The adipose tissue cell type enriched transcriptome
Adipose tissue (fat) is a highly specialized loose connective tissue composed primarily of adipocytes. It has a range of physiological functions, such as the insulation and protection of organs, the storage of energy, and secretion of metabolic hormones; indeed, it is increasingly considered a central metabolic organ in the regulation of whole-body energy homeostasis. Fat is mainly located either around the organs (visceral adipose tissue) or beneath the skin (subcutaneous adipose tissue).
Visceral adipose tissue
Subcutaneous adipose tissue
Adipose tissue cell type enriched transcriptome: Summary
Genes with predicted cell type specificity within visceral or subcutaneous adipose tissue are detailed below (Table 1 and 2, respectively). Identified enriched genes are subdivided into 3 specificity categories, based on the difference between the enrichment score in the corresponding cell type, compared to the other cell types profiled in the tissue (see Methods Summary page for details):
Table 1. Number of genes in each specificity category in the visceral adipose cell types.
Figure 1. Bar plot of the number of enriched genes in the cell types of the visceral adipose tissue, divided by specificity category
Table 2. Number of genes in each specificity categoriy in the subcutaneous adipose cell types.
Figure 2. Bar plot of the number of enriched genes in the cell types of the subcutaneous adipose tissue, divided by specificity category
Adipose tissue cell type enriched transcriptome: Illustrative examples
Adipocytes are the predominant cell type found in adipose tissue. The mature adipocyte has several tasks, the most obvious being storage of lipids as an energy depot and metabolism of these lipids according to systemic needs. The lipids are stored as lipid droplets, that contribute to the cells’ high buoyancy, large size and round shape. Adipocytes also secrete several important signaling molecules, known as adipokines. Through adipokine secretion, adipocytes play an important role in regulating the energy balance and metabolic pathways in the body.
The active processes of lipogenesis and lipolysis in adipocytes is reflected by the high expression of lipid metabolic pathway genes, such as Acyl-CoA Synthetase Long Chain Family Member 1 (ACSL1), and other accessory molecules, such as Perilipin 4 (PLIN4), which coats lipid droplets to protect them from lipases. Protein Kinase CAMP-Dependent Type II Regulatory Subunit Beta (PRKAR2B), a signaling regulatory subunit involved in adipocyte maturation, and Microsomal Glutathione S-Transferase 1 (MGST1), an antioxidant enzyme that protects cells from lipid peroxidation, were also classified as having specificity in adipocytes in both visceral and subcutaneous adipose tissue.
Adipose progenitor cells
Adipose progenitor cells are a transient cell type, with characteristics of both mesenchymal/fibroblastic cells and adipocytes. Adipose progenitor cells is a broad term, with cells in this category covering multiple developmental stages within the adipogenic pathway. These cells resemble an early stage of adipocyte development, before markers of stem cells or multipotent progenitors are lost following differentiation.
Genes classified as having specificity in adipose progenitor cells include Mannose Receptor C Type 2 (MRC2), which plays a role in extracellular matrix remodeling, Cadherin 11 (CDH11), which plays a role in calcium-dependent cell-cell adhesion, as well as genes with unknown function such as Deleted In Primary Ciliary Dyskinesia Homolog (DPCD).
Mesothelial cells form a flattened and squamous monolayer that line both the serosal cavities and the organs within. They have several protective functions, including creating a low friction surface so that organs can move freely, as well as protecting organs within the cavity by transporting required molecules and cells across the serous membrane, while keeping microbes and other potential dangers out. They have also been found to be involved in inflammatory responses and wound healing, in addition to expressing pro- and anti -coagulant factors. In adipose tissue, mesothelial cells have only been identified in visceral depots and seem to be absent in subcutaneous depots.
Mesothelial cells are of mesoderm origin, and genes classified as having specificity include keratins such as KRT8 and KRT19, kallikreins such as KLK7 and KLK8, as well as genes related to coagulation such as the anticoagulant protein Annexin A8 (ANXA8).
The immunohistochemistry images of adipose tissue featured in the Human Protein Atlas do not currently include examples from the greater omentum section of the visceral peritoneum.