The dramatic rise in obesity has led to a global increase in diabetes and cardiovascular disease. Excess calories in mammals are stored as triglyceride molecules in white adipose tissue (also known as white fat). By contrast, brown adipose tissue (BAT; brown fat) burns fat and glucose molecules to generate heat in a process called adaptive non-shivering thermogenesis1. Activating BAT thermogenesis is a potential strategy for combating obesity, so there is great interest in understanding the development and physiology of this tissue. Writing in Nature, Zeng et al.2 report that the protein calsyntenin 3β (CLSTN3β), found in brown fat cells, acts as a crucial regulator of BAT innervation and thermogenesis by controlling the secretion of a growth factor, the protein S100b.
A key regulator of BAT thermogenesis is the sympathetic nervous system3, which mediates neuronal and hormonal responses to stress — better known as the fight-or-flight response. Sympathetic nerves innervate BAT much more abundantly than they do white fat, and release noradrenaline molecules that bind to and activate adrenergic receptors on BAT heat-producing cells (brown adipocytes; Fig. 1). Active adrenergic receptors produce heat by triggering biochemical events in mitochondria — the cell’s energy-generating organelles — with the help of a specialized mitochondrial protein, known as uncoupling protein 1 (UCP1).
Zeng et al. set out to investigate the processes that control the innervation of BAT. By analysing all the RNA transcripts produced in brown adipocytes, the authors identified a previously uncharacterized, mammalian-specific gene — Clstn3b — that is highly and selectively expressed in these cells. They observed that the expression of Clstn3b is also induced in beige fat by exposure to cold; beige fat consists of adipocytes in white fat that can be triggered to become thermogenic by cold and other stimuli4.
Strikingly, when the researchers genetically engineered ‘knockout’ mice that lack Clstn3b, the animals rapidly developed hypothermia in response to acute cold, which is indicative of defective thermogenesis. These mice were also heavier than their wild-type counterparts, had increased fat deposits and decreased oxidative activity in brown adipocytes, and became obese and had raised levels of blood glucose (glucose intolerance) on a high-fat diet. Conversely, genetically engineered mice that overexpress Clstn3b in brown adipocytes had lower body weights than wild-type mice and improved cold tolerance, and were resistant to diet-induced obesity and glucose intolerance. Together, these studies indicate that Clstn3b is necessary and sufficient for BAT thermogenesis and whole-body energy expenditure.
How does calsyntenin 3β control BAT function? Despite the altered cold sensitivity of the Clstn3b-knockout mice, the animals’ isolated brown adipocytes showed normal respiratory responses (oxygen consumption in mitochondria) when treated acutely with noradrenaline. Similar results were obtained at the whole-body level when the knockout mice were treated with an adrenergic-receptor activator, indicating that the noradrenaline-responsive machinery is intact in these animals. However, Zeng et al. observed that sympathetic innervation in BAT is affected by the level of expression of Clstn3b: compared with wild-type mice, there was a decrease in the density of sympathetic nerve fibres in Clstn3b-knockout mice, but an increase in Clstn3b-overexpressing mice.
The authors observed that calsyntenin 3β is found in the membrane of the endoplasmic reticulum (ER) of brown adipocytes; the ER is an intracellular organelle in which many secreted and membrane proteins are synthesized. So how does it influence BAT innervation? The researchers found that the protein that is most strongly downregulated in the BAT of Clstn3b-knockout mice is S100b, which is abundant in brain cells called astrocytes5. Intriguingly, Zeng et al. find that calsyntenin 3β interacts physically with S100b, and thereby acts as a chaperone protein that directs and aids the secretion of newly formed S100b from the ER.
The authors found that when they treated cultured sympathetic neurons with soluble S100b, extension of nerve projections was promoted. Moreover, BAT innervation in mice lacking S100b was reduced compared with that in wild-type mice, whereas innervation of other organs, such as the salivary glands, was unaffected. Remarkably, forced expression of S100b in the brown adipocytes of Clstn3b-knockout mice is sufficient to correct the deficits in sympathetic innervation and thermogenesis in these animals. The emerging picture is of an adipocyte-derived mechanism that mediates crosstalk between brown adipocytes and innervating sympathetic fibres (Fig. 1).
Zeng and colleagues’ findings raise several questions. For example, how is calsyntenin 3β expression regulated in brown adipocytes? The authors found that one upstream regulator of this protein is the enzyme lysine-specific demethylase 1 (LSD1), which mediates the differentiation of brown and beige fat. Adipose-specific loss of LSD1 in mice causes downregulation of BAT-specific genes and aberrant induction of genes typically found in white adipose tissue6.
The growth of sympathetic neurons mostly relies on a protein called nerve growth factor (NGF). NGF-deficient mice have pronounced disruption of sympathetic innervation of multiple peripheral tissues7, but BAT innervation has not been specifically examined in this context. It remains to be seen whether NGF acts together with S100b to coordinate BAT innervation, or is dispensable, as in the innervation of the trachea7. Could other factors derived from peripheral tissue control the sympathetic innervation of specific targets? If so, they could be useful tools for manipulating such innervation and studying its role in individual tissues.
Sympathetic innervation controls the expression of genes involved in thermogenesis and the differentiation of brown and beige fat4. However, Zeng et al. found that loss of Clstn3b does not affect the expression of genes involved in adipocyte development, thermogenesis or mitochondrial function, including the gene that encodes UCP1. Only the expression of the Dio2 gene, which encodes an enzyme that produces the active form of a thyroid hormone, is significantly altered by the loss or overexpression of Clstn3b. The Dio2 protein is abundant in BAT, and, as is seen in the Clstn3b-knockout mice, mice lacking Dio2 have impaired thermogenesis and decreased cold tolerance, and are susceptible to diet-induced obesity8. Altered thyroid signalling might therefore contribute to BAT dysfunction in the absence of calsyntenin 3β.
The final issue arising from Zeng and colleagues’ work is how calsyntenin 3β promotes the secretion of S100b. The synthesis of proteins destined for cellular secretion is initiated in the cytoplasm. The nascent proteins typically contain an amino-acid sequence known as a signal peptide, which directs them from the cytoplasm to the ER for the start of their secretory journey while they are still being synthesized. However, some secreted proteins lack signal peptides and are imported to the ER after they have been synthesized9. S100b lacks a peptide signal, and so the new findings raise the question of whether calsyntenin 3β functions as a general ER chaperone for the secretion of proteins that lack signal peptides, or is specific for S100b.
In humans, BAT had been thought to exist only in infants, until imaging studies revealed deposits of thermogenic brown fat in adults10. Clstn3b is expressed in human adipose tissue, and might be involved in the regulation of BAT innervation, as it is in mice. Zeng and colleagues’ study might therefore inform therapeutic strategies to enhance sympathetic innervation, and thereby harness the thermogenic potential of BAT to combat obesity and its metabolic consequences.
Nature 569, 196-197 (2019)