The cellular and functional complexity of thermogenic fat

Abstract

Brown and beige adipocytes are mitochondria-enriched cells capable of dissipating energy in the form of heat. These thermogenic fat cells were originally considered to function solely in heat generation through the action of the mitochondrial protein uncoupling protein 1 (UCP1). In recent years, significant advances have been made in our understanding of the ontogeny, bioenergetics and physiological functions of thermogenic fat. Distinct subtypes of thermogenic adipocytes have been identified with unique developmental origins, which have been increasingly dissected in cellular and molecular detail. Moreover, several UCP1-independent thermogenic mechanisms have been described, expanding the role of these cells in energy homeostasis. Recent studies have also delineated roles for these cells beyond the regulation of thermogenesis, including as dynamic secretory cells and as a metabolic sink. This Review presents our current understanding of thermogenic adipocytes with an emphasis on their development, biological functions and roles in systemic physiology.

Fig. 1 |. characteristics and anatomical distribution of thermogenic adipocytes.

a | Mammals possess several types of adipocytes with distinct metabolism, morphology, location and developmental timing. Brown adipocytes are a constitutive, embryonic (dermomyotome)-origin cell type and cluster in designated depots, such as interscapular brown adipose tissue (BAT) depots of mice and infants. On the other hand, beige adipocytes are a recruitable and non-dermomyotome-derived cell type often seen in white adipose tissue (WAT) depots in the postnatal stage. As beige adipocytes are generally surrounded by white adipocytes, beige adipocytes are thought of as intermediate in colour between white and brown adipocytes. Regardless, both brown and beige adipocytes dissipate energy in the form of heat, whereas white adipocytes are responsible for energy storage. The abundance of mitochondria is the major determinant of fat colour (brown/beige and white). Brown and beige adipocytes contain substantially higher amounts of cristae-dense mitochondria with high levels of iron, which give the tissue containing these cells its brown colour. By contrast, white adipocytes possess relatively few mitochondria. These cells also differ in the morphology of their lipid droplets. Brown and beige adipocytes have numerous small lipid droplets (multilocular lipid droplets), whereas white adipocytes contain a large lipid droplet (unilocular lipid droplet) for lipid storage. b | Adult mice possess several depots of BAT.The interscapular BAT is the largest BAT depot in mice, and its development is completed prenatally. In addition to these depots, certain signals (cold, adrenergic signalling) can promote emergence of beige adipocytes in anterior subcutaneous and inguinal WAT in mice. Suprascapular fat depot has also been characterized as beige fat. Of note, beige adipocyte biogenesis is regulated distinctly from brown adipocytes studied in the interscapular BAT. Thermogenic fat cells in mice have also been identified in perivascular adipose tissue surrounding the thoracic aorta and in epicardial adipose tissue surrounding the coronary arteries^, (not shown). It remains unclear whether these cells are brown or beige adipocytes, or another thermogenic adipocyte altogether. Humans also possess BAT depots in, at least, six anatomic regions, including cervical, supraclavicular, axillary, mediastinal, paraspinal and abdominal. The interscapular BAT depot is most prominent in infants and gradually declines with age. In adults, some of the BAT depots are anatomically analogous to those in mice.

Fig. 2 |. Beige fat biogenesis.

Emergence of beige adipocytes in white adipose tissue (beiging) is an inducible process stimulated by external beiging stimuli (for example, cold, adrenergic ligands). This can involve either de novo differentiation engaging progenitor cells (path 1) or reinstallation of the thermogenic phenotype by dormant cells (path 3). Key molecular markers for the different cell types associated with these transitions are indicated. De novo differentiation of beige adipocytes involves designated progenitors, which undergo cell proliferation, fate commitment to preadipocytes and differentiation into adipocytes. Progenitors and preadipocytes contain small amounts of mitochondria relative to differentiated adipocytes. When external beiging stimuli are withdrawn, mitochondria-enriched beige adipocytes transform into dormant adipocytes that resemble white adipocytes (whitening or beige-to-white fat conversion) (path 2). The whitening of beige fat is initiated by active mitochondrial clearance through selective autophagy (mitophagy), and does not involve de-differentiation/re-differentiation of adipocytes. Many of those dormant adipocytes can reinstall the thermogenic programme in response to re-exposition to beiging stimuli (path 3). This process involves active mitochondrial biogenesis. SCA1, stem cell antigen 1; UCP1, uncoupling protein 1.

Fig. 3 |. Regulation and heterogeneity of thermogenic fat.

a | Norepinephrine, released from sympathetic nerve terminals, binds to β3-adrenergic receptor (β3-AR) in differentiated adipocytes. This triggers reinstallation of the thermogenic programme in dormant cells and release of fatty acids and paracrine/autocrine factors (for example, fibroblast growth factor 21 (FGF21)) from adipocytes that stimulate de novo differentiation of beige adipocytes from progenitor cells. Hormonal cues, including IL-6, bone morphogenetic proteins (BMPs), FGF21 and atrial natriuretic peptide (ANP), are shown to promote beige adipocyte biogenesis, although the extent to which each beiging hormone promotes de novo differentiation or white to beige conversion needs further investigation. The beiging hormones have been summarized elsewhere. Even in regions that are not densely innervated, and hence not exposed to β3-adrenergic signals, connection of cells via the gap junction channel formed by connexin 43 (Cx43) has been shown to propagate intracellular thermogenic signals to promote thermogenic fat emergence. b | Recent advances with single-cell analysis revealed that thermogenic fat is heterogeneous, harbouring adipocyte populations with different molecular and functional characteristics. For example, the interscapular brown adipose tissue (BAT) of mice contains two functionally distinct subtypes of brown adipocytes with high or low expression of adiponectin (Adipoq^high and Adipoq^low). In inguinal white adipose tissue (WAT) of mice, induction of beige fat is generally under the control of β-AR signalling. However, a β-AR signalling-independent population of beige adipocytes has been identified in inguinal WAT that is driven by signalling downstream of the nicotinic acetylcholine receptor subunit CHRNA2. Notably, these adipocytes primarily use glucose as their metabolic fuel, in contrast to fatty acids that are the main fuel for thermogenic adipocytes, and hence have been termed g-beige adipocytes. They are also known to be derived from a distinct population of adipocyte progenitor cells (APCs) that, instead of being positive for smooth muscle actin (αSMA), show expression of PDGFRα and MYOD, the latter of which which does not typically associate with either white or beige adipocyte lineage. UCP1, uncoupling protein 1.

Fig. 4 |. Thermogenic mechanisms in adipocytes.

a | Mitochondrial respiration uncoupling via uncoupling protein 1 (UCP1). UCP1 is located in the mitochondrial inner membrane and uncouples the proton (H⁺) gradient from ATP synthesis. UCP1 activity is inhibited by purine nucleotides, whereas long-chain free fatty acids (LCFAs; marked with a red tail), which are negatively charged at the carboxyl end, bind to UCP1 and trigger the transfer of H⁺ into the matrix as a fatty acid anion/H⁺ symporter. b | One of the mechanisms of UCP1-independent thermogenesis in beige adipocytes is futile Ca²⁺ cycling in and out of the endoplasmic reticulum. This involves Ca²⁺ uptake into the endoplasmic reticulum by sarcoplasmic/endoplasmic reticulum calcium ATPase 2B (SERCA2B) and its release by ryanodine receptor 2 (RYR2) and inositol trisphosphate receptor (IP3R), which is coupled to ATP hydrolysis by SERCA2B and heat generation. Activation of α1-adrenergic receptor (α1-AR) and β3-AR, in response to norepinephrine, triggers cellular Ca²⁺ flux and its futile cycling. Under certain conditions, such as increased cytosolic Ca²⁺ levels (for example, enhanced Ca²⁺release from RYR2 or IP3R), ATP abundance (high ATP/ADP ratio) or reduced binding affinity of SERCA2 to Ca²⁺ (often regulated by micropeptides), ATP hydrolysis by SERCA2 is uncoupled from Ca²⁺ uptake, thereby being highly exothermic. c | Another UCP1-independent thermogenic mechanism is creatine substrate cycling, which involves ATP-dependent phosphorylation of creatine by mitochondria-localized creatine kinase (Mi-CK) to phosphocreatine (PCr) and PCr dephosphorylation by unknown diphosphatases (Enz_1–n). d | Lipolysis of triglycerides (TAGs) generates glycerol and fatty acids, which can be re-esterified back to TAG (TAG–fatty acid cycling). This process involves ATP-dependent conversion of glycerol to glycerol 3-phosphate (G3P) by glycerol kinase (GyK). G3P is also a key component of the NADH–G3P shuttle, which involves interconversion of G3P and dihydroxyacetone phosphate (DHAP) and allows for rapid ATP synthesis in the mitochondria. This cycle is promoted by thiazolidinediones, which activate GyK, as well as by cold exposure and satiety hormone leptin, which promote lipolysis. AAC, ADP/ATP carrier.

Fig. 5 |. The multifaceted roles of brown and beige fat.

One of the biological roles of brown and beige fat is thermogenesis that involves uncoupling protein 1 (UCP1)-dependent and UCP1-independent mechanisms (FIG. 4). Besides thermogenesis, brown and beige fat function as a metabolic sink for glucose, fatty acids, cholesterol and branched-chain amino acids (BCAAs), thereby controlling metabolite clearance in the circulation. At the tissue level, enhanced beige fat biogenesis is coupled with reduced fibrosis, reduced inflammation and increased angiogenesis within adipose tissues (supporting tissue homeostasis). Also, brown and beige fat secrete various molecules (also known as batokines, as listed in the figure) that mediate communication with central and peripheral organs (endocrine) and cell to cell communication within adipose tissues (autocrine/paracrine). FGF21, fibroblast growth factor 21.

Fig. 6 |. Harnessing thermogenic fat activity for precision medicine.

Nutritional supplementation with branched-chain amino acids (BCAAs) has differential outcomes in humans depending on their metabolic state: BCAAs are known to enhance energy expenditure, and, paradoxically, higher circulating levels of BCAAs are found in individuals with obesity and/or individuals with diabetes who exhibit lower energy expenditure^,,. BCAAs are actively imported into brown fat mitochondria and stimulate thermogenesis, and thus BCAA supplementation enhances energy expenditure for those individuals who are capable of oxidizing BCAAs in their brown adipose tissue (BAT) depots. By contrast, impaired BAT activity as a BCAA sink, as often seen in obesity and ageing, reduces BCAA clearance, thereby increasing circulating BCAA levels, leading to the overflow of BCAAs into skeletal muscle and resulting in insulin resistance. Hence, stratification of human individuals based on their BAT activity (using ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET) or magnetic resonance imaging (MRI)) can be explored as a component of precision medicine to tailor therapeutic or dietary interventions to the patient’s metabolic state.