Novel Subpopulation of Adipose Tissue Macrophages IMAMs and Their Role in Obesity and Metabolic Disorders

Background

Obesity is an increasingly severe health issue worldwide, often accompanied by a state of chronic low-grade inflammation. This condition is driven by the infiltration and dysfunction of immune cells within adipose tissue and other metabolic organs. Adipose tissue macrophages (ATMs) are of particular interest due to their significant role in regulating metabolic inflammation in obese populations. Research indicates that obesity-related metabolic inflammation is a major driver of insulin resistance and type 2 diabetes. However, current therapeutic approaches primarily alleviate symptoms without fundamentally addressing the immunopathological disorders associated with obesity and related metabolic disorders. Therefore, exploring the immune mechanisms involved in obesity development is key to formulating effective therapeutic strategies.

Macrophages exhibit high functional heterogeneity and phenotypic plasticity. In lean individuals, ATMs predominantly display an anti-inflammatory M2 phenotype. In contrast, in obesity, ATMs form a “crown-like structure” around necrotic adipocytes, exhibiting a pro-inflammatory M1 phenotype. The traditional M1/M2 classification fails to fully encapsulate the complexity of obesity-related macrophages, necessitating the identification of truly pathogenic ATM subpopulations to develop targeted treatments.

Overview of the Study

This study, conducted by researchers from institutions including Tongji Medical College of Huazhong University of Science and Technology and Shanxi Medical University, utilized single-nucleus RNA sequencing (snRNA-seq) to discover a unique ATM subpopulation in obese individuals called ATF4^hiPDIA3^hiACSL4^hiCCL2^hi inflammatory and metabolically activated macrophages (IMAMs). These IMAMs are enriched in obese patients, showing significant migratory and pro-inflammatory characteristics, reliant on the expression of Protein Disulfide Isomerase 3 (PDIA3).

The study found that the transcription factor ATF4 promotes the expression of PDIA3 under metabolic stress, and PDIA3 regulates RhoA activity through redox mechanisms, enhancing the pro-inflammatory and migratory capacities of IMAMs. Furthermore, by employing a PDIA3 small interfering RNA (siRNA) encapsulated liposome delivery strategy, the study successfully inhibited high-fat diet-induced metabolic disorders in obese mouse models, offering new insights for clinical treatment of obesity and metabolic diseases.

Research Process

1. Identification of the IMAMs Subpopulation

The researchers first used snRNA-seq to analyze adipose tissue from lean and obese individuals, identifying eight distinct ATM subpopulations. The IMAMs of group 1 were significantly enriched in obese individuals but almost undetectable in lean individuals. IMAMs exhibited high expression of ATF4, PDIA3, ACSL4, and CCL2, markers that were noticeably under-expressed in ATMs from lean individuals. Further pseudo-time analysis indicated that the differentiation trajectory of IMAMs in obese samples differed from that in lean individuals, suggesting that IMAMs are a pathogenic ATM subpopulation in obesity development.

2. Mechanism of PDIA3 in IMAMs

PDIA3 plays a crucial role in maintaining the pro-inflammatory and migratory characteristics of IMAMs. Experiments revealed that ATF4, upon sensing metabolic stress, regulates PDIA3 expression, and PDIA3 controls RhoA activity through redox signaling, enhancing the migratory and pro-inflammatory properties of IMAMs. Elevated PDIA3 levels correlate positively with body mass index (BMI), blood glucose, total cholesterol, and triglyceride levels, highlighting its important role in obesity-related metabolic disorders. Additionally, PDIA3 is significantly upregulated in adipose tissue of obese mice, primarily located in ATMs.

3. Protection Against Obesity-Induced Metabolic Disorders by PDIA3 Deletion

By developing a mouse model with specific PDIA3 deletion, the researchers found that PDIA3-deficient mice exhibited notable anti-obesity traits when subjected to a high-fat diet (HFD)-induced obesity model. These traits included slow weight gain, improved insulin sensitivity, reduced adipocyte size, and decreased serum insulin and triglyceride levels. Moreover, PDIA3-deficient mice demonstrated higher oxygen consumption and carbon dioxide output, indicating enhanced metabolic capacity. The deficiency in PDIA3 also reduced the proportion of M1 macrophages in adipose tissue, weakening the expression of pro-inflammatory factors such as IL-6 and TNF-α, thereby alleviating obesity-related chronic low-grade inflammation.

4. Molecular Mechanism of PDIA3 Interaction with RhoA

PDIA3 interacts with RhoA, maintaining its active state by preventing the formation of intramolecular disulfide bonds in RhoA. In macrophages, active RhoA promotes the YAP signaling pathway, thereby enhancing the pro-inflammatory and migratory capabilities of macrophages. Experiments showed that the YAP signaling pathway is inhibited in macrophages from PDIA3-deficient mice, further confirming the critical role of PDIA3 in macrophage function regulation.

5. Inhibition of Obesity and Metabolic Disorders with PDIA3 siRNA Liposomes

To explore the clinical application potential of PDIA3 inhibitors, the research team conducted experiments using PDIA3 siRNA liposomes in obese mice. The results showed that PDIA3 siRNA liposomes effectively inhibited weight gain in obese mice, improved blood glucose and insulin resistance, reduced the proportion of M1 macrophages in adipose tissue, and increased the proportion of anti-inflammatory M2 macrophages, indicating potential therapeutic effects of PDIA3 siRNA liposomes on obesity and metabolic disorders.

Conclusion of the Study

This study identified a specific IMAMs subpopulation in obese individuals through single-cell RNA sequencing. The pro-inflammatory and migratory characteristics of this subpopulation depend on the expression of PDIA3. The research confirmed that ATF4, as a metabolic stress sensor, transcribes PDIA3 expression, forming the PDIA3^hiIMAMs subpopulation, leading to the development of obesity and insulin resistance. Targeting PDIA3 expression or function in a clinical setting could mitigate inflammation and metabolic disorders related to obesity.

Significance of the Study

This study is the first to reveal the existence of the IMAMs subpopulation and its pathogenic role in obesity and metabolic disorders, expanding our understanding of adipose tissue macrophage heterogeneity and providing an important theoretical foundation for PDIA3-targeted obesity immunotherapy. The successful application of PDIA3 siRNA liposomes, in particular, opens new avenues for clinical treatment of obesity and metabolic diseases, demonstrating great clinical transformation potential.

Highlights of the Study

1. Discovery of a Novel Pathogenic Macrophage Subpopulation IMAMs: IMAMs are enriched in obese patients and closely associated with metabolic inflammation.
2. Key Role of PDIA3 in IMAMs: PDIA3 enhances the pro-inflammatory and migratory capabilities of IMAMs by regulating RhoA activity, serving as a key molecule for the maintenance of IMAMs.
3. Clinical Potential of PDIA3 siRNA Liposomes: PDIA3 siRNA liposomes effectively inhibit weight gain and metabolic disorders in obese mice, providing new insights for future treatment of obesity and metabolic diseases.

Limitations and Future Prospects

Despite the significant progress, this study has limitations. For instance, the LysM-Cre PDIA3 knockout model simultaneously deletes PDIA3 in neutrophils, potentially affecting the phenotype of obese mice. Furthermore, since PDIA3 is also present in serum, future research should explore its potential roles as an endocrine signal on other cell types.

This study offers new insights into targeting specific macrophage subpopulations, potentially bringing innovative therapeutic strategies for obesity-related metabolic disorders.