Karen Koeninger Ryan

Objective A growing literature suggests manipulating dietary protein status decreases sweet consumption in rodents and in humans. Underlying neurocircuit mechanisms have not yet been determined, but previous work points towards hedonic rather than homeostatic pathways. Here we hypothesized that a history of protein restriction reduces sucrose seeking by altering mesolimbic dopamine signaling. Methods We tested this hypothesis using established behavioral tests of palatability and motivation, including the ‘palatability contrast’ and conditioned place preference (CPP) tests. We used modern optical sensors for measuring real-time nucleus accumbens (NAc) dopamine dynamics during sucrose consumption, via fiber photometry, in male C57/Bl6J mice maintained on low-protein high-carbohydrate (LPHC) or control (CON) diet for ∼5 weeks. Results A history of protein restriction decreased the consumption of a sucrose ‘dessert’ in sated mice by ∼50% compared to controls [T-test, p< 0.05]. The dopamine release in NAc during sucrose consumption was reduced, also by ∼50%, in LPHC-fed mice compared to CON [T-test, p< 0.01]. Furthermore, LPHC-feeding blocked the sucrose-conditioned place preference we observed in CON-fed mice [paired T-test, p< 0.05], indicating reduced motivation. This was accompanied by a 33% decrease in neuronal activation of the NAc core, as measured by c-Fos immunolabeling from brains collected directly after the CPP test. Conclusions Despite ongoing efforts to promote healthier dietary habits, adherence to recommendations aimed at reducing the intake of added sugars and processed sweets remains challenging. This study highlights chronic dietary protein restriction as a nutritional intervention that suppresses the motivation for sucrose intake, via blunted sucrose-evoke dopamine release in NAc.

Sex is key variable influencing body composition and substrate utilization. At rest, females maintain greater adiposity than males and resist the mobilization of fat. Males maintain greater lean muscle mass and mobilize fat readily. Determining the mechanisms that direct these sex-dependent effects is important for both reproductive and metabolic health. Here, we highlight the fundamental importance of sex in shaping metabolic physiology and assess growing evidence that the hepatokine fibroblast growth factor-21 (FGF21) plays a mechanistic role to facilitate sex-dependent responses to a changing nutritional environment. First, we examine the importance of sex in modulating body composition and substrate utilization. We summarize new data that point toward sex-biased effects of pharmacologic FGF21 administration on these endpoints. When energy is not limited, metabolic responses to FGF21 mirror broader sex differences; FGF21-treated males conserve lean mass at the expense of increased lipid catabolism, whereas FGF21-treated females conserve fat mass at the expense of reduced lean mass. Next, we examine the importance of sex in modulating the endogenous secretion of FGF21 in response to changing macronutrient and energy availability. During the resting state when energy is not limited, macronutrient imbalance increases the secretion of FGF21 more so in males than females. When energy is limited, the effect of sex on both the secretion of FGF21 and its metabolic actions may be reversed. Altogether, we argue that a growing literature supports FGF21 as a plausible mechanism contributing to the sex-dependent mobilization vs preservation of lipid storage and highlight the need for further research.

Fibroblast growth factor-21 (FGF21) is an intercellular signaling molecule secreted by metabolic organs, including skeletal muscle, in response to intracellular stress. FGF21 crosses the blood brain barrier and acts via the nervous system to coordinate aspects of the adaptive starvation response, including increased lipolysis, gluconeogenesis, fatty acid oxidation, and activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Given its beneficial effects for hepatic lipid metabolism, pharmaceutical FGF21 analogues are in clinical trials treatment of fatty liver disease. We predicted pharmacologic treatment with FGF21 increases HPA axis activity and skeletal muscle glucocorticoid signaling and induces skeletal muscle atrophy in mice. Here we found a short course of systemic FGF21 treatment decreased muscle protein synthesis and reduced tibialis anterior weight; this was driven primarily by its effect in female mice. Similarly, intracerebroventricular FGF21 reduced TA muscle fiber cross sectional area; this was more apparent among female mice compared to male littermates. In agreement with the reduced muscle mass, the topmost enriched metabolic pathways in plasma collected from FGF21-treated females were related to amino acid metabolism, and the relative abundance of plasma proteinogenic amino acids were increased up to three-fold. FGF21 treatment increased hypothalamic Crh mRNA, plasma corticosterone, and adrenal weight, and increased expression of glucocorticoid receptor target genes known to reduce muscle protein synthesis and/or promote degradation. Given the proposed use of FGF21 analogues for the treatment of metabolic disease, the study is both physiologically relevant and may have important clinical implications.

Pharmacological administration of Fibroblast growth factor 21 (FGF21) alters food choice, including that it decreases the consumption of sucrose and other sweet tastants. Conversely, endogenous secretion of FGF21 by the liver is modulated by diet, such that plasma FGF21 is increased after eating foods that have a low dietary protein: total energy (P: E) ratio. Together, these findings suggest a strategy to promote healthy eating, in which the macronutrient content of a pre-load meal could reduce the later consumption of sweet desserts. Here, we tested the prediction that individuals eating a low P: E pre-load meal, and next offered a highly palatable sweet ‘dessert’, would eat less of the sugary snack compared to controls, due to increased FGF21 signaling. In addition to decreasing sweet intake, FGF21 increases the consumption of dietary protein. Thus, we predicted that individuals eating a low protein pre-load meal, and subsequently offered a very high-protein pellet as ‘dessert’ or snack, would eat more of the high protein pellet compared to controls, and that this depends on FGF21. We tested this in C57Bl/6J, and liver-specific FGF21-null (FGF21 ^ΔL ) null male and female mice and littermate controls. Contrary to expectation, eating a low protein pre-load did not reduce the later consumption of a sweet solution in either males or females, despite robustly increasing plasma FGF21. Rather, eating the low protein pre-load increased later consumption of a high protein pellet. This was more apparent among males and was abrogated in the FGF21 ^ΔL mice. We conclude that physiologic induction of hepatic FGF21 by a low protein pre-load is not sufficient to reduce later consumption of sweet dessert, though it effectively increases the subsequent intake of dietary protein in male mice.

Background: Fibroblast growth factor-21 (FGF21) is an intercellular signaling molecule secreted by metabolic organs, including skeletal muscle, in response to intracellular stress. FGF21 crosses the blood brain barrier and acts via the nervous system to coordinate aspects of the adaptive starvation response, including increased lipolysis, gluconeogenesis, hepatic fatty acid oxidation, and activation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Given its beneficial effects for hepatic lipid metabolism, pharmaceutical FGF21 analogues are in clinical trials treatment of fatty liver disease. We predicted pharmacologic treatment with FGF21 in-creases HPA axis activity and skeletal muscle glucocorticoid signaling and induces skeletal muscle atrophy in mice. Methods: We treated male and female mice with FGF21 or saline, delivered either pe-ripherally or directly to the brain, to determine its effect on skeletal muscle. To identify metabolic pathways affected by FGF21, we analyzed untargeted primary metabolites measured in plasma by GCTOF-MS. To determine mechanisms underlying sex-and FGF21-dependent changes in muscle mass, we measured hormonal and molecular mediators of muscle protein synthesis and degradation. We performed stable isotope labeling with deuterium oxide to directly measure muscle protein synthesis. Results: A short course of systemic FGF21 treatment decreased muscle protein synthe-sis ( P < 0.001) and reduced tibialis anterior weight ( P < 0.05); this was driven primarily by its effect in female mice ( P < 0.05). Similarly, intracerebroventricular FGF21 reduced TA muscle fiber cross sectional area ( P < 0.01); this was more apparent among female mice compared to male littermates ( P < 0.05). In agreement with the reduced muscle mass, the topmost enriched meta-bolic pathways in FGF21-treated females were related to amino acid metabolism, and the relative abundance of plasma proteinogenic amino acids were increased up to three-fold ( P < 0.05). FGF21 treatment increased hypothalamic Crh mRNA ( P < 0.01), plasma corticosterone ( P < 0.01), and adrenal weight ( P < 0.05), and increased expression of glucocorticoid receptor target genes known to reduce muscle protein synthesis and/or promote degradation including Foxo1 , Redd1 , and Klf15 (P < 0.05). Again, these changes were driven primarily by effects of FGF21 in females ( P < 0.05). Conclusions: FGF21 increased plasma amino acids and decreased skeletal muscle mass, together with activation of the HPA axis and glucocorticoid receptor target genes in skeletal muscle—and female mice were more sensitive to all these outcomes. Given the proposed use of FGF21 analogues for the treatment of metabolic disease, the study is both physiologically relevant and may have important clinical implications.

The liver regulates energy partitioning and use in a sex-dependent manner, coupling hepatic substrate availability to female reproductive status. Fibroblast growth factor 21 (FGF21) is a hepatokine produced in response to metabolic stress that adaptively directs systemic metabolism and substrate use to reduce hepatic lipid storage. Here we report that FGF21 altered hepatic transcriptional and metabolic responses, and reduced liver triglycerides, in a sex-dependent manner. FGF21 decreased hepatic triglycerides in obese male mice in a weight loss–independent manner; this was abrogated among female littermates. The effect of FGF21 on hepatosteatosis is thought to derive, in part, from increased adiponectin secretion. Accordingly, plasma adiponectin and its upstream adrenergic receptor → cAMP → exchange protein directly activated by cAMP signaling pathway was stimulated by FGF21 in males and inhibited in females. Both ovariectomized and reproductively senescent old females responded to FGF21 treatment by decreasing body weight, but liver triglycerides and adiponectin remained unchanged. Thus, the benefit of FGF21 treatment for improving hepatosteatosis depends on sex but not on a functional female reproductive system. Because FGF21 provides a downstream mechanism contributing to several metabolic interventions, and given its direct clinical importance, these findings may have broad implications for the targeted application of nutritional and pharmacological treatments for metabolic disease.