Liver Danger Signal Triggers TREM2+ Macrophage-Mediated MS4A7-Dependent Inflammasome Activation Driving Steatohepatitis

Background and Research Motivation

In recent years, with the increasing incidence of metabolic dysfunction-associated steatohepatitis (MASH), its pathological mechanisms have garnered significant attention. MASH, formerly known as nonalcoholic steatohepatitis (NASH), represents an advanced stage of metabolic fatty liver disease, characterized by persistent liver injury, inflammation, and fibrosis. The pathogenesis of MASH primarily focuses on immune responses triggered by hepatocellular damage. Although recent studies have suggested that TREM2+ NASH-associated macrophages (NAMs) play important roles in various disease states, their specific role in MASH pathogenesis remains unclear.

The main objective of this study (by Zhou et al.) was to identify and understand the specific pathological role of NAMs in the course of MASH. Through analyses of mouse and human samples, the researchers discovered that the membrane-spanning 4-domains A7 (MS4A7) acts as a specific pathogenic factor that promotes the progression of MASH.

Source of Study

This paper was authored by researchers Linkang Zhou, Xiaoxue Qiu, Ziyi Meng, and others from multiple research institutions, including the Life Sciences Institute, the Department of Cell and Developmental Biology, and the Department of Molecular and Integrative Physiology at the University of Michigan, USA. The paper was published on March 13, 2024, in Science Translational Medicine.

Research Process

Subjects and Methods

The study utilized a mouse model with diet-induced MASH and conducted comprehensive gene and protein expression analyses. The specific process is as follows:

1. Single-cell RNA sequencing (scRNA-seq): Generated single-cell RNA sequencing data of non-parenchymal cells (NPCs) in healthy and diet-induced MASH mouse livers to identify macrophages with high TREM2 expression.
2. Immunohistochemistry and fluorescence staining (RNA Scope): Used to validate MS4A7 expression and identify cells co-expressing TREM2.
3. Gene knockout mouse model: Established whole-body and myeloid-specific MS4A7 gene knockout mice to study the function of MS4A7 in MASH progression.
4. Lipid droplet (LD) exposure experiment: Investigated the effects on liver macrophages by injecting lipid droplets extracted from fatty liver.
5. NLPR3 inflammasome activation experiment: Explored the role of MS4A7 in NLPR3 inflammasome activation and analyzed its specific molecular mechanisms in macrophages.

Data Analysis Algorithms

Various data analysis algorithms were used in the study, including single-cell RNA sequencing data analysis, quantitative polymerase chain reaction (qPCR) for gene expression, and Western Blot for protein validation. Bioinformatics analyses were also performed to identify differentially expressed genes and conduct pathway enrichment analysis.

Main Results

1. Identification of MS4A7: MS4A7 was significantly induced in MASH livers of both mice and humans and correlated with the severity of liver damage. In diet-induced MASH mouse livers, MS4A7 mRNA and protein levels gradually increased, whereas its expression significantly decreased after MASH resolution (when diet was switched to standard).

2. MS4A7 deletion alleviates liver injury: MS4A7-deficient mice showed reduced liver fibrosis and liver injury markers, with lower serum concentrations of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) in the diet-induced MASH model.

3. Cell reprogramming: Single-cell RNA sequencing showed that disease-associated macrophages (NAMs) and monocyte-derived macrophages (MDMs) had reduced presence in MS4A7-deficient mice, and the proportion of activated hepatic stellate cells was also lower, suggesting that myeloid cell MS4A7 signaling elicits significant pathological changes in diet-induced MASH.

4. Lipid droplets and liver injury: Lipid droplet-induced macrophage congestion and liver injury were significantly alleviated in MS4A7-deficient mice, indicating that MS4A7 plays an important role in lipid droplet-mediated liver injury.

5. NLPR3 inflammasome activation: MS4A7 played a key role in NLPR3 inflammasome activation. MS4A7 deficiency significantly attenuated inflammasome markers such as IL-1β induced by ATP and lipopolysaccharide (LPS) stimulation.

Conclusion and Research Significance

Through in-depth study of MS4A7, this paper proposes a novel understanding of the critical role of TREM2+ macrophages in MASH pathogenesis via the MS4A7-dependent inflammasome pathway. Specifically, the MS4A7 signaling axis initiated by lipid droplets promotes NLPR3 inflammasome activation and the shaping of the pathological state of cells.

Highlights of the paper include:

1. Revealing the role of MS4A7 as a NAM-specific pathogenic factor in MASH, offering a potential target for future therapeutic strategies.
2. Demonstrating the antagonistic roles of the TREM2 and MS4A7 signaling pathways in disease progression, deepening the understanding of the complexity of the immune environment in liver disease.
3. Expanding the understanding of the mechanism of lipid droplets as danger signals in metabolic liver disease.

This study, combining comprehensive animal experiments and single-cell transcriptomic analyses, provides unique insights into the pathogenesis of complex liver diseases, offering significant scientific and practical value!