GFRAL mediates metabolic responses to mitochondrial stress in brown fat

In a discovery that reshapes how scientists understand the conversation between stressed fat tissue and the brain, researchers at the University of Iowa have demonstrated that a little-known brainstem receptor called GFRAL is essential for some of the most striking metabolic benefits that arise when brown fat mitochondria are under stress. The study, published in the Journal of Molecular Medicine, reveals a previously unrecognized signaling axis running from brown adipose tissue, through the blood-borne cytokine GDF15, to GFRAL-expressing neurons in the hindbrain, and it shows that this pathway partially protects mice from obesity while safeguarding their ability to maintain body temperature in the cold.

The work centers on OPA1, a mitochondrial fusion protein whose loss in brown adipocytes triggers a cascade of stress responses. In earlier studies, the group led by Renata O. Pereira found that deleting OPA1 selectively in brown fat activates the transcription factor ATF4, which in turn ramps up production of two secreted signaling molecules: fibroblast growth factor 21 (FGF21) and growth differentiation factor 15 (GDF15). Both act as so-called batokines, hormones released by brown adipose tissue that travel through the circulation to influence metabolism elsewhere in the body. Mice lacking OPA1 in brown fat resist diet-induced obesity, burn more energy, and tolerate cold exposure remarkably well. But a key question remained open: which of these effects actually require GDF15 to act on its known receptor, GFRAL?

GDF15 is a divergent member of the TGF-β superfamily, a cytokine produced by nearly every cell type in response to stress. Its concentrations rise in the blood during aging, pregnancy, cancer, obesity, and cardiovascular disease, and its reputation as a regulator of appetite and body weight has made it one of the most closely watched molecules in metabolic research. In 2017, several research groups independently identified GFRAL, the GDNF family receptor α-like protein, as the receptor responsible for GDF15’s effects. GFRAL is expressed almost exclusively in discrete regions of the hindbrain, including the area postrema and the nucleus of the solitary tract, where GDF15 binding activates the GDF15-GFRAL-RET complex and suppresses food intake while increasing energy expenditure.

To test whether this brainstem pathway was truly responsible for the metabolic rewiring seen in the brown-fat stress model, the Iowa team, including first author Ayushi Sood, crossed OPA1 brown-fat knockout mice with mice lacking GFRAL throughout the body. The resulting double knockouts, which the researchers call DKO mice, lacked both the mitochondrial fusion protein in brown adipocytes and the GDF15 receptor in the hindbrain. The experiment provided a clean genetic test of whether GFRAL-dependent signaling was necessary for the improved metabolism previously attributed to GDF15.

At the molecular level, the cross worked exactly as designed. OPA1 mRNA was markedly reduced in brown fat of both the single and double knockouts, while Gfral mRNA was depleted from the hindbrain of mice carrying the GFRAL deletion. Importantly, deleting GFRAL did not interfere with the upstream induction of the stress response: Fgf21 and Gdf15 mRNA levels in brown fat rose in both OPA1 BKO and DKO mice, and circulating GDF15 concentrations were equally elevated in both groups compared with controls. Thermogenic gene expression in brown fat remained impaired in the double knockouts just as in the single mutants, confirming that GFRAL signaling is not required for the induction of GDF15 secretion or for the local thermogenic changes within brown fat itself.

Under ordinary chow feeding, the receptor turned out to be largely irrelevant to the baseline benefits of the OPA1 deletion. Six-week-old DKO mice weighed the same as their wild-type, OPA1 BKO, and GFRAL KO littermates, with comparable fat mass, lean mass, glucose tolerance, and fasting glucose. Moreover, the compensatory browning of inguinal white adipose tissue, in which white fat depots acquire thermogenic properties and begin expressing the uncoupling protein UCP1, proceeded undiminished in DKO mice. Levels of UCP1 protein in their white fat were elevated to the same degree as in OPA1 BKO animals. This finding dovetails with earlier work showing that the browning response under baseline conditions is driven primarily by FGF21 rather than GDF15, and it establishes that GFRAL signaling is dispensable for this particular adaptation.

The picture changed dramatically when the mice were challenged with a high-fat diet. Over twelve weeks of feeding a diet in which 60 percent of calories came from fat, OPA1 BKO mice once again resisted weight gain, staying leaner than all other groups. GFRAL knockout mice, lacking the stress context, gained weight just like wild-type controls. The DKO animals displayed an intermediate phenotype: they weighed significantly less than wild-type mice at the end of the diet and accumulated less total fat, yet they were substantially heavier and fatter than the OPA1 BKO mice. Fat depots including inguinal and gonadal white adipose tissue told the same story, with DKO mice falling between the two extremes. Indirect calorimetry added a crucial detail: energy expenditure, oxygen consumption, carbon dioxide production, and food intake were statistically indistinguishable across the genotypes, indicating that the partial loss of obesity resistance in DKO mice was not explained by altered caloric intake. The investigators also observed a trend toward attenuated browning of white fat in DKO mice under obesogenic conditions, suggesting that GDF15 acting through GFRAL helps sustain the UCP1-mediated energy expenditure that keeps OPA1 BKO mice lean on a fatty diet.

Even more striking was the dissociation between body weight and metabolic health. Although DKO mice accumulated less fat than wild-type controls, they failed to gain the glucose and liver benefits that make the OPA1 BKO phenotype so remarkable. Glucose tolerance tests and insulin tolerance tests showed that DKO mice were no better than wild-type animals, and their fasting glucose and circulating insulin levels were similarly unimproved. Liver triglyceride content, which is reduced in OPA1 BKO mice, remained at wild-type levels in the double knockouts. This finding implies that GFRAL-mediated signaling may have direct effects on glucose homeostasis, insulin sensitivity, and hepatic lipid metabolism that are independent of the amount of fat the animals carry, although the authors caution that the greater fat mass in DKO mice could itself be sufficient to blunt these improvements. Notably, GFRAL deletion alone produced no changes in body weight, food intake, glucose handling, or liver triglycerides under these experimental conditions, contrasting with some earlier reports and likely reflecting differences in genetic background and diet protocols.

The cold-exposure experiments delivered perhaps the most dramatic result. When mice adapted to 30 degrees Celsius were suddenly shifted to 4 degrees, DKO animals became severely hypothermic, losing the enhanced cold tolerance that characterizes OPA1 BKO mice, while GFRAL knockout mice maintained their core body temperature as well as wild-type controls. This suggests that GFRAL signaling becomes critical for thermoregulation only in the context of mitochondrial stress in brown fat. Surprisingly, the molecular analysis did not reveal an obvious culprit: thermogenic gene expression and UCP1 protein levels in both brown and white fat were similarly altered in OPA1 BKO and DKO mice after cold exposure, and sympathetic activation, estimated by tyrosine hydroxylase levels, did not differ between the two stressed groups. The researchers speculate that GFRAL-dependent, UCP1-independent heat production, such as the futile calcium cycling in skeletal muscle recently linked to GDF15 signaling, may underlie the hypothermia phenotype, though this remains to be tested.

The broader implications are considerable. GDF15 has attracted intense pharmaceutical interest as a potential anti-obesity therapy, and long-acting formulations of the cytokine have shown weight-loss effects extending from mice to monkeys. But a full picture of GFRAL-dependent versus GFRAL-independent actions of GDF15 is essential for designing such therapies, and the new data significantly expand the known reach of the GFRAL pathway. Beyond appetite suppression in the hindbrain, the receptor now appears to modulate energy expenditure, white fat browning, glucose homeostasis, liver lipid handling, and cold-induced thermoregulation, at least in the setting of brown-fat mitochondrial stress. The authors also note an important caveat: because GFRAL was deleted throughout development, it remains possible that remodeling of thermoregulatory neural circuits contributes to the phenotypes observed.

Taken together, the study defines a novel BAT-GDF15-GFRAL axis, a three-node communication system in which stressed brown fat secretes GDF15 as an endocrine signal, the hindbrain receives it through GFRAL, and the brain then orchestrates systemic adaptations that resist obesity and defend body temperature. The discovery that different stress contexts engage this pathway differently, with physiological stress leaving thermoregulation intact while mitochondrial stress makes it indispensable, suggests a layer of regulatory specificity that could be exploited therapeutically. As obesity and its comorbidities continue their global rise, understanding precisely how a signal from burning fat reaches the brainstem and reshapes whole-body metabolism may prove to be more than an elegant piece of physiology; it may point the way toward the next generation of metabolic medicines.