Novel insights into adipose tissue heterogeneity

When normalized to volume, adipose tissue is comprised mainly of large lipid metabolizing and storing cells called adipocytes. Strikingly, the numerical representation of non-adipocytes, composed of a wide variety of cell types found in the so-called stromal vascular fraction (SVF), outnumber adipocytes by far. Besides its function in energy storage, adipose tissue has emerged as a versatile organ that regulates systemic metabolism and has therefore constituted an attractive target for the treatment of metabolic diseases. Recent high-resolution single cells/nucleus RNA seq data exemplify an intriguingly profound diversity of both adipocytes and SVF cells in all adipose depots, and the current data, while limited, demonstrate the significance of the intra-tissue cell composition in shaping the overall functionality of this tissue. Due to the complexity of adipose tissue, our understanding of the biological relevance of this heterogeneity and plasticity is fractional. Therefore, establishing atlases of adipose tissue cell heterogeneity is the first step towards generating an understanding of these functionalities. In this review, we will describe the current knowledge on adipose tissue cell composition and the heterogeneity of single-cell RNA sequencing, including the technical limitations.

Effects of co-incubation of LPS-stimulated RAW 264.7 macrophages on leptin production by 3T3-L1 adipocytes: A method for co-incubating distinct adipose tissue cell lines.

Adipose tissue is a major endocrine organ capable of releasing inflammatory adipokines. Inflammatory adipokine release is linked to the changes occurring in adipose tissue in the overfed state, where tissue remodeling results in hypertrophic adipocytes that recruit monocytes to infiltrate the tissue and take on an inflammatory phenotype. As the proportion of inflammatory macrophages increases there is a concurrent increase in release of macrophage-specific inflammatory mediators, further contributing to the inflamed state and setting an inflammatory loop between the macrophages and adipocytes. Although most inflammatory adipokines are released by macrophages, adipocytes can also release immunomodulatory adipokines, such as leptin. The objective of this research was to determine if co-incubation of activated macrophages with mature adipocytes, using Transwell inserts, affected leptin release by mature adipocytes. We also examined if there were differences in the amount of cell-secreted products quantified in cell-conditioned media collected from macrophage-containing (Transwell insert) and adipocyte-containing (well) compartments. Mature adipocytes (differentiated 3T3-L1 murine fibroblasts) were co-incubated with control and lipopolysaccharide-stimulated (0.01 μg/ml) murine macrophages (RAW264.7), and nitric oxide, interleukin-6, and leptin levels were quantified in the cell-conditioned media from the two compartments. Activation status of the macrophages did not affect leptin release by the adipocytes. We observed higher amounts of leptin in wells compared to Transwells. Nitric oxide and interleukin-6 levels were similar between Transwells and wells, suggesting that these adipokines are traveling through the Transwell inserts and reaching equilibrium between the two compartments. There was a weak negative relationship between nitric oxide release by macrophages and leptin release by adipocytes. Our results suggest that co-incubating activated macrophages and adipocytes using Transwell inserts can result in distinct microenvironments in the different cellular compartments and that separate sampling of these compartments is required to detect the subtle signaling dynamics that exist between these cells.

Assessing the Contribution of Relative Macrophage Frequencies to Subcutaneous Adipose Tissue

Background: Macrophages play an important role in regulating adipose tissue function, while their frequencies in adipose tissue vary between individuals. Adipose tissue infiltration by high frequencies of macrophages has been linked to changes in adipokine levels and low-grade inflammation, frequently associated with the progression of obesity. The objective of this project was to assess the contribution of relative macrophage frequencies to the overall subcutaneous adipose tissue gene expression using publicly available datasets.Methods: Seven publicly available microarray gene expression datasets from human subcutaneous adipose tissue biopsies (n = 519) were used together with TissueDecoder to determine the adipose tissue cell-type composition of each sample. We divided the subjects in four groups based on their relative macrophage frequencies. Differential gene expression analysis between the high and low relative macrophage frequencies groups was performed, adjusting for sex and study. Finally, biological processes were identified using pathway enrichment and network analysis.Results: We observed lower frequencies of adipocytes and higher frequencies of adipose stem cells in individuals characterized by high macrophage frequencies. We additionally studied whether, within subcutaneous adipose tissue, interindividual differences in the relative frequencies of macrophages were reflected in transcriptional differences in metabolic and inflammatory pathways. Adipose tissue of individuals with high macrophage frequencies had a higher expression of genes involved in complement activation, chemotaxis, focal adhesion, and oxidative stress. Similarly, we observed a lower expression of genes involved in lipid metabolism, fatty acid synthesis, and oxidation and mitochondrial respiration.Conclusion: We present an approach that combines publicly available subcutaneous adipose tissue gene expression datasets with a deconvolution algorithm to calculate subcutaneous adipose tissue cell-type composition. The results showed the expected increased inflammation gene expression profile accompanied by decreased gene expression in pathways related to lipid metabolism and mitochondrial respiration in subcutaneous adipose tissue in individuals characterized by high macrophage frequencies. This approach demonstrates the hidden strength of reusing publicly available data to gain cell-type-specific insights into adipose tissue function.

Adipose tissue in health and disease through the lens of its building blocks

BackgroundHighly specialized cells work in synergy forming tissues to perform functions required for the survival of organisms. Understanding this tissue-specific cellular heterogeneity and homeostasis is essential to comprehend the development of diseases within the tissue and also for developing regenerative therapies. Cellular subpopulations in the adipose tissue have been related to disease development, but efforts towards characterizing the adipose tissue cell type composition are limited due to lack of robust cell surface markers, limited access to tissue samples, and the labor-intensive process required to identify them.ResultsWe propose a framework, identifying cellular heterogeneity while providing state-of-the-art cellular markers for each cell type present in tissues using transcriptomics level analysis. We validate our approach with an independent dataset and present the most comprehensive study of adipose tissue cell type composition to date, determining the relative amounts of 21 different cell types in 779 adipose tissue samples detailing differences across four adipose tissue depots, between genders, across ranges of BMI and in different stages of type-2 diabetes. We also highlight the heterogeneity in reported marker-based studies of adipose tissue cell type composition and provide novel cellular markers to distinguish different cell types within the adipose tissue.ConclusionsOur study provides a systematic framework for studying cell type composition in a given tissue and valuable insights into adipose tissue cell type heterogeneity in health and disease.

ПОРІВНЯЛЬНА ХАРАКТЕРИСТИКА ФЕНОТИПОВИХ ЗМІН КУЛЬТУР КЛІТИН ЖИРОВОЇ ТКАНИНИ ТА КІСТКОВОГО МОЗКУ В ПРОЦЕСІ КУЛЬТИВУВАННЯ

У статті описані дані щодо зміни фенотипу культур клітин жирової тканини (ККЖТ) та кісткового мозку (КККМ) у процесі культивування. Дослідження первинних культур клітин кісткового мозку та жирової тканини щура показали, що вони морфологічно гетерогенні, у їх склад входили: невелика кількість клітин полігональної форми, а основну масу складали фібробластоподібні. За подальшого культивування відмічали процес переходу від гетерогенних культур на нульовому пасажі до найбільш гомогенних у кінці дослідження. Нами були відмічені відмінності у імунофенотипі культур клітин кісткового мозку та жирової тканини, які не зникали з пасажами. This article describes the changes in phenotype of cultures of adipose tissue cells (ATCC) and bone marrow cells (BMCC) in the process of cultivation. Study of primary cultures of cells of the bone marrow and adipose tissue of rat has shown that they are morphologically heterogeneous, they included: a small number of cells of polygonal shape, and the bulk was fìbroblast-like cells. Process of transition from the heterogeneous cultures at zero passaging to the most homogeneous at the end of the study was noted during further cultivation. We noted differences in immunophenotype of bone marrow and adipose tissue cell cultures that did not disappear with passaging.

Visceral adipose tissue activated macrophage content and inflammatory adipokine secretion is higher in pre-eclampsia than in healthy pregnancys

Obesity increases pre-eclampsia (PE) risk. Adipose tissue inflammation may contribute to the clinical syndrome of PE. We compared adipose tissue macrophage infiltration and release of pro-inflammatory adipokines in PE and healthy pregnancy. Subcutaneous and visceral adipose tissue biopsies were collected from healthy (n=13) and PE (n=13) mothers. Basal and lipopolysaccharide (LPS) stimulated adipocyte TNFα, IL-6, CCL-2, and CRP release was measured. Adipose tissue cell densities of activated (cfms+) and total (CD68+) macrophages were determined. In PE only, visceral adipose tissue TNFα release was increased after LPS stimulation (57 [76] versus 81 [97] pg/ml/µg DNA, P=0.030). Basal TNFα release was negatively correlated insulin sensitivity of visceral adipocytes (r = −0.61, P=0.030) in PE. Visceral adipocyte IL-6 release was increased after LPS stimulation in PE only (566 [696] versus 852 [914] pg/ml/µg DNA, P=0.019). Visceral adipocyte CCL-2 basal (67 [61] versus 187 [219] pg/ml/µgDNA, P=0.049) and stimulated (46 [46] versus 224 [271] pg/ml/µg DNA, P=0.003) release was greater than in subcutaneous adipocytes in PE only. In PE, median TNF mRNA expression in visceral adipose tissue was higher than controls (1.94 [1.13–4.14] versus 0.8 [0.00–1.27] TNF/PPIA ratio, P=0.006). In visceral adipose tissue, CSF1R (a marker of activated macrophages) mRNA expression (24.8[11.0] versus 51.0[29.9] CSF1R/PPIA ratio, P=0.011) and activated (cfms+) macrophage count (6.7[2.6] versus 15.2[8.8] % cfms+/adipocyte, P=0.031) were higher in PE than in controls. In conclusion, our study demonstrates dysregulation of inflammatory pathways predominantly in visceral adipose tissue in PE. Inflammation of visceral adipose tissue may mediate many of the adverse metabolic effects associated with PE.