Single cell full-length transcriptome of human subcutaneous adipose tissue reveals unique and heterogeneous cell populations

Variant-to-function approaches for adipose tissue: Insights into cardiometabolic disorders

Genome-wide association studies (GWASs) have identified thousands of genetic loci associated with cardiometabolic disorders. However, the functional interpretation of these loci remains a daunting challenge. This is particularly true for adipose tissue, a critical organ in systemic metabolism and the pathogenesis of various cardiometabolic diseases. We discuss how variant-to-function (V2F) approaches are used to elucidate the mechanisms by which GWAS loci increase the risk of cardiometabolic disorders by directly influencing adipose tissue. We outline GWAS traits most likely to harbor adipose-related variants and summarize tools to pinpoint the putative causal variants, genes, and cell types for the associated loci. We explain how large-scale perturbation experiments, coupled with imaging and multi-omics, can be used to screen variants' effects on cellular phenotypes and how these phenotypes can be tied to physiological mechanisms. Lastly, we discuss the challenges and opportunities that lie ahead for V2F research and propose a roadmap for future studies.

Towards a consensus atlas of human and mouse adipose tissue at single-cell resolution

Adipose tissue (AT) is a complex connective tissue with a high relative proportion of adipocytes, which are specialized cells with the ability to store lipids in large droplets. AT is found in multiple discrete depots throughout the body, where it serves as the primary repository for excess calories. In addition, AT has an important role in functions as diverse as insulation, immunity and regulation of metabolic homeostasis. The Human Cell Atlas Adipose Bionetwork was established to support the generation of single-cell atlases of human AT as well as the development of unified approaches and consensus for cell annotation. Here, we provide a first roadmap from this bionetwork, including our suggested cell annotations for humans and mice, with the aim of describing the state of the field and providing guidelines for the production, analysis, interpretation and presentation of AT single-cell data.

Obesity, white adipose tissue and cancer

White adipose tissue (WAT) is crucial for whole-body energy homeostasis and plays an important role in metabolic and hormonal regulation. While healthy WAT undergoes controlled expansion and contraction to meet the body's requirements, dysfunctional WAT in conditions like obesity is characterized by excessive tissue expansion, alterations in lipid homeostasis, inflammation, hypoxia, and fibrosis. Obesity is strongly associated with an increased risk of numerous cancers, with obesity-induced WAT dysfunction influencing cancer development through various mechanisms involving both systemic and local interactions between adipose tissue and tumors. Unhealthy obese WAT affects circulating levels of free fatty acids and factors like leptin, adiponectin, and insulin, altering systemic lipid metabolism and inducing inflammation that supports tumor growth. Similar mechanisms are observed locally in an adipose-rich tumor microenvironment (TME), where WAT cells can also trigger extracellular matrix remodeling, thereby enhancing the TME's ability to promote tumor growth. Moreover, tumors reciprocally interact with WAT, creating a bidirectional communication that further enhances tumorigenesis. This review focuses on the complex interplay between obesity, WAT dysfunction, and primary tumor growth, highlighting potential targets for therapeutic intervention.

Pericytes Promote More Vascularization than Stromal Cells via an Interleukin-6-Dependent Mechanism in Microfluidic Chips

Pericytes are a key player in vascularization, protecting endothelial cells from external harm and promoting the formation of new vessels when necessary. However, pericytic identity and its relationship with other cell types, such as mesenchymal stromal/stem cells, is highly debated. This study compares the role of pericytes and unselected stromal cells in vascularization using multichannel microfluidic chips. In both angiogenesis and vasculogenesis, pericytes promote more vessel formation than stromal cells. Pericytes can wrap around endothelial vessels acting as mural cells, while stromal cells remain separated. Whole-transcriptome sequencing confirms an upregulation of pro-vascularization genes in endothelial cell-pericyte co-cultures, while metabolism increases and inflammation decreases in stromal cell co-cultures. Treatment of stromal-endothelial cell co-cultures with either conditioned media or isolated extracellular vesicles from pericytes replicates the increase in vasculogenesis of the direct co-cultures. Cytokine quantification reveals that interleukin 6 (IL-6) is significantly increased in pericyte conditions. Blocking it with siltuximab results in a reduction of pericyte vasculogenic potential comparable to stromal cell levels, revealing that pericyte pro-vascularization is mediated by IL-6. This study provides new insights into the relationship between pericytes and endothelial cells and the elusive identity of mesenchymal stromal cells. These findings are relevant for both vascular biology and tissue engineering.

Metabolic dysfunction-associated steatotic liver disease (MASLD): Exploring systemic impacts and innovative therapies

Metabolic dysfunction-associated steatotic liver disease (MASLD), which includes the inflammatory subtype metabolic dysfunction-associated steatohepatitis, is a prominent cause of chronic liver disease with systemic effects. Insulin resistance, obesity, and dyslipidaemia produce MASLD in over 30 % of adults. It is a global health issue. From MASLD to MASH, hepatic inflammation and fibrosis grow, leading to cirrhosis, hepatocellular cancer, and extrahepatic complications such CVD, CKD, and sarcopenia. Effects of MASLD to MASH are mediated through mechanisms that include inflammation, oxidative stress, dysbiosis, and predisposition through genetic makeup. Advances in diagnostic nomenclature in the past few years have moved the emphasis away from NAFLD to MASLD, focusing on the metabolic etiology and away from the stigma of an alcoholic-related condition. Epidemiological data show a large geographical variability and increasing prevalence in younger populations, particularly in regions with high carbohydrate-rich diets and central adiposity. Lifestyle modification is considered as the main management of MASLD currently. This may include dietary intervention, exercise, and weight loss management. Pharmaceutical management is primarily aimed at metabolic dysfunction with promising findings for GLP-1 receptor agonists, pioglitazone and SGLT-2 inhibitors, which can correct both hepatic and systemic outcome. However, it still depends on well-integrated multidisciplinary care models by considering complex relationships between MASLD and its effects on extrahepatic organs. Determining complications at an early stage; developing precision medicine strategies; exploring new therapeutic targets will represent crucial factors in improving their outcomes. This review discuss the systemic nature of MASLD and calls for multiple collaborations to reduce its far-reaching health impacts and our quest for understanding its pathological mechanisms. Thus, collective efforts that are required to address MASLD are under the public health, clinical care, and research angles toward effectively containing its rapidly increasing burden.

A dataset of single-cell transcriptomic atlas of Bama pig and potential marker genes across seven tissues

The use of single-cell sequencing technology for single-cell transcriptomics studies in pigs is expanding progressively. However, the comprehensive classification of cell types across different anatomical tissues and organs of pig in multiple datasets remains relatively limited. This study employs single-cell and single-nucleus sequencing technologies in Bama pig to identify unique marker genes and their corresponding transcriptomic profiles across diverse cell types in various anatomical tissues and organs, including subcutaneous fat, visceral fat, psoas major muscle, liver, spleen, lung, and kidney. Through detailed data analyses, we observed widespread cellular diversity across various anatomical tissues and organs of Bama pig. This work contributes a comprehensive dataset that supports physiological studies and aids in the identification and prediction of potential marker genes through single-cell transcriptomics of these tissues. The methodologies and data employed in this study are designed to improve the accuracy of cell type identification and ensure consistent cell type allocation.

Aging human abdominal subcutaneous white adipose tissue at single cell resolution

White adipose tissue (WAT) is a robust energy storage and endocrine organ critical for maintaining metabolic health as we age. Our aim was to identify cell-specific transcriptional aberrations that occur in WAT with aging. We leveraged full-length snRNA-Seq and histology to characterize the cellular landscape of human abdominal subcutaneous WAT in a prospective cohort of 10 younger (≤30 years) and 10 older individuals (≥65 years) balanced for sex and body mass index (BMI). The older group had greater cholesterol, very-low-density lipoprotein, triglycerides, thyroid stimulating hormone, and aspartate transaminase compared to the younger group (p < 0.05). We highlight that aging WAT is associated with adipocyte hypertrophy, increased proportions of lipid-associated macrophages and mast cells, an upregulation of immune responses linked to fibrosis in pre-adipocyte, adipocyte, and vascular populations, and highlight CXCL14 as a biomarker of these processes. We show that older WAT has elevated levels of senescence marker p16 in adipocytes and identify the adipocyte subpopulation driving this senescence profile. We confirm that these transcriptional and phenotypical changes occur without overt fibrosis and in older individuals that have comparable WAT insulin sensitivity to the younger individuals.

Mitochondrial Function and Dysfunction in White Adipocytes and Therapeutic Implications

For a long time, white adipocytes were thought to function as lipid storages due to the sizeable unilocular lipid droplet that occupies most of their space. However, recent discoveries have highlighted the critical role of white adipocytes in maintaining energy homeostasis and contributing to obesity and related metabolic diseases. These physiological and pathological functions depend heavily on the mitochondria that reside in white adipocytes. This article aims to provide an up-to-date overview of the recent research on the function and dysfunction of white adipocyte mitochondria. After briefly summarizing the fundamental aspects of mitochondrial biology, the article describes the protective role of functional mitochondria in white adipocyte and white adipose tissue health and various roles of dysfunctional mitochondria in unhealthy white adipocytes and obesity. Finally, the article emphasizes the importance of enhancing mitochondrial quantity and quality as a therapeutic avenue to correct mitochondrial dysfunction, promote white adipocyte browning, and ultimately improve obesity and its associated metabolic diseases. © 2024 American Physiological Society. Compr Physiol 14:5581-5640, 2024.

Dissecting human adipose tissue heterogeneity using single-cell omics technologies

Single-cell omics technologies that profile genes (genomic and epigenomic) and determine the abundance of mRNA (transcriptomic), protein (proteomic and secretomic), lipids (lipidomic), and extracellular matrix (matrisomic) support the dissection of adipose tissue heterogeneity at unprecedented resolution in a temporally and spatially defined manner. In particular, cell omics technologies may provide innovative biomarkers for the identification of rare specific progenitor cell subpopulations, assess transcriptional and proteomic changes affecting cell proliferation and immunomodulatory potential, and accurately define the lineage hierarchy and differentiation status of progenitor cells. Unraveling adipose tissue complexity may also provide for the precise assessment of a dysfunctional state, which has been associated with cancer, as cancer-associated adipocytes play an important role in shaping the tumor microenvironment supporting tumor progression and metastasis, obesity, metabolic syndrome, and type 2 diabetes mellitus. The information collected by single-cell omics has relevant implications for regenerative medicine because adipose tissue is an accessible source of multipotent cells; alternative cell-free approaches, including the use of adipose tissue stromal cell-conditioned medium, extracellular vesicles, or decellularized extracellular matrix, are clinically valid options. Subcutaneous white adipose tissue, which is generally harvested via liposuction, is highly heterogeneous because of intrinsic biological variability and extrinsic inconsistencies in the harvesting and processing procedures. The current limited understanding of adipose tissue heterogeneity impinges on the definition of quality standards appropriate for clinical translation, which requires consistency and uniformity of the administered product. We review the methods used for dissecting adipose tissue heterogeneity and provide an overview of advances in omics technology that may contribute to the exploration of heterogeneity and dynamics of adipose tissue at the single-cell level.

Adipose-derived mesenchymal stromal cells in clinical trials: Insights from single-cell studies

Mesenchymal Stromal Cells (MSCs) offer tremendous potential for the treatment of various diseases and their healing properties have been explored in hundreds of clinical trials. These trails primarily focus on immunological and neurological disorders, as well as regenerative medicine. Adipose tissue is a rich source of mesenchymal stromal cells and methods to obtain and culture adipose-derived MSCs (AD-MSCs) have been well established. Promising results from pre-clinical testing of AD-MSCs activity prompted clinical trials that further led to the approval of AD-MSCs for the treatment of complex perianal fistulas in Crohn's disease and subcutaneous tissue defects. However, AD-MSC heterogeneity along with various manufacturing protocols or different strategies to boost their activity create the need for standardized quality control procedures and safety assessment of the intended cell product. High-resolution transcriptomic methods have been recently gaining attention, as they deliver insight into gene expression profiles of individual cells, helping to deconstruct cellular hierarchy and differentiation trajectories, and to understand cell-cell interactions within tissues. This article presents a comprehensive overview of completed clinical trials evaluating the safety and efficacy of AD-MSC treatment, together with current single-cell studies of human AD-MSC. Furthermore, our work emphasizes the increasing significance of single-cell research in elucidating the mechanisms of cellular action and predicting their therapeutic effects.

Dynamics of single-nuclei transcriptomic profiling of adipose tissue from diverse anatomical locations during mouse aging process

Adipose tissue plays critical roles in an individual's aging process. In this research, we use single-nucleus RNA sequencing to create highly detailed transcriptional maps of subcutaneous adipose tissue and visceral adipose tissue in young and aged mice. We comprehensively identify the various cell types within the white adipose tissue of mice, our study has elucidated seven distinct cell types within this tissue. Further analyses focus on adipocytes, fibro-adipogenic progenitors, and immune cells, revealing age-related declines in the synthetic metabolic activity of adipocytes, diminished immune regulation, and reduced maturation or proliferation of fibroblasts in undifferentiated adipocytes. We confirm the presence of distinct subpopulations of adipocytes, highlighting decreases in adipogenesis subgroups due to aging. Additionally, we uncover a reduction in immune cell subpopulations, driven by age-associated immune system dysregulation. Furthermore, pseudo-time analyses indicate that Adipocyte1 represents the 'nascent' phase of adipocyte development, while Adipocyte2 represents the 'mature' phase. We use cell-cell interaction to explore the age-dependent complexities of the interactions between FAPs and adipocytes, and observed increased expression of the inflammation-related Retn-Tlr4 interaction in older mice, while the anti-inflammatory Angpt1-Tek interaction was only detected in young mice. These transcriptional profiles serve as a valuable resource for understanding the functional genomics underlying metabolic disorders associated with aging in human adipose tissue.

Distinct subpopulations of human subcutaneous adipose tissue precursor cells revealed by single-cell RNA sequencing

Adipose-derived stem cells (ADSCs) play an important role in the differential capacity for excess energy storage between upper body abdominal (ABD) adipose tissue (AT) and lower body gluteofemoral (GF) AT. We cultured ADSCs from subcutaneous ABD AT and GF AT isolated from eight women with differential body fat distribution and performed single-cell RNA sequencing. Six populations of ADSCs were identified and segregated according to their anatomical origin. The three ADSC subpopulations in GF AT were characterized by strong cholesterol/fatty acid (FA) storage and proliferation signatures. The two ABD subpopulations, differentiated by higher expression of committed preadipocyte marker genes, were set apart by differential expression of extracellular matrix and ribosomal genes. The last population, identified in both depots, was similar to smooth muscle cells and when individually isolated and cultured in vitro they differentiated less than the other subpopulations. This work provides important insight into the use of ADSC as an in vitro model of adipogenesis and suggests that specific subpopulations of GF-ADSCs contribute to the more robust capacity for GF-AT to expand and grow compared with ABD-AT in women.NEW & NOTEWORTHY Identification of distinct subpopulations of adipose-derived stem cells (ADSCs) in upper body abdominal subcutaneous (ABD) and lower body gluteofemoral subcutaneous (GF) adipose tissue depots. In ABD-ADSCs, subpopulations are more committed to adipocyte lineage. GF-ADSC subpopulations are enriched for genes involved in lipids and cholesterol metabolism. Similar depot differences were found in stem cell population identified in freshly isolated stoma vascular fraction. The repertoire of ADSCs subpopulations was different in apple-shaped versus pear-shaped women.

White adipocyte dysfunction and obesity-associated pathologies in humans

The prevalence of obesity and associated chronic diseases continues to increase worldwide, negatively impacting on societies and economies. Whereas the association between excess body weight and increased risk for developing a multitude of diseases is well established, the initiating mechanisms by which weight gain impairs our metabolic health remain surprisingly contested. In order to better address the myriad of disease states associated with obesity, it is essential to understand adipose tissue dysfunction and develop strategies for reinforcing adipocyte health. In this Review we outline the diverse physiological functions and pathological roles of human white adipocytes, examining our current knowledge of why white adipocytes are vital for systemic metabolic control, yet poorly adapted to our current obesogenic environment.

Adipocyte-endothelial cell interplay in adipose tissue physiology

Adipose tissue (AT) expansion through hyperplasia or hypertrophy requires vascular remodeling that involves angiogenesis. There is quite some evidence that obese white AT (WAT) displays altered vasculature. Some studies suggest that this is associated with hypoxia, which is thought to play a role in inducing inflammatory activation of the excessively expanding WAT. Increasing evidence, based on genetic manipulations or treatments with inhibitory or activator pharmaceuticals, demonstrates that AT angiogenesis is crucial for AT metabolic function, and thereby for whole body metabolism and metabolic health. Despite some contradiction between studies, disturbance of WAT angiogenesis in obesity could be an important factor driving WAT dysfunction and the comorbidities of obesity. Endothelial cells (ECs) contribute to healthy WAT metabolism via transport of fatty acids and other plasma components, secretory signaling molecules, and extracellular vesicles (EVs). This communication is crucial for adipocyte metabolism and underscores the key role that the AT endothelium plays in systemic energy homeostasis and healthy metabolism. Adipocytes communicate towards the neighboring endothelium through several mechanisms. The pro-inflammatory status of hypertrophic adipocytes in obesity is reflected in ECs activation, which promotes chronic inflammation. On the other hand, adiponectin secreted by the adipocytes is important for healthy endothelial function, and adipocytes also secrete other pro- or anti-angiogenic effector molecules and a wealth of EVs - however, their detailed roles in signaling towards the endothelium are yet poorly understood. To conclude, targeting AT angiogenesis and promoting the healthy communication between adipocytes and ECs represent potentially promising strategies to treat obesity and its comorbidities.

Steatotic liver disease, MASLD and risk of chronic kidney disease

With the rising tide of fatty liver disease related to metabolic dysfunction worldwide, the association of this common liver disease with chronic kidney disease (CKD) has become increasingly evident. In 2020, the more inclusive term metabolic dysfunction-associated fatty liver disease (MAFLD) was proposed to replace the old term non-alcoholic fatty liver disease (NAFLD). In 2023, a modified Delphi process was led by three large pan-national liver associations. There was consensus to change the fatty liver disease nomenclature and definition to include the presence of at least one of five common cardiometabolic risk factors as diagnostic criteria. The name chosen to replace NAFLD was metabolic dysfunction-associated steatotic liver disease (MASLD). The change of nomenclature from NAFLD to MAFLD and then MASLD has resulted in a reappraisal of the epidemiological trends and associations with the risk of developing CKD. The observed association between MAFLD/MASLD and CKD and our understanding that CKD can be an epiphenomenon linked to underlying metabolic dysfunction support the notion that individuals with MASLD are at substantially higher risk of incident CKD than those without MASLD. This narrative review provides an overview of the literature on (a) the evolution of criteria for diagnosing this highly prevalent metabolic liver disease, (b) the epidemiological evidence linking MASLD to the risk of CKD, (c) the underlying mechanisms by which MASLD (and factors strongly linked with MASLD) may increase the risk of developing CKD, and (d) the potential drug treatments that may benefit both MASLD and CKD.

The evolving functions of the vasculature in regulating adipose tissue biology in health and obesity

Adipose tissue is an endocrine organ and a crucial regulator of energy storage and systemic metabolic homeostasis. Additionally, adipose tissue is a pivotal regulator of cardiovascular health and disease, mediated in part by the endocrine and paracrine secretion of several bioactive products, such as adipokines. Adipose vasculature has an instrumental role in the modulation of adipose tissue expansion, homeostasis and metabolism. The role of the adipose vasculature has been extensively explored in the context of obesity, which is recognized as a global health problem. Obesity-induced accumulation of fat, in combination with vascular rarefaction, promotes adipocyte dysfunction and induces oxidative stress, hypoxia and inflammation. It is now recognized that obesity-associated endothelial dysfunction often precedes the development of cardiovascular diseases. Investigations have revealed heterogeneity within the vascular niche and dynamic reciprocity between vascular and adipose cells, which can become dysregulated in obesity. Here we provide a comprehensive overview of the evolving functions of the vasculature in regulating adipose tissue biology in health and obesity.

The Multifaceted Roles of Bone Marrow Adipocytes in Bone and Hematopoietic Homeostasis

Bone marrow adipose tissue (BMAT) makes up a significant portion of the marrow space, ranging from 50% to 70%, in healthy adults. It expands with aging, obesity, anorexia nervosa, and irradiation, which are conditions associated with skeletal complications or hematopoietic disorders. Therefore, BMAT has been viewed as a negative component of the bone marrow niche for decades, although the mechanisms and causative relationships have not been well-addressed. Of note, recent studies have revealed that BMAT is a multifaceted tissue that can serve as an energy reservoir to fuel osteoblasts and hematopoietic cells under stressful situations, and also acts as an endocrine/paracrine organ to suppress bone formation and support hematopoiesis at steady-state conditions. In this review, we summarize the uniqueness of BMAT, the complex findings of previous studies, and update our understanding of the physiological roles of BMAT in bone and hematopoietic metabolism based on a newly established bone marrow adipocyte-specific mouse model.

The Emerging Importance of Mitochondria in White Adipocytes: Neither Last nor Least

The growing recognition of mitochondria's crucial role in the regulation of white adipose tissue remodeling and energy balance underscores its significance. The marked metabolic diversity of mitochondria provides the molecular and cellular foundation for enabling adipose tissue plasticity in response to various metabolic cues. Effective control of mitochondrial function at the cellular level, not only in thermogenic brown and beige adipocytes but also in energy-storing white adipocytes, exerts a profound influence on adipose homeostasis. Furthermore, mitochondria play a pivotal role in intercellular communication within adipose tissue via production of metabolites with signaling properties. A more comprehensive understanding of mitochondrial regulation within white adipocytes will empower the development of targeted and efficacious strategies to enhance adipose function, leading to advancements in overall metabolic health.

White Adipose Tissue Heterogeneity in the Single-Cell Era: From Mice and Humans to Cattle

Adipose tissue is a major modulator of metabolic function by regulating energy storage and by acting as an endocrine organ through the secretion of adipokines. With the advantage of next-generation sequencing-based single-cell technologies, adipose tissue has been studied at single-cell resolution, thus providing unbiased insight into its molecular composition. Recent single-cell RNA sequencing studies in human and mouse models have dissected the transcriptional cellular heterogeneity of subcutaneous (SAT), visceral (VAT), and intramuscular (IMAT) white adipose tissue depots and revealed unique populations of adipose tissue progenitor cells, mature adipocytes, immune cell, vascular cells, and mesothelial cells that play direct roles on adipose tissue function and the development of metabolic disorders. In livestock species, especially in bovine, significant gaps of knowledge remain in elucidating the roles of adipose tissue cell types and depots on driving the pathogenesis of metabolic disorders and the distinct fat deposition in VAT, SAT, and IMAT in meat animals. This review summarizes the current knowledge on the transcriptional and functional cellular diversity of white adipose tissue revealed by single-cell approaches and highlights the depot-specific function of adipose tissue in different mammalian species, with a particular focus on recent findings and future implications in cattle.

Adipose 'neighborhoods' collaborate to maintain metabolic health

Body fat is stored in anatomically distinct adipose depots that vary in their cell composition and play specialized roles in systemic metabolic homeostasis via secreted products. Their local effects on nearby tissues (e.g. the gut and visceral adipose tissues) are increasingly recognized and this local crosstalk is being elucidated. The major subcutaneous fat depots, abdominal and gluteal-femoral, exert opposite effects on the risk of metabolic disease. The pace of research into developmental, sex, and genetic determinants of human adipose depot growth and function is rapidly accelerating, providing insight into the pathogenesis of metabolic dysfunction in persons with obesity.

Adipose tissue endothelial cells: insights into their heterogeneity and functional diversity

Cardiovascular disease is the leading cause of death globally. Endothelial cells (ECs), the key units of all vascular segments, have a significant impact on the health and disease of organisms. Adipose tissue is vital to cardiovascular health, therefore, understanding adipose EC (AdEC) biology is important. Recent data have highlighted the presence of distinct AdEC subpopulations that govern adipose tissue homeostasis. In addition to their role in nutrient metabolism and transport, AdECs are involved in bidirectional cellular communication with adipocytes, among other cells. These interactions are mainly mediated by paracrine factors, including noncoding RNAs. In this review, we highlight recent results showcasing the functions of AdECs in adipose tissue biology, metabolic homeostasis, and changes occurring in obesity.

Adipocyte gene expression in obesity - insights gained and challenges ahead

White adipocytes possess extraordinary plasticity with an unparalleled capacity to expand in size with nutritional overload. Several lines of evidence indicate that limitations to this plasticity, as found in both lipodystrophy and obesity, drive several of the comorbidities of these disease, thereby underscoring the need to understand the mechanisms of healthy and unhealthy adipose expansion. Recent single-cell technologies and studies of isolated adipocytes have allowed researchers to gain insight into the molecular mechanisms of adipocyte plasticity. Here, we review current insight into the effect of nutritional overload on white adipocyte gene expression and function. We review the role of adipocyte size and heterogeneity and discuss the challenges and future directions.

Master of disguise: deconvoluting adipose tissue heterogeneity and its impact on metabolic health

Adipose tissue in its different forms: white, brown, and beige, while essential in day-to-day bodily functions, leads to several disorders when present in overabundance, including obesity and type-2 diabetes. Adipose tissue function/dysfunction is largely mediated by the diversity of its cell composition, within adipocytes and cells in its stromal fraction. Owing to its heterogeneous nature, recent studies have focused on intercalating the effects of cellular diversity with adipose tissue function, particularly by employing sequencing technologies. In this review, we highlight the recent advances in utilizing single-cell and single-nuclei RNA sequencing technologies to discover novel adipose tissue cell types or subtypes, and to determine their role in mediating tissue, as well as whole-body metabolism and function.

Integrated analysis of plasma proteome and cortex single-cell transcriptome reveals the novel biomarkers during cortical aging

Background

With the increase of age, multiple physiological functions of people begin gradually degenerating. Regardless of natural aging or pathological aging, the decline in cognitive function is one of the most obvious features in the process of brain aging. Brain aging is a key factor for several neuropsychiatric disorders and for most neurodegenerative diseases characterized by onset typically occurring late in life and with worsening of symptoms over time. Therefore, the early prevention and intervention of aging progression are particularly important. Since there is no unified conclusion about the plasma diagnostic biomarkers of brain aging, this paper innovatively employed the combined multi-omics analysis to delineate the plasma markers of brain aging.

Methods

In order to search for specific aging markers in plasma during cerebral cortex aging, we used multi-omics analysis to screen out differential genes/proteins by integrating two prefrontal cortex (PFC) single-nucleus transcriptome sequencing (snRNA-seq) datasets and one plasma proteome sequencing datasets. Then plasma samples were collected from 20 young people and 20 elder people to verify the selected differential genes/proteins with ELISA assay.

Results

We first integrated snRNA-seq data of the post-mortem human PFC and generated profiles of 65,064 nuclei from 14 subjects across adult (44-58 years), early-aging (69-79 years), and late-aging (85-94 years) stages. Seven major cell types were classified based on established markers, including oligodendrocyte, excitatory neurons, oligodendrocyte progenitor cells, astrocytes, microglia, inhibitory neurons, and endotheliocytes. A total of 93 cell-specific genes were identified to be significantly associated with age. Afterward, plasma proteomics data from 2,925 plasma proteins across 4,263 young adults to nonagenarians (18-95 years old) were combined with the outcomes from snRNA-seq data to obtain 12 differential genes/proteins (GPC5, CA10, DGKB, ST6GALNAC5, DSCAM, IL1RAPL2, TMEM132C, VCAN, APOE, PYH1R, CNTN2, SPOCK3). Finally, we verified the 12 differential genes by ELISA and found that the expression trends of five biomarkers (DSCAM, CNTN2, IL1RAPL2, CA10, GPC5) were correlated with brain aging.

Conclusion

Five differentially expressed proteins (DSCAM, CNTN2, IL1RAPL2, CA10, GPC5) can be considered as one of the screening indicators of brain aging, and provide a scientific basis for clinical diagnosis and intervention.

A single nuclei atlas of aging human abdominal subcutaneous white adipose tissue

White adipose tissue (WAT) is a robust energy storage and endocrine organ critical for maintaining metabolic health as we age. Our aim was to identify cell-specific transcriptional aberrations that occur in WAT with aging. We leveraged full-length snRNA-Seq to characterize the cellular landscape of human subcutaneous WAT in a prospective cohort of 10 Younger (≤ 30 years) and 10 Older individuals (≥ 65 years) balanced for sex and body mass index (BMI). We highlight that aging WAT is associated with adipocyte hypertrophy, increased proportions of resident macrophages (M2), an upregulated innate immune response and senescence profiles in specific adipocyte populations, highlighting CXCL14 as a biomarker of this process. We also identify novel markers of pre-adipocytes and track their expression levels through pre-adipocyte differentiation. We propose that aging WAT is associated with low-grade inflammation that is managed by a foundation of innate immunity to preserve the metabolic health of the WAT.

New discoveries in the field of metabolism by applying single-cell and spatial omics

Single-cell multi-Omics (SCM-Omics) and spatial multi-Omics (SM-Omics) technologies provide state-of-the-art methods for exploring the composition and function of cell types in tissues/organs. Since its emergence in 2009, single-cell RNA sequencing (scRNA-seq) has yielded many groundbreaking new discoveries. The combination of this method with the emergence and development of SM-Omics techniques has been a pioneering strategy in neuroscience, developmental biology, and cancer research, especially for assessing tumor heterogeneity and T-cell infiltration. In recent years, the application of these methods in the study of metabolic diseases has also increased. The emerging SCM-Omics and SM-Omics approaches allow the molecular and spatial analysis of cells to explore regulatory states and determine cell fate, and thus provide promising tools for unraveling heterogeneous metabolic processes and making them amenable to intervention. Here, we review the evolution of SCM-Omics and SM-Omics technologies, and describe the progress in the application of SCM-Omics and SM-Omics in metabolism-related diseases, including obesity, diabetes, nonalcoholic fatty liver disease (NAFLD) and cardiovascular disease (CVD). We also conclude that the application of SCM-Omics and SM-Omics approaches can help resolve the molecular mechanisms underlying the pathogenesis of metabolic diseases in the body and facilitate therapeutic measures for metabolism-related diseases. This review concludes with an overview of the current status of this emerging field and the outlook for its future.

Exploring the heterogeneity of white adipose tissue in mouse and man

Adipose tissue is a heterogeneous organ, comprising cell types, including mature adipocytes, progenitor cells, immune cells, and vascular cells. Here, we discuss the heterogeneity of human and mouse white adipose tissue in general and white adipocytes specifically, focusing on how our understanding of adipocyte subpopulations has expanded with the advent of single nuclear RNA sequencing and spatial transcriptomics. Furthermore, we discuss critical remaining questions regarding how these distinct populations arise, how their functions differ from one another, and which potentially contribute to metabolic pathophysiology.

Isolation of nuclei from frozen human subcutaneous adipose tissue for full-length single-nuclei transcriptional profiling

Automated single-cell dispensing is incompatible with white adipose tissue (WAT) due to lipid-laden adipocytes. Single-nuclei RNA-Seq permits transcriptional profiling of all cells from WAT. Human WAT faces unique technical challenges in isolating nuclei compared to rodent tissue due to greater extra-cellular matrix content and larger lipid droplets. In this protocol, we detail how to isolate nuclei from frozen subcutaneous human WAT for single-nuclei RNA-Seq. For complete information on the generation and use of this protocol, please refer to Whytock et al. (2022).1.

Adipose tissue at single-cell resolution

Adipose tissue exhibits remarkable plasticity with capacity to change in size and cellular composition under physiological and pathophysiological conditions. The emergence of single-cell transcriptomics has rapidly transformed our understanding of the diverse array of cell types and cell states residing in adipose tissues and has provided insight into how transcriptional changes in individual cell types contribute to tissue plasticity. Here, we present a comprehensive overview of the cellular atlas of adipose tissues focusing on the biological insight gained from single-cell and single-nuclei transcriptomics of murine and human adipose tissues. We also offer our perspective on the exciting opportunities for mapping cellular transitions and crosstalk, which have been made possible by single-cell technologies.

Heterogeneity of white adipocytes in metabolic disease

PURPOSE OF REVIEW

This review aims to discuss the most recent evidence identifying the presence of distinct white adipocyte subpopulations in white adipose tissue (WAT) and how these may be altered with increasing adiposity and/or metabolic disease. We conceptualize how changes in adipocyte subpopulations may contribute to alterations in WAT function and the development of metabolic diseases such as type 2 diabetes mellitus (T2DM), nonalcoholic fatty liver disease (NAFLD) and cardiovascular disease (CVD).

RECENT FINDINGS

Studies utilizing novel analytical approaches support the existence of distinct white adipocyte subpopulations in both human and murine WAT. Adipocyte subtypes are potentially functionally distinct and may have different roles in WAT function and obesity-associated metabolic diseases.

SUMMARY

The exploration of white adipocyte heterogeneity using novel analytical technologies, has unveiled a new layer of complexity in the study of WAT biology. Interrogation of potential functional differences between adipocyte subpopulations and their role in the function of different WAT depots is now needed. Through understanding the mechanisms regulating white adipocyte subtype development and potential pathophysiological consequences of changes in the presence of adipocyte subpopulations, studies could provide novel therapeutic targets for the treatment of T2DM, NAFLD, and CVD.

Intermuscular adipose tissue in metabolic disease

Intermuscular adipose tissue (IMAT) is a distinct adipose depot described in early reports as a 'fatty replacement' or 'muscle fat infiltration' that was linked to ageing and neuromuscular disease. Later studies quantifying IMAT with modern in vivo imaging methods (computed tomography and magnetic resonance imaging) revealed that IMAT is proportionately higher in men and women with type 2 diabetes mellitus and the metabolic syndrome than in people without these conditions and is associated with insulin resistance and poor physical function with ageing. In parallel, agricultural research has provided extensive insight into the role of IMAT and other muscle lipids in muscle (that is, meat) quality. In addition, studies using rodent models have shown that IMAT is a bona fide white adipose tissue depot capable of robust triglyceride storage and turnover. Insight into the importance of IMAT in human biology has been limited by the dearth of studies on its biological properties, that is, the quality of IMAT. However, in the past few years, investigations have begun to determine that IMAT has molecular and metabolic features that distinguish it from other adipose tissue depots. These studies will be critical to further decipher the role of IMAT in health and disease and to better understand its potential as a therapeutic target.