RNA sequencing of inflamed skin showed that Slc7a11 was upregulated in innate immune cells. In a mouse model of excisional skin wounding, inhibition or loss of SLC7A11 expression accelerated healing dynamics and reduced the apoptotic cell load in the wound. Mechanistic studies revealed a link between SLC7A11, glucose homeostasis and diabetes. SLC7A11-deficient dendritic cells were dependent on aerobic glycolysis using glucose derived from glycogen stores for increased efferocytosis; also, transcriptomics of efferocytic SLC7A11-deficient dendritic cells identified increased expression of genes linked to gluconeogenesis and diabetes. Further, Slc7a11 expression was higher in the wounds of diabetes-prone db/db mice, and targeting SLC7A11 accelerated their wound healing. The faster healing was also linked to the release of the TGFβ family member GDF15 from efferocytic dendritic cells. In sum, SLC7A11 is a negative regulator of efferocytosis, and removing this brake improves wound healing, with important implications for wound management in diabetes.

Coping With Stress: The Mitokine GDF-15 as a Biomarker of COVID-19 Severity

Growth differentiation factor 15 (GDF-15) is a transforming growth factor (TGF)-β superfamily cytokine that plays a central role in metabolism regulation. Produced in response to mitochondrial stress, tissue damage or hypoxia, this cytokine has emerged as one of the strongest predictors of disease severity during inflammatory conditions, cancers and infections. Reports suggest that GDF-15 plays a tissue protective role via sympathetic and metabolic adaptation in the context of mitochondrial damage, although the exact mechanisms involved remain uncertain. In this review, we discuss the emergence of GDF-15 as a distinctive marker of viral infection severity, especially in the context of COVID-19. We will critically review the role of GDF-15 as an inflammation-induced mediator of disease tolerance, through metabolic and immune reprogramming. Finally, we discuss potential mechanisms of GDF-15 elevation during COVID-19 cytokine storm and its limitations. Altogether, this cytokine seems to be involved in disease tolerance to viral infections including SARS-CoV-2, paving the way for novel therapeutic interventions.