FGF 21 Mouse
Description
Introduction to Fibroblast Growth Factor 21
Fibroblast growth factor 21 (FGF21) is a member of the fibroblast growth factor family that functions as an endocrine hormone rather than a traditional growth factor. Unlike classical FGFs, FGF21 does not promote cell proliferation, making it unique within its family. The function of FGF21 remained unknown until 2005 when it was identified as a novel metabolic regulator with potential anti-diabetic properties. Researchers discovered that FGF21 is a potent activator of glucose uptake in adipocytes and protects animals from diet-induced obesity when overexpressed in transgenic mice .
Normal physiological FGF21 levels in mice vary considerably depending on strain, age, nutritional status, and time of day. In published studies, serum FGF21 concentrations in chow-fed mice range from undetectable to 3000 ng/mL, reflecting significant variability across experimental conditions . Under standard conditions, FGF21 concentrations in mouse biological samples have been measured as follows:
| Sample Type | Average Concentration |
|---|---|
| Serum | 409 pg/mL |
| Plasma (citrate) | 397 pg/mL |
| Plasma (EDTA) | 358 pg/mL |
| Plasma (heparin) | 348 pg/mL |
| L-929 cell culture supernatant | 59 pg/mL |
These values increase significantly during certain physiological states. For example, during pregnancy (day 18 of gestation), serum FGF21 levels can rise dramatically to 9,193 pg/mL compared to 401 pg/mL in non-pregnant mice .
Signaling Pathways
In adipocytes, FGF21 activates AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1), leading to enhanced mitochondrial oxidative function. This activation requires serine/threonine kinase 11 (STK11/LKB1), which directly phosphorylates Thr-172 of AMPK and activates its kinase activity .
The molecular pathway proceeds as follows: FGF21 treatment increases cellular NAD+ levels, leading to activation of SIRT1 and subsequent deacetylation of downstream targets, including peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) and histone 3. This activation enhances mitochondrial oxidative capacity as demonstrated by increases in oxygen consumption, citrate synthase activity, and induction of key metabolic genes .
Inhibition of AMPK, SIRT1, and PGC-1α activities attenuates the effects of FGF21 on oxygen consumption and gene expression, indicating that FGF21 regulates mitochondrial activity through an AMPK–SIRT1–PGC1α–dependent mechanism in adipocytes .
Regulation of Expression
The expression of FGF21 in mice is regulated by various factors, particularly nutritional status. In the liver, FGF21 expression is primarily regulated by:
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The peroxisome proliferator-activated receptor-α (PPARα) pathway, which is activated by free fatty acids (FFAs) and/or protein insufficiency
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The cyclic adenosine monophosphate (EPAC)/protein kinase A (PKA) pathway, which is activated by a signaling cascade stimulated by the hepatic glucagon receptor
FGF21 has a remarkably short half-life in mouse circulation, estimated to be only 1-2 hours. This rapid turnover is due to enzymatic degradation by fibroblast activation protein α (FAP), a serine protease that cleaves and inactivates FGF21, as well as renal clearance . This short half-life must be considered when designing experiments or therapeutic approaches involving FGF21 administration.
Glucose Metabolism
FGF21 plays a crucial role in regulating glucose metabolism in mice. Administration of recombinant FGF21 protein to rodents with diet-induced or genetic obesity and diabetes exerts strong antihyperglycemic effects . In mouse 3T3-L1 adipocytes, FGF21 treatment significantly increases glucose uptake by inducing the expression of glucose transporter GLUT1 but not GLUT4 .
The glucose-lowering effects of FGF21 are particularly striking in diabetic mouse models. When administered to ob/ob mice for 7 days, FGF21 normalized fed glucose levels in a dose-dependent manner. Similar glucose-lowering effects were observed in db/db mice and ZDF rats . Importantly, a single dose of FGF21 administered to obese mice can rapidly increase insulin sensitivity and reduce blood glucose levels by more than 50% .
The mechanism of FGF21's acute insulin-sensitizing effects involves direct signaling to brown adipocytes. In contrast, long-term administration of FGF21 increases energy expenditure and results in weight loss, thereby increasing insulin sensitivity indirectly .
Lipid Metabolism
FGF21 exerts potent effects on lipid metabolism in mice. Administration of FGF21 to obese mice significantly reduces plasma triglyceride levels in a dose-dependent manner . During fasting or protein restriction, FGF21 mediates the adaptive response by inducing ketogenesis, gluconeogenesis, lipolysis, and lipid β-oxidation .
In the liver, FGF21 regulates the expression of lipogenic genes. Studies have shown that protein restriction reduces the expression of liver lipogenic genes (Scd1, Srebp1, Fas), and this effect is maintained in FGF21-knockout mice, suggesting parallel pathways .
Energy Homeostasis
FGF21 is a key regulator of energy homeostasis in mice. It enhances mitochondrial oxidative capacity in adipocytes, as demonstrated by increases in oxygen consumption, citrate synthase activity, and induction of key metabolic genes . This effect is mediated through the AMPK–SIRT1–PGC1α pathway.
FGF21 transgenic mice display remarkable resistance to diet-induced obesity. When challenged with a high-fat/high-carbohydrate diet for 15 weeks, these mice consumed almost twice as much food as wild-type littermates when normalized to body weight, yet they did not gain as much weight and were resistant to obesity. They also exhibited lower fasted glucose levels, less fat in the liver, retained more brown adipose tissue, and had smaller subcutaneous adipocytes .
Protein Restriction and Longevity
FGF21 plays a critical role in mediating the beneficial effects of protein restriction in mice. Protein restriction increases lifespan, reduces frailty, lowers body weight and adiposity, improves physical performance, and improves glucose tolerance in wild-type mice .
Remarkably, these beneficial effects of protein restriction are entirely dependent on FGF21. Mice lacking FGF21 (Fgf21 KO) not only fail to exhibit the health benefits of protein restriction but actually show detrimental effects, including early-onset weight loss, increased frailty, and reduced lifespan when fed a low-protein diet .
The liver mRNA expression of FGF21 is persistently increased for the full duration of protein restriction, suggesting a sustained response. Interestingly, protein restriction also significantly reduces circulating IGF-1 levels in both control and Fgf21 KO mice, indicating that this well-known effect of protein restriction is not FGF21-dependent .
Cardiac Protection
FGF21 serves an important protective function in the heart. Fgf21−/− mice exhibit an increased relative heart weight and develop enhanced signs of dilatation and cardiac dysfunction in response to isoproterenol infusion, indicating eccentric hypertrophy development .
In addition, Fgf21−/− mice show enhanced induction of cardiac hypertrophy markers and pro-inflammatory pathways and greater repression of fatty acid oxidation. Many of these alterations are present in Fgf21−/− mice from the neonatal stage, suggesting that FGF21 plays a role in normal cardiac development and function .
FGF21 is expressed in the heart and released by cardiomyocytes. This locally produced FGF21 protects cardiac cells against hypertrophic insults, suggesting both endocrine and autocrine/paracrine roles for FGF21 in cardiac protection .
FGF21 Knockout Mice
FGF21 knockout (Fgf21−/−) mice have been instrumental in understanding the physiological roles of FGF21. These mice exhibit several phenotypic characteristics:
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Cardiac abnormalities: Fgf21−/− mice show increased relative heart weight and enhanced susceptibility to cardiac dysfunction when challenged with hypertrophic stimuli .
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Metabolic vulnerability during protein restriction: When fed a low-protein diet, Fgf21−/− mice exhibit early-onset weight loss, increased frailty, and reduced lifespan compared to wild-type mice on the same diet. Specific markers of frailty in these mice include alopecia, poor coat condition, and kyphosis .
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Altered IGF-1 signaling: Fgf21−/− mice have increased circulating IGF-1 levels regardless of diet, suggesting an interaction between FGF21 and the growth hormone-IGF-1 axis .
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Normal response to fasting: Despite initial hypotheses that FGF21 might be required for fasting adaptation, Fgf21−/− mice do not exhibit an impaired response to fasting, suggesting redundant or compensatory mechanisms .
FGF21 Transgenic Mice
FGF21 transgenic mice that overexpress human FGF21 from the liver using the apoE promoter provide valuable insights into the potential benefits of increased FGF21 signaling:
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Metabolic health: FGF21 transgenic mice exhibit improved glucose clearance and insulin sensitivity relative to control littermates. At 9 months of age, these mice have lower fasted glucose levels compared to wild-type mice, suggesting protection against age-related metabolic decline .
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Resistance to obesity: When challenged with a high-fat/high-carbohydrate diet for 15 weeks, FGF21 transgenic mice are resistant to diet-induced weight gain and fat accumulation, despite consuming almost twice as much food when normalized to body weight .
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Body composition: FGF21 transgenic mice have less fat in the liver, retain more brown adipose tissue, and have smaller subcutaneous adipocytes compared to wild-type mice .
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Absence of proliferative effects: Unlike mice overexpressing other FGF family members, FGF21 transgenic mice do not develop liver tumors or show evidence of any tissue hyperplasia up to 10 months of age, confirming the non-mitogenic nature of FGF21 .
Mouse Disease Models
Various mouse disease models have been used to study the therapeutic potential of FGF21:
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ob/ob and db/db mice: Administration of FGF21 to these genetic models of obesity and diabetes for 7 days normalized fed glucose levels and reduced plasma triglycerides in a dose-dependent manner .
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Diet-induced obesity models: Mice with diet-induced obesity show improved metabolic parameters and weight loss when treated with FGF21 .
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Isoproterenol-induced cardiac hypertrophy: FGF21 treatment protects against cardiac hypertrophy in mice challenged with isoproterenol, a model of pathological cardiac remodeling .
ELISA Assays for Mouse FGF21
Several ELISA (Enzyme-Linked Immunosorbent Assay) kits are available for the specific measurement of mouse FGF21 levels in various samples:
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SimpleStep ELISA kits: These provide a sensitive method for quantifying mouse FGF21 with a detection limit of 1.47 pg/mL. The assay uses a simple Mix-Wash-Read protocol with just one incubation and wash step, providing results in 90 minutes .
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Colorimetric assays: These offer a 450nm readout that works on any plate reader and provide reliable quantification of mouse FGF21 in various sample types .
The recovery rates for mouse FGF21 ELISA across different sample types show excellent reliability:
| Sample Type | Recovery Range |
|---|---|
| Cell culture supernatant | 91 - 97% |
| Serum | 99 - 107% |
| EDTA Plasma | 92 - 108% |
| Heparin Plasma | 88 - 104% |
| Citrate plasma | 94 - 106% |
Physiological Variations in FGF21 Levels
FGF21 levels in mice exhibit significant variations under different physiological conditions:
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Fasting: FGF21 levels increase during fasting in mice, with the protein serving as a starvation-induced factor that regulates fuel utilization .
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Pregnancy: FGF21 levels are dramatically increased during pregnancy in mice, with levels rising from approximately 401 pg/mL in normal mouse serum to 9,193 pg/mL in pregnant mouse serum at day 18 of gestation .
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Diet: Different diets significantly affect FGF21 levels. Protein restriction particularly increases hepatic FGF21 production, while high-fat diets can also elevate FGF21 levels in mice .
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Age: Baseline FGF21 levels and response to metabolic challenges can vary with age, with older mice showing different patterns of FGF21 expression compared to younger animals .
Metabolic Disease Treatment
FGF21 shows remarkable therapeutic potential in mouse models of metabolic disease:
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Diabetes treatment: Administration of FGF21 to diabetic ob/ob mice for 7 days normalized blood glucose levels and reduced plasma triglycerides in a dose-dependent manner. Similar glucose-lowering effects were observed in db/db mice and ZDF rats .
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Obesity management: FGF21 treatment or overexpression protects against diet-induced obesity in mice. Transgenic mice overexpressing FGF21 are resistant to weight gain even when consuming more calories than wild-type mice .
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Insulin resistance amelioration: FGF21 improves insulin sensitivity in obese and diabetic mouse models. A single dose can rapidly increase insulin sensitivity in obese mice, while long-term administration leads to sustained improvements .
The therapeutic effects of FGF21 in these models persist for at least 24 hours following the cessation of administration, indicating a prolonged pharmacological action despite its short half-life .
Lifespan Extension
FGF21 plays a critical role in mediating the lifespan-extending effects of protein restriction in mice:
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Wild-type mice on protein restriction show increased lifespan, reduced frailty, improved physical performance, and improved glucose tolerance .
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These beneficial effects are entirely lost in FGF21-knockout mice, which instead show increased frailty, early-onset weight loss, and reduced lifespan when fed a low-protein diet .
This demonstrates that FGF21 is essential for the pro-longevity effects of protein restriction and highlights its potential as a longevity-promoting factor.
Cardiac Protection
FGF21 has significant cardioprotective effects in mice:
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FGF21-knockout mice develop enhanced cardiac hypertrophy and dysfunction in response to hypertrophic stimuli .
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Treatment with FGF21 can reverse cardiac hypertrophy both in vivo and in cultured cardiomyocytes .
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The heart appears to be both a source and target of FGF21, suggesting both endocrine and autocrine/paracrine mechanisms of cardiac protection .
Pharmacokinetic Challenges
Despite its therapeutic promise, FGF21 faces several limitations as a potential therapeutic agent:
Unresolved Questions
Several aspects of FGF21 biology in mice remain to be fully elucidated:
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Tissue-specific roles: While liver-derived FGF21 is well-studied, the specific functions of FGF21 produced by other tissues (heart, adipose tissue, brain) require further investigation.
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Developmental roles: The observation that FGF21-knockout mice show cardiac alterations from the neonatal stage suggests potential developmental functions that remain unexplored.
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Interaction with other hormonal systems: The relationship between FGF21 and other metabolic hormones (insulin, leptin, adiponectin) requires further study to understand the integrated hormonal regulation of metabolism.
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Translation to human applications: While FGF21 shows promising effects in mouse models, additional research is needed to determine how these findings translate to humans and to develop improved FGF21-based therapeutics.
Product Specs
The FGF family consists of over 20 small (~17-26 kDa) secreted peptides, originally recognized for their ability to stimulate fibroblast proliferation. This mitogenic activity was found to be mediated through FGF receptors (FGFRs) 1, 2, or 3. A fourth related tyrosine kinase receptor, FGFR4, displayed binding affinity for FGFs but did not elicit a mitogenic response.
FGFs exert their cellular effects through at least five distinct subfamilies of high-affinity FGFRs: FGFR-1, -2, -3, and -4, all possessing intrinsic tyrosine kinase activity (with multiple splice isoforms except for FGFR-4), and FGFR-5, which lacks an intracellular kinase domain. Emerging evidence suggests the importance of FGFRs in regulating glucose and lipid homeostasis. Overexpression of a dominant-negative FGFR-1 in pancreatic beta cells has been linked to diabetes in mice, implying a crucial role for FGF signaling in normal beta cell function and blood sugar regulation. FGFR-2 appears vital during pancreatic development. Furthermore, FGFR-4 has been implicated in cholesterol metabolism and bile acid synthesis.
Studies indicate that FGF-19 promotes resistance to diet-induced obesity, improves glucose tolerance, and enhances lipid profiles in diabetic rodents. Given that these effects are partly mediated through observed changes in metabolic rates, FGF-19 can be regarded as a regulator of energy expenditure.
While FGF-21 exhibits preferential expression in the liver, a comprehensive understanding of its bioactivity and mechanism of action has been limited. Notably, FGF-21 acts as a potent activator of glucose uptake in adipocytes, offering protection against diet-induced obesity when overexpressed in transgenic mice. Moreover, therapeutic administration of FGF-21 to diabetic rodents has been shown to reduce blood glucose and triglyceride levels.
Q&A
What is FGF21 and what are its primary functions in mice?
FGF21 is a hormone primarily secreted by the liver during fasting that elicits diverse aspects of the adaptive starvation response. In mice, FGF21 induces hepatic fatty acid oxidation and ketogenesis, increases insulin sensitivity, blocks somatic growth, and causes bone loss . It functions as a potent metabolic regulator that coordinates multiple physiological processes in response to nutritional challenges.
FGF21 requires the co-receptor β-Klotho (Klb) for its signaling activity, particularly in the brain and adipose tissue. Its expression is highly responsive to nutritional status, with fasting being a primary inducer of hepatic FGF21 production .
How does FGF21 affect lifespan in mouse models?
Transgenic mice overexpressing FGF21 show markedly extended lifespan compared to wild-type littermates. Research has demonstrated that FGF21 transgenic mice exhibit approximately 30% increase in median survival time in males and 40% in females without requiring caloric restriction. This lifespan extension is comparable to that achieved by caloric restriction .
The mechanism appears to involve blunting of the growth hormone/insulin-like growth factor-1 (GH/IGF-1) signaling pathway in the liver. FGF21-Tg mice share phenotypic similarities with long-lived dwarf mice, including small size, reduced circulating insulin and IGF-1 concentrations, increased adiponectin levels, and female infertility .
How does FGF21 influence glucose and lipid metabolism in mice?
FGF21 induces profound improvements in glucose and lipid metabolism in mouse models:
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Transgenic mice overexpressing FGF21 exhibit dramatically enhanced insulin sensitivity, with increased glucose infusion rates during hyperinsulinemic-euglycemic clamp studies .
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FGF21 treatment significantly enhances insulin-stimulated suppression of hepatic glucose production and activation of whole-body glucose disposal .
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Administration of FGF21 to diabetic ob/ob and db/db mice reduces plasma glucose and triglyceride levels to near normal, with effects persisting for at least 24 hours after cessation of treatment .
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FGF21 regulates energy homeostasis in adipocytes through activation of AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1), resulting in enhanced mitochondrial oxidative function .
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Younger FGF21-Tg mice (<8 months old) show significant decreases in serum insulin, IGF-1, glucose, triglycerides, cholesterol, and hepatic triglyceride levels .
How does FGF21 affect macronutrient preference in mice?
FGF21 plays a critical role in regulating macronutrient preference, particularly protein intake:
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Systemic FGF21 treatment significantly increases protein intake and decreases carbohydrate intake without altering fat or total food intake in macronutrient choice tests .
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When either fat or carbohydrate is fixed in diet choice experiments, FGF21 significantly increases consumption of protein-rich diets .
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FGF21 signaling in the brain is necessary for mice to exhibit adaptive increases in protein intake during protein restriction. This physiological shift in macronutrient preference is lost in mice lacking either FGF21 or β-Klotho in the brain .
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Direct administration of FGF21 into the brain is sufficient to shift preference toward higher-protein diets, demonstrating that central FGF21 action mediates this effect .
What molecular mechanisms mediate FGF21's effects on metabolism and longevity?
FGF21 activates multiple signaling pathways that contribute to its metabolic and longevity effects:
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GH/IGF-1 Signaling Pathway: Transcriptomic analysis suggests that FGF21 acts primarily by blunting the growth hormone/insulin-like growth factor-1 signaling pathway in liver. This mechanism parallels that seen in other long-lived mouse models, including pituitary loss-of-function Ames and Snell strains and GH receptor/binding protein-knockout mice .
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AMPK and SIRT1 Activation: FGF21 regulates energy homeostasis in adipocytes through activation of AMP-activated protein kinase (AMPK) and sirtuin 1 (SIRT1), enhancing mitochondrial oxidative function. AMPK phosphorylation levels increase with FGF21 treatment in adipocytes and white adipose tissue .
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β-Klotho-Dependent Central Effects: FGF21 requires the co-receptor β-Klotho for its actions, particularly in the brain where it affects macronutrient preference. This brain-specific signaling is essential for the protein preference phenotype observed with FGF21 administration .
Interestingly, FGF21's lifespan extension effects appear to be independent of three pathways typically associated with longevity: NAD+ metabolism, AMP kinase, and mTOR signaling, suggesting a novel mechanism for extending lifespan .
How does exercise influence FGF21 expression and function in mice?
Acute exercise has significant effects on FGF21 expression and function in mice:
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A single bout of acute exercise increases serum FGF21 levels in mice .
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Exercise-induced FGF21 elevation is accompanied by increased serum levels of free fatty acids, glycerol, and ketone bodies, suggesting a relationship with exercise-mediated alterations in energy substrate utilization .
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FGF21 expression increases in skeletal muscle under certain conditions, such as in muscle-specific Akt1 transgenic mice that exhibit protection from high-fat diet-induced obesity and insulin resistance .
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Muscle-derived FGF21 may function as a myokine that contributes to improvements in obesity and insulin resistance, as observed in muscle-specific Atg7-deficient mice fed high-fat diet compared to control mice .
This exercise-induced regulation of FGF21 represents an important physiological context that differs from fasting-induced FGF21 and may contribute to the metabolic benefits of physical activity.
What are the potential adverse effects of chronic FGF21 elevation in mice?
Despite its beneficial effects on metabolism and lifespan, chronic FGF21 elevation in mice is associated with several adverse effects:
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Bone Loss: FGF21 transgenic mice and mice treated with recombinant FGF21 show reduced bone mass. This is caused in part by increased differentiation of marrow adipocytes and a corresponding decrease in osteoblast differentiation .
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Growth Inhibition: FGF21 inhibits GH action directly at the growth plate and causes GH resistance, resulting in smaller mice with blocked somatic growth .
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Female Infertility: Female FGF21 transgenic mice exhibit infertility, which shares similarities with other long-lived dwarf mouse models .
These adverse effects represent important considerations for potential therapeutic applications of FGF21. Any clinical development would need to address these issues, particularly bone loss, which could limit its use in humans .
How do genetic models of obesity and diabetes respond to FGF21 treatment?
Genetic models of obesity and diabetes show remarkable responses to FGF21 treatment:
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ob/ob Mice: Administration of FGF21 to ob/ob mice (125 or 750 μg/kg/day for 7 days) significantly lowers blood glucose, with normalization of fed glucose levels after 7 days of treatment. Plasma triglyceride levels also show dose-dependent reduction .
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db/db Mice: Similar glucose-lowering effects are observed in db/db mice treated with FGF21, demonstrating efficacy across different genetic models of diabetes .
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Dose-Response Relationship: Both low (125 μg/kg/day) and high (750 μg/kg/day) doses of FGF21 effectively normalize glucose levels in ob/ob mice, though triglyceride reduction shows dose-dependency .
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Persistent Effects: The glucose-lowering effects of FGF21 persist for at least 24 hours following cessation of administration, indicating sustained metabolic improvements .
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Safety Profile: Importantly, FGF21 does not induce mitogenicity, hypoglycemia, or weight gain at any tested dose in diabetic or healthy animals, suggesting a favorable therapeutic profile .
What are the optimal methods for measuring FGF21 levels and activity in mouse models?
Accurate assessment of FGF21 biology in mice requires careful consideration of measurement techniques:
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Circulating FGF21 Measurement:
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Enzyme-linked immunosorbent assay (ELISA) is the standard method for quantifying FGF21 protein in serum/plasma
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Sample collection timing is critical as FGF21 levels fluctuate with feeding/fasting cycles
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For consistent results, standardize fasting duration (typically 16-24 hours) before collection
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Include appropriate controls for nutritional status and diurnal variations
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Tissue Expression Analysis:
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Quantitative PCR (qPCR) for measuring FGF21 mRNA expression in liver, adipose tissue, and other relevant tissues
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Western blotting for tissue-specific protein expression
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Immunohistochemistry for localization studies
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FGF21 Signaling Assessment:
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Metabolic Phenotyping:
Combining multiple measurement approaches provides the most comprehensive assessment of FGF21 biology in experimental settings.
How should researchers design FGF21 transgenic and knockout mouse experiments?
Effective genetic manipulation studies of FGF21 in mice require careful experimental design:
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Transgenic Overexpression Models:
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Tissue-specific promoters are crucial for targeting FGF21 expression to relevant tissues
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The apoE promoter has been successfully used for hepatocyte-specific FGF21 expression
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The albumin promoter provides an alternative for liver-specific expression
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Include wild-type littermate controls to account for genetic background effects
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Verify transgene expression levels and circulating FGF21 concentrations (typically 5-10 fold higher than fasting levels in FGF21-Tg mice)
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Knockout Models:
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Global FGF21 knockout mice help establish the necessity of endogenous FGF21
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Tissue-specific knockouts using Cre-loxP systems can differentiate source-specific effects
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Conditional knockouts allow temporal control of FGF21 deletion
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Careful phenotyping across multiple metabolic parameters is essential
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Co-receptor Studies:
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Experimental Controls:
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Include age-matched, sex-matched wild-type controls
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Consider both heterozygous and homozygous transgenic lines to assess dose effects
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Perform comprehensive phenotyping at multiple ages to capture developmental and age-related effects
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Longevity Studies:
What protocols are most effective for administering recombinant FGF21 in mouse experiments?
Optimal administration of recombinant FGF21 in mice requires consideration of several key parameters:
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Dosing Regimens:
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Effective doses typically range from 125-750 μg/kg/day for metabolic studies
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Single daily subcutaneous injection is commonly used and effective
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Administration for 7 days is sufficient to observe significant glucose and triglyceride-lowering effects in diabetic models
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Longer-term studies may require alternative delivery methods
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Administration Routes:
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Formulation Considerations:
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Recombinant protein stability should be verified before administration
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Vehicle composition should be carefully controlled and reported
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Protein concentration verification is essential for accurate dosing
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Timing of Assessments:
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Controls and Comparisons:
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Vehicle-treated controls with identical administration protocol
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Dose-response studies to establish minimum effective dose
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Positive control groups (e.g., established insulin sensitizers) when appropriate
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These protocols have been validated in multiple models including wild-type, ob/ob, and db/db mice, demonstrating consistent metabolic improvements with FGF21 administration .
How can researchers effectively study FGF21's effects on macronutrient preference?
Studying FGF21's effects on macronutrient preference requires specialized experimental approaches:
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Three-Macronutrient Choice Test:
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Diet Pair Selection Tests:
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Protein Restriction Models:
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Central Administration Studies:
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Experimental Controls:
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Control for palatability differences between diets
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Account for potential neophobia when introducing novel diets
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Consider diurnal variations in feeding behavior
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Monitor total caloric intake in addition to macronutrient selection
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These approaches have established that FGF21 signaling in the brain is both necessary and sufficient for adaptive increases in protein intake during protein restriction .
How do the metabolic effects of FGF21 in mice compare to its effects in other species?
FGF21's metabolic effects show important similarities and differences across species:
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Rodents vs. Non-human Primates:
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FGF21 administration exerts strong antihyperglycemic and triglyceride-lowering effects in both mice and rhesus monkeys with diet-induced or genetic obesity and diabetes
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Body weight reduction is observed in both species with FGF21 treatment
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These cross-species similarities suggest conservation of core FGF21 functions in metabolism
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Species-Specific Responses:
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The magnitude of effects may differ between species
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Dosing requirements likely vary based on species-specific pharmacokinetics
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Tissue-specific expression patterns and regulatory mechanisms may show species differences
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The lifespan extension observed in FGF21 transgenic mice has not been demonstrated in other species
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Mechanistic Conservation:
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The requirement for β-Klotho co-receptor appears consistent across species
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Effects on insulin sensitivity and glucose homeostasis are observed across multiple species
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The protein-specific appetite regulation may be evolutionarily conserved, though this requires further confirmation in non-rodent species
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The conservation of FGF21's metabolic effects across mice and non-human primates suggests potential for human translation, though species-specific differences must be carefully considered in study design and interpretation .
What are the key challenges in translating FGF21 findings from mouse models to potential human therapeutics?
Several critical challenges must be addressed before mouse FGF21 findings can be effectively translated to human therapeutics:
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Adverse Effect Profile:
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Pharmacokinetic Limitations:
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Native FGF21 has a relatively short half-life, necessitating frequent administration
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Optimal dosing regimens established in mice may not directly translate to humans
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Tissue-specific delivery remains challenging
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Biological Complexity:
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The biological roles of FGF21 in humans may differ in some aspects from mice
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Human genetic variation may influence individual responses to FGF21-based therapies
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Comorbidities common in metabolic disease patients may alter FGF21 responsiveness
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Mechanistic Understanding:
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The precise mechanisms mediating FGF21's beneficial effects are not fully elucidated
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Understanding which effects are direct versus indirect requires further clarification
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The contribution of various tissues to FGF21 production and response needs better characterization
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Therapeutic Optimization:
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Developing tissue-selective FGF21 analogs that maintain beneficial effects while minimizing adverse effects
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Identifying biomarkers to predict and monitor response to FGF21-based therapies
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Determining optimal combinations with other metabolic therapies
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These challenges highlight the need for continued mechanistic studies in both animal models and humans to realize the therapeutic potential of FGF21 while mitigating risks .
Product Science Overview
FGF-21 signals through FGF receptor 1c and 4, using β-Klotho as a cofactor, to activate the PI3K and MAPK pathways . The expression of FGF-21 is regulated by tissue-specific peroxisome proliferator-activated receptors (PPARs). For instance, PPAR-α stimulation leads to FGF-21 production in the liver, while PPAR-γ activation results in its production in adipose tissue .
FGF-21 has several important biological functions:
- Metabolic Regulation: It promotes insulin-independent glucose uptake and lipid accumulation in primary human adipocytes and mouse 3T3-L1 cells . In pancreatic islets and INS-1 cells, it inhibits glucose-mediated glucagon release and stimulates insulin production .
- Thermogenesis: FGF-21 regulates thermogenesis in white and brown adipose tissue .
- Cell Proliferation: Unlike other FGFs, FGF-21 does not induce proliferation in immortalized cell lines .
