CHRNA2: a new paradigm in beige thermoregulation and metabolism

Abstract

The contribution of thermogenic adipocytes to maintain systemic metabolic homeostasis has been increasingly appreciated in recent years. It is now recognized that different types (e.g., brown, beige) and subtypes of thermogenic adipocytes may arise from various developmental origins. In addition to the adrenergic pathway, other signals can activate thermogenesis, including paracrine communication between immune cells within the adipose tissue niche and thermogenic adipocytes. In this opinion article we highlight the recently discovered beige-selective signaling between acetylcholine from immune cells and cholinergic receptor nicotinic alpha 2 subunit (CHRNA2) in activated beige adipocytes. We present our current knowledge of how this previously unrecognized adipose non-neuronal cholinergic signaling pathway mediates beige thermoregulation, and discuss its impact on whole-body fitness and its therapeutic potential as a novel target for combating metabolic disease.

Harnessing the power of beige fat through CHRNA2

Fueled by ever-worsening obesity epidemic, metabolic disorders including cardiovascular diseases, type 2 diabetes, dyslipidemia, non-alcoholic steatohepatitis, and hypertension are all on the rise globally [1]. Thermogenic adipocytes (see Glossary), ever since their rediscovery in human adults, have been under intense investigation for their potential in counteracting obesity by burning stored chemical energy as heat [2–5]. It is now recognized that there are several different types and subtypes of thermogenic fat cells. These can be activated during cold or upon overfeeding, and such activation can be controlled through neuronal adrenergic signaling or adrenergic-independent mechanisms [2,3]. The delicate, tightly controlled energy homeostasis dictates the complexity of thermogenic fat biology. In this opinion article we introduce and discuss a recently identified signaling pathway mediated via CHRNA2, which functionally distinguishes thermogenic beige adipocytes from other types of adipocytes, and is present and functional in human subcutaneous fat [6,7]. Through paracrine crosstalk with cholinergic immune cells, CHRNA2 can be activated by both cold and excessive calorie intake, leading to both uncoupling protein 1 (UCP1)-dependent and -independent thermogenesis [6–9]. Loss-of-function studies in rodents clearly revealed that CHRNA2 signaling helps to attenuate obesity-related metabolic disorders [6,7]. Many drug discovery efforts have targeted acetylcholine receptors for treatments against neuronal diseases [10]. This newly discovered cholinergic circuitry within adipose tissue may bring fresh drug candidates to the table and ultimately help to materialize the therapeutic potential of adaptive thermogenesis in adipocytes against metabolic disorders in human.

Diverse origins of thermogenic adipocytes

This has been a very exciting decade for the adipocyte field: the long-assumed paradigm that ‘all fat cells are born equal’ has been fundamentally challenged and disproven by revolutionary discoveries. From revealing the unexpected – the shared lineage between brown fat cells and skeletal muscle [11] – to the identification of beige adipocytes as a distinct type of fat cells [12], in vivo pulse-chase studies demonstrating de novo beige adipogenesis [13], and the recent identification of various types of adipocytes and preadipocytes by using single-cell and single-nucleus RNA sequencing (scRNA-seq, snRNA-seq) approaches, the field has come a long way [14–24].

The increasingly complex picture of origins of thermogenic adipocytes is reminiscent of the classification of hematopoietic stem cells. Functionally speaking, adipocytes can be generally divided into white adipocytes that store chemical energy, and thermogenic adipocytes that mediate energy dissipation [2]. So-called thermogenic classical brown fat is characterized by a high density of mitochondria and multiple lipid droplets, whereas the architecture of beige fat is similar to that of white adipocytes under basal conditions [2]. Upon thermogenic stimulation such as cold exposure, catecholamine treatment, acute high-calorie intake, and exercise, beige adipocytes are activated and share many of the thermogenesis-related morphological and biochemical characteristics of brown adipocytes [2]. These two types of thermogenic fat cells, that arise from different developmental lineages, are localized at separate anatomical locations: in rodents most classical brown fat can be found at interscapular depots, whereas inducible beige adipocytes largely reside within subcutaneous fat depots such as the inguinal depot [2]. Thermogenic adipocytes detected in human adults have been reported to resemble either murine brown or beige adipocytes through molecular signature analysis [12,25–28]. The discrepancy is possibly caused by many factors such as different biopsy sites and the heterogeneity of human genetic makeup. These energy-dissipating adipocytes in human will be referred to here as thermogenic adipocytes for clarity.

In recent years new technologies, including scRNA-seq and snRNA-seq, have been used to investigate the heterogeneous nature of thermogenic adipocytes from a fresh angle [14,15,17–24]. Consistent with what was observed in previous investigations using fluorescence-activated cell sorting (FACS) and in vivo lineage-tracing approaches [29,30], scRNA-seq and snRNA-seq studies indicate that platelet-derived growth factor (PDGF) receptors PDGFRα and PDGFRβ are present in various types or subtypes of preadipocytes, many of which later give rise to thermogenic adipocytes [14–17,19,22,23]. It is worth noting that stimulus-induced beige activation may differ between these subpopulations of precursors: for example, Pdgfra precursors undergo proliferation and beige adipogenesis following β3-adrenergic activation [29], whereas Pdgfrb⁺ progenitors contribute to the formation of beige fat only after prolonged cold exposure [30]. A subpopulation of precursor cells expressing CD81 within the subcutaneous fat have been shown to give rise to a large fraction of beige adipocytes after prolonged cold exposure [19]. Significantly, CD81 is also functionally involved in thermogenic activation of beige adipocytes, and responds to irisin by forming a complex with αV/β1 and αV/β5 integrins [19,31]. Close examination of thermogenic adipocytes from various adipose depots, including interscapular and perivascular adipose tissues, revealed a potential connection with the smooth muscle lineage [22,23]. The expanding list of functional markers for various subtypes of thermogenic adipocytes will greatly facilitate the classification of thermogenic fat cells in humans.

Activation of adaptive thermogenesis: adrenergic signaling and beyond

To harness its therapeutic potential against metabolic disorders, many research efforts have focused on the activation mechanisms of thermogenic fat. It has been well characterized that cold exposure leads to catecholamine release from the sympathetic nervous system (SNS), resulting in the recruitment and activation of thermogenic fat through β-adrenergic receptors (β-ARs) [2]. In rodents, it is generally thought that β1-AR contributes to the proliferation of brown and beige precursors after cold exposure, whereas β3-AR is predominantly expressed in mature thermogenic adipocytes and mediates the activation of thermogenesis [32]. In human, the functional involvement of each β-AR in regulating thermogenic fat has been intensively examined in recent years. The debate over whether the metabolic benefits outweigh undesirable side effects, or which receptor plays the more dominant role, remains contentious [33–36].

An additional hurdle for agonists targeting β-ARs being put to clinical use is catecholamine resistance, which has been well documented in people with various illnesses, obesity, or advanced age [37–39]. Furthermore, several regulatory pathways independent of β-adrenergic signaling have been shown to regulate thermogenic brown and beige fat function (Figure 1) [3,6,40–42]. It has been demonstrated that cardiac natriuretic peptides induce thermogenesis in brown and beige adipocytes after binding to natriuretic peptide receptor A [40]. Activation of the A2A receptor by adenosine also increases the thermogenic activity of brown fat and recruits beige adipocytes within subcutaneous fat [41]. A recent study reported that GPR3, a G protein-coupled receptor, regulates thermogenesis through its intrinsic signaling activity in the absence of any external ligand [42]. Intriguingly, a subtype of beige adipocytes was identified in β-less mice in which all subtypes of β-ARs were deleted. The so-called glycolytic beige fat (g-beige fat) emerged upon cold exposure in the absence of β-adrenergic signaling [43]. Further investigation showed that g-beige fat originated from Myod-expressing precursors after stimulation of transcription factor GABPα, which differs from the pathway leading to canonical beige adipocytes [43].

Figure 1. Activation and thermogenic mechanisms in beige fat.

Upon cold exposure or acute high-fat diet (HFD) feeding, thermogenic beige adipocytes can be activated through several different pathways. In addition to canonical signaling through β-adrenergic receptors (β-ARs) by norepinephrine (NE), these cells can be activated via cholinergic receptor nicotinic alpha 2 subunit (CHRNA2) by acetylcholine (ACh), the A_2A receptor by adenosine, G protein-coupled receptor 3 (GPR3) and natriuretic peptide receptor A (NPR-A) by natriuretic peptide (NP). This leads to the activation of second messengers that initiate thermogenic gene transcription and ultimately enhance thermogenic capacity. In addition to uncoupling protein 1 (UCP1)-dependent heat production, thermogenesis can also be mediated through peptidase M20 domain-containing 1 (PM20D1)-induced N-acyl amino acids (NAAs), calcium futile cycling regulated by sarcoplasmic/endoplasmic reticulum calcium ATPase 2b (SERCA2b) and ryanodine receptor 2 (RYR2) in the endoplasmic reticulum, and creatine futile cycling. Abbreviations: ANT, adenine nucleotide translocator; cGMP, cyclic GMP; CK, creatine kinase; ETC, electron transport chain; PKA, protein kinase A; PKG, protein kinase G; TCA cycle, tricarboxylic acid cycle.

Collectively, these upstream signaling pathways activate beige adipocytes and enhance the thermogenic capacity of these cells. Canonical thermogenesis is mediated via UCP1 which uncouples the proton gradient from ATP formation [32]. In recent years several UCP1-independent thermogenic pathways have been identified in adipocytes, including creatine futile cycling [44–46] and calcium futile cycling mediated by sarco/endoplasmic reticulum Ca²⁺-ATPase 2b (SERCA2b) [47]. Mitochondrial uncoupling can also be mediated by adenine nucleotide translocators [48], which can be stimulated by peptidase M20 domain-containing 1 (PM20D1) induced N-acyl amino acids [49].

In addition to cold, thermogenic fat is known to be stimulated when energy intake rapidly exceeds expenditure, as a part of the response referred to as diet-induced thermogenesis (DIT). Even though β-ARs are involved in mediating DIT in adipose tissue, and compromised DIT and obesity are observed in β-less mice [50], additional regulatory signals have also been implicated in this process [51]. It has been demonstrated that bile acids activate thermogenesis in both brown and beige adipocytes [52–54], suggesting that postprandial elevation of bile acids may contribute to DIT. PPARγ-regulated lipid sensing in vagal afferents and creatine futile cycling within adipocytes have been shown to be functionally involved in DIT regulation and the maintenance of energy homeostasis [55,56]. Recent studies have also shed light on how DIT influences human thermogenic fat activities and whole-body metabolism [57,58]. The gut hormone secretin has been reported to regulate DIT in murine brown fat and human thermogenic adipocytes and to further induce satiation [59,60]. It is tempting to speculate that any therapeutic strategies exploiting thermogenic activation caused by high calorie intake, if at all possible, would be much more likely to be adhered to by participants compared with cold exposure.

Communications between immune cells and thermogenic fat

Aside from central control directly mediated through the SNS, thermogenic activity in adipocytes can be further influenced via paracrine signaling from immune cells within the tissue niche. Many types of adipose tissue-resident immune cells, including macrophages, B cells, group 2 innate lymphoid cells, γδ T cells, mast cells, and invariant natural killer T cells, have been shown to be involved in communication and regulating its activity [18,61,62]. Owing to space limitations, we focus here on the crosstalk between macrophages and thermogenic adipocytes; the roles of other types of immune cells have been broadly discussed elsewhere [62,63].

The key role that macrophages play in regulating adipose tissue function has long been appreciated, particularly in the context of obesity and metabolic syndromes [64]. In thermoregulation, it has been proposed and later challenged that macrophages activate brown and beige adipocytes through the secretion of catecholamines [65,66]. It is now believed that particular subtypes of macrophages may be involved in the removal of norepinephrine (NE) and termination of signaling [67,68]. A subpopulation of macrophages residing within both interscapular and subcutaneous fat depots, called sympathetic neuron-associated macrophages, were shown to express both the NE transporter SLC6A2 and the degradation enzyme monoamine oxidase A [67]. Catecholamine catabolism in adipose macrophages was also observed during aging in an independent study [68]. In addition, brown fat-resident macrophages have been reported to regulate sympathetic innervation through the PlexinA4–Sema6a axis [69]. However, this effect was not observed in the subcutaneous fat depot [69].

The crosstalk between macrophages and thermogenic brown and beige fat goes beyond fine-tuning neuronal influence. A recent scRNA-seq study revealed that macrophages residing in subcutaneous adipose tissue can promote thermogenesis independently of the β-adrenergic pathway [70]. Brown adipocyte-secreted chemokine C-X-C motif chemokine ligand-14 (CXCL14) promotes the recruitment of macrophages into adipose tissue and leads to activation of both brown and beige adipocytes [71]. High-fat diet feeding induces PDGFcc secretion from white adipose tissue-resident macrophages and leads to increased fat storage. Blockade of this signaling tilts the energy balance towards expenditure through thermogenic activation of brown but not subcutaneous beige fat [72]. Interaction between macrophages and adipocytes via integrin α4 and its counter-receptor vascular cell adhesion molecule 1 (VCAM-1) inhibits beige adipogenesis and leads to inflammation-related ‘whitening’ of subcutaneous adipose tissue in obesity [73].

Macrophages residing in different organs are known to exhibit microenvironment-specific functions. These cells may arise from distinct progenitors during embryonic hematopoiesis, and are self-maintained within the tissue niche in addition to being recruited from circulating monocytes [74]. Single-cell genomics has greatly accelerated the evolution of this area [75]. It is reasonable to anticipate significant advances in the next couple of years in our understanding of the pleiotropic functions and heterogeneous nature of various subtypes of adipose macrophages in the interscapular brown and subcutaneous fat depots, revealing deeper insights how these cells may directly or indirectly affect neighboring thermogenic adipocytes.

Cholinergic signaling beyond neuronal function

Among all the signaling molecules secreted from immune cells that regulate fat function, one of the unexpected newcomers is acetylcholine (discussed in more detail in the next two sections). As a neurotransmitter, the essential function of acetylcholine in the autonomic nervous system and the central nervous system has been well appreciated [76]. In addition, the so-called non-neuronal cholinergic system (NNCS) has been increasingly investigated in various disease conditions [77]. Accumulating evidence indicates that various types of non-neuronal cells contain cellular machinery for acetylcholine synthesis (choline acetyltransferase, ChAT), transport (vesicular acetylcholine transporter, VaChT), secretion, and degradation (acetylcholinesterase, AChE; and butyrylcholinesterase, BChE), and NNCS influences important physiological functions of various key organs [78–80]. Acetylcholine-producing immune cells have been shown to mediate blood pressure [81], innate immunity [82], antiviral response [83], and attenuate cytokine release from macrophages via CHRNA7 [84,85]. This last pathway, termed cholinergic anti-inflammatory signaling, has been shown to be regulated by acetylcholine produced by parasympathetic ganglion or ChAT⁺ T cells [84–87]. It is conceivable that in many (if not most) of these cases, acetylcholine from both neuronal and non-neuronal sources could contribute to the activation of nicotinic acetylcholine receptors (nAChRs) and muscarinic acetylcholine receptors (mAChRs) expressed in the nonexcitable cell types. Further study in this area may shed light on how the homeostasis between neuronal control, inflammation, and metabolism is maintained under both steady-state and disease conditions.

Immune-beige adipocyte communication via CHRNA2

Once it became apparent that there are at least two different types of thermogenic adipocytes: developmentally preformed brown fat between the shoulder blades and inducible beige adipocytes interspersed within subcutaneous white adipose tissue [12], one of the obvious next goals was to reveal unique functions of different types of thermogenic fat, if such unique functions exist. A global transcriptomic survey revealed that Chrna2, encoding one of the subunits of nAChRs, was among the genes with increased expression in activated primary subcutaneous adipocyte culture [6]. Taking advantage of the fact that nAChRs form ligand-gated ion channels, calcium imaging assay was carried out using primary adipocyte cultures isolated from Ucp1–cre–Rfp mice. This functional assay at single-cell level resolution demonstrated that the response to either acetylcholine or nicotine through CHRNA2-containing ion channels is beige fat-selective, and is absent in either white or brown adipocytes. Furthermore, activation of CHRNA2 signaling was observed in human differentiated primary adipose stromal cells isolated from subcutaneous fat, but no response was seen in human primary perirenal adipocytes, indicating that CHRNA2 may also functionally distinguish different types of human thermogenic adipocytes [6]. As additional subpopulations of adipocytes are identified, it is conceivable that the response to CHRNA2 agonists would be an effective readout to reveal the possible functional involvement of CHRNA2 signaling within various types or subtypes of thermogenic adipocytes.

The identification of a nAChR within subcutaneous fat indicates that acetylcholine may be produced within the tissue niche. Close examination using reporter mouse lines that trace ChAT expression in vivo revealed that ChAT-expressing cells are predominantly CD45⁺ immune cells [6,8], and not neurons, supporting the consensus that little or no parasympathetic innervation occurs in subcutaneous fat [88]. It is worth noting that a drastically higher abundance of ChAT⁺ immune cells was observed in the subcutaneous adipose depot in comparison to the interscapular brown fat and visceral white adipose depots [8], consistent with the notion that this circuitry is cell type/tissue-specific. Upon cold exposure, ChAT⁺ macrophages, but not T cells or B cells, are responsible for increased production of acetylcholine in subcutaneous fat [8]. This subpopulation of macrophages were termed cholinergic adipose macrophages (ChAMs). Signaling through β2-adrenergic receptors activates ChAMs, and genetic ablation of ChAT in macrophages compromises the adaptive thermogenic response in subcutaneous fat [8]. A later independent study showed that this crosstalk between macrophages and beige adipocytes through acetylcholine–CHRNA2 may also interface with other key thermogenic regulatory signaling, such as the fibroblast growth factor 21 pathway [9].

CHRNA2 signaling mediates beige fat activation and systemic metabolism

Upon cold exposure, CHRNA2 signaling is activated in beige fat, and mice with either global or adipocyte-specific deletion of Chrna2 demonstrated significantly impaired thermogenic capacity in subcutaneous fat and reduced mitochondrial respiration (Figure 2) [6,7]. Similarly, acute high calorie intake activates CHRNA2 signaling together with β-adrenergic pathways as part of the DIT response [7]. The futile creatine cycle has been identified as a key pathway in beige fat that mediates energy dissipation during both cold exposure and DIT, particularly in the absence of UCP1 [44–46,56,89]. Activation of CHRNA2 signaling via nicotine treatment elevates creatine metabolism gene expression in both murine and human primary subcutaneous adipocyte cultures [7]. Furthermore, adipocyte-specific Chrna2 deletion abrogates DIT activation of creatine metabolism in subcutaneous fat [7].

Figure 2. Activation of beige adipocytes via cholinergic receptor nicotinic alpha 2 subunit (CHRNA2) signaling.

CHRNA2 signaling is selectively functional in beige adipocytes where it is activated by acetylcholine produced by immune cells. In addition to increased mitochondrial respiration driven by uncoupling protein 1 (UCP1), creatine futile cycling, which involves phosphorylation of creatine by mitochondria-localized creatine kinase (Mi-CK), is also elevated upon CHRNA2 activation. Furthermore, CHRNA2 signaling plays a regulatory role in glycolytic beige adipocytes which arise from MyoD⁺ progenitors. It has been shown that fibroblast growth factor 21 (FGF21) upregulated by miRNA-182–5p in adipocytes may activate this acetylcholine–CHRNA2 circuitry through a paracrine mechanism. Abbreviations: AAC, ADP/ATP carrier; ETC, electron transport chain; GLUT, glucose transporter; PCr, phosphocreatine; TCA cycle, tricarboxylic acid cycle.

The recently identified g-beige fat, a subset of beige adipocytes, originates from Myod1-expressing precursors and displays enhanced glycolysis [43]. Agonist treatments revealed that functional CHRNA2 signaling is present in both primary inguinal MyoD⁺ g-beige fat isolated from reporter mice and a cell line with ectopic expression of g-beige fat regulator GABPα, indicating that CHRNA2 signaling regulates not only canonical beige fat as shown previously [6], but also these newly discovered glycolytic beige adipocytes [7]. Activation of CHRNA2 signaling in cultured g-beige fat increases the expression of glucose metabolism genes and elevates lactate levels [7]. How CHRNA2 signaling influences glucose metabolism is particularly of interest, given the increasing attention and debates regarding questions such as – what is the dominant fuel source for thermogenic fat under physiological conditions, and what triggers or controls the metabolic switch between oxidative phosphorylation and glycolysis in both health and disease? [90,91].

The absence of CHRNA2 signaling greatly compromises beige fat function, which in turn has detrimental consequences at the whole-body metabolism level when facing dietary challenges. Deletion of Chrna2 globally or in adipocytes results in higher body and fat tissue weights and reduced expression of thermogenic genes in subcutaneous fat after chronic high-fat diet feeding [6,7]. Moreover, mice lacking Chrna2 display impaired glucose tolerance and reduced insulin sensitivity [6,7]. Considering that the ‘classical’ thermogenic organs – brown fat and skeletal muscle – are fully functional in these mice, and that β-adrenergic signaling in subcutaneous fat tissue is also intact, these results highlight the essential role of CHRNA2 signaling in energy metabolism, and suggest that targeting either the activation of CHRNA2 or inhibition of acetylcholine degradation specifically within subcutaneous adipose tissue niche may present promising therapeutic opportunities for combating obesity and metabolic disorders (Figure 3, Key figure).

Figure 3.

Within subcutaneous (SubQ) fat, norepinephrine (NE) binds to the β2 adrenergic receptor (β2-AR) and leads to the release of acetylcholine (ACh) from macrophages containing functional ChAT. ACh selectively interacts with CHRNA2 in activated beige adipocytes. Mechanistic understanding of the ACh–CHRNA2 axis may enable functional assays, including calcium imaging, to be used to identify small molecules that either activate CHRNA2 signaling or inhibit ACh degradation, and which may ultimately become drug candidates against obesity and its associated metabolic disorders such as hyperglycemia, cardiovascular disease, and fatty liver disease. Abbreviations: AChE, acetylcholinesterase; BChE, butyrylcholinesterase.

Key figure.

Therapeutic potential of the choline acetyltransferase (ChAT)-cholinergic receptor nicotinic alpha 2 subunit (CHRNA2) signaling pathway

Concluding remarks

It has been only a little more than 50 years since Jean-Pierre Changeux and colleagues identified the first neurotransmitter receptor, nAChR [92]. The broad spectrum of physiological functions of acetylcholine and its receptors, both neuronal and non-neuronal, has been recognized in the context of many human diseases [10]. The discovery of acetylcholine–CHRNA2 signaling between immune cells and activated beige adipocytes within subcutaneous adipose depots has brought attention on cholinergic regulation back to fat, where it had previously been debated, concluding that parasympathetic nerves do not play a role [88] (see Outstanding questions). nAChRs are homo- or hetero-pentameric complexes made up of α and/or β subunits [93]. The native composition of nAChR subunits in different cell types has been shown to dictate the cell type-specific pharmacologic characteristics of the receptor (affinities for agonists, competitive antagonists, and allosteric effectors). The assembly of functional nAChRs is tightly regulated [10]. Future investigations focusing on the beige fat-specific composition of CHRNA2-containing ion channels, and their affiliated accessory components, may enable productive drug screening to identify molecules that function specifically in beige adipocytes, thus harnessing metabolic benefits without unwanted off-target effects.

Outstanding questions.

In addition to ChAMs, are other types of ChAT⁺ immune cells, particularly ChAT-expressing lymphocytes (T cells and B cells), involved in regulating beige fat function within subcutaneous fat depots?

The half-life of acetylcholine is estimated at milliseconds because of rapid hydrolysis by acetylcholinesterase and butyrylcholinesterase. Therefore, the paracrine regulation of CHRNA2 is restricted to the vicinity of ChAT⁺ immune cells. Does the distribution of these ChAT⁺ cells within the heterogeneous adipose tissue change in response to environmental stimuli?

In addition to cold and high calorie intake, many other internal or environmental factors influence adipose tissue function, such as aging and exercise. How is ChAT–CHRNA2 signaling affected by these processes?

Is the ChAT–CHRNA2 axis functionally involved in other major metabolic organs? If so, does the ChAT–CHRNA2 axis drive a distinct metabolic response in each metabolic organ, and how?

Highlights.

Non-neuronal cholinergic signaling via the choline acetyltransferase (ChAT)-cholinergic receptor nicotinic α2 subunit (CHRNA2) axis is present in subcutaneous fat and mediates beige fat function, glucose homeostasis, systemic energy metabolism, and whole-body health.

Functional CHRNA2 signaling in adipose tissue is a unique feature of beige adipocytes in both mice and humans.

CHRNA2 signaling regulates both canonical and glycolytic beige fat function.

Upon cold exposure, acetylcholine secreted from cholinergic macrophages within the subcutaneous fat depot activates CHRNA2 signaling in beige adipocytes.

CHRNA2 signaling mediates thermogenesis through both uncoupling protein 1 (UCP1)- and creatine-dependent pathways.

Glossary

Acetylcholine

an organic chemical that is released by neurons or other non-neuronal cells, including T cells and macrophages. Acetylcholine functions as a neurotransmitter and plays an essential role in regulating nervous system function. Acetylcholine from non-neuronal sources regulates a spectrum of physiological functions, including anti-inflammation and energy homeostasis

β-Adrenergic receptors (β-ARs)

a family of G protein-coupled receptors whose ligands are catecholamines, such as norepinephrine secreted from sympathetic nerves. There are several subtypes of β-ARs, including β1, β2, and β3. In particular, β3-adrenergic receptors are mainly expressed in differentiated thermogenic adipocytes

Calcium imaging assay

a technique that optically measures the concentration of calcium in cultured cells or cells in situ by analyzing fluorescence intensity changes using either chemical indicators (e.g., Fura-2AM) or genetically encoded calcium indicators (GECIs)

Choline acetyltransferase (ChAT)

the rate-limiting enzyme in the synthesis of acetylcholine in neurons and other non-neuronal cells, including T cells and macrophages. ChAT catalyzes the transfer of an acetyl group from acetyl-CoA to choline

Cholinergic receptor nicotinic alpha 2 subunit (CHRNA2)

a subunit of the nicotinic acetylcholine receptor (nAChR) family which forms pentameric, ligand-gated ion channels. CHRNA2-containing ion channels respond to both acetylcholine and nicotine. CHRNA2 signaling in activated beige adipocytes regulates both the thermogenic response and systemic energy homeostasis upon environmental stimuli

Diet-induced thermogenesis (DIT)

a process that restores systemic energy homeostasis by increasing energy expenditure after acute excessive calorie intake. DIT has been shown to occur in adipose tissue through both UCP1-dependent and -independent mechanisms

Thermogenic adipocytes

adipocytes that convert chemical energy into heat through UCP1-dependent or -independent mechanisms upon activation. Broadly, thermogenic adipocytes can be categorized as preformed classical brown adipocytes and inducible beige adipocytes

Uncoupling protein 1 (UCP1)

a mitochondrial inner-membrane protein that is highly expressed in brown adipocytes and activated beige adipocytes. UCP1, when activated by long-chain fatty acids, causes proton leakage and disconnects oxidative phosphorylation from ATP synthesis, thus accelerating fuel oxidation and leading to the dissipation of energy as heat