Miriam Granado

Leptin regulates glutamate and glucose transporters in hypothalamic astrocytes

Glial cells perform critical functions that alter the metabolism and activity of neurons, and there is increasing interest in their role in appetite and energy balance. Leptin, a key regulator of appetite and metabolism, has previously been reported to influence glial structural proteins and morphology. Here, we demonstrate that metabolic status and leptin also modify astrocyte-specific glutamate and glucose transporters, indicating that metabolic signals influence synaptic efficacy and glucose uptake and, ultimately, neuronal function. We found that basal and glucose-stimulated electrical activity of hypothalamic proopiomelanocortin (POMC) neurons in mice were altered in the offspring of mothers fed a high-fat diet. In adulthood, increased body weight and fasting also altered the expression of glucose and glutamate transporters. These results demonstrate that whole-organism metabolism alters hypothalamic glial cell activity and suggest that these cells play an important role in the pathology of obesity.

Differential Acute and Chronic Effects of Leptin on Hypothalamic Astrocyte Morphology and Synaptic Protein Levels

Leptin induces inverse effects on astrocyte structural proteins and synaptic proteins in the hypothalamus, with opposite effects of acute and chronic exposure.

Fenofibrate, a PPARα agonist, decreases atrogenes and myostatin expression and improves arthritis-induced skeletal muscle atrophy

Arthritis is a chronic inflammatory illness that induces cachexia, which has a direct impact on morbidity and mortality. Fenofibrate, a selective PPARα activator prescribed to treat human dyslipidemia, has been reported to decrease inflammation in rheumatoid arthritis patients. The aim of this study was to elucidate whether fenofibrate is able to ameliorate skeletal muscle wasting in adjuvant-induced arthritis, an experimental model of rheumatoid arthritis. On day 4 after adjuvant injection, control and arthritic rats were treated with 300 mg/kg fenofibrate until day 15, when all rats were euthanized. Fenofibrate decreased external signs of arthritis and liver TNFα and blocked arthritis-induced decreased in PPARα expression in the gastrocnemius muscle. Arthritis decreased gastrocnemius weight, which results from a decrease in cross-section area and myofiber size, whereas fenofibrate administration to arthritic rats attenuated the decrease in both gastrocnemius weight and fast myofiber size. Fenofibrate treatment prevented arthritis-induced increase in atrogin-1 and MuRF1 expression in the gastrocnemius. Neither arthritis nor fenofibrate administration modify Akt-FoxO3 signaling. Myostatin expression was not modified by arthritis, but fenofibrate decreased myostatin expression in the gastrocnemius of arthritic rats. Arthritis increased muscle expression of MyoD, PCNA, and myogenin in the rats treated with vehicle but not in those treated with fenofibrate. The results indicate that, in experimental arthritis, fenofibrate decreases skeletal muscle atrophy through inhibition of the ubiquitin-proteasome system and myostatin.

Adipose tissue loss in adjuvant arthritis is associated with a decrease in lipogenesis, but not with an increase in lipolysis

Adjuvant-induced arthritis is a model of rheumatoid arthritis that induces cachexia. In other cachectic situations, there is an increase in lipolysis resulting in a loss of adipose tissue mass. The aim of this work was to analyse the effect of chronic arthritis, induced by adjuvant injection, on white adipose tissue (WAT). For this purpose, rats were killed 10 days after adjuvant injection, when the first external symptoms appeared, on days 15 and 22 when the external signs of the illness reach their severest level. As arthritis decreases food intake, a pair-fed group was also included. Serum concentrations of insulin, leptin, adiponectin, glycerol and nitrites, as well as gene expression of leptin, adiponectin, hormone-sensitive lipase (HSL), fatty acid synthase (FAS), tumour necrosis factor alpha and zinc-alpha(2)-glycoprotein (ZAG) were determined. Arthritis decreased food intake between days 5 and 16, but not during the last 5 days of the experiment. There was a marked decrease in relative adipose tissue weight and in serum leptin and adiponectin as well as in their gene expression in WAT in arthritic rats. Arthritis decreased the gene expression of FAS in the WAT. However, none of these effects was found in pair-fed rats. Arthritis did not increase lipolysis, since arthritic rats have lower serum concentrations of glycerol, HSL mRNA in WAT, as well as liver ZAG mRNA than the pair-fed or control rats. These data suggest that in chronic arthritis the decrease in white adipose mass is secondary to a reduced adipose lipogenesis, and this effect is not mainly due to the decrease in food intake.

Effects of Acute Changes in Neonatal Leptin Levels on Food Intake and Long-Term Metabolic Profiles in Rats

In rodents there is a rise in serum leptin levels between postnatal days (PND) 5 and 14, with this neonatal leptin surge reported to modulate the maturation of hypothalamic circuits involved in appetite regulation. We hypothesized that acute changes in neonatal leptin levels have different long-term metabolic effects depending on how and when this surge is modified. To advance the timing of the normal leptin peak, male Wistar rats were injected with leptin (sc, 3 μg/g) on PND 2. To ablate the leptin peak on PND 10, a pegylated leptin antagonist (sc, 9 μg/g) was injected. Controls received vehicle. All rats were allowed to eat ad libitum until PND 150. Increased leptin on PND 2 reduced food intake (P<0.01) after 3 months of age with no effect on body weight. Levels of total ghrelin were reduced (P<0.001) and acylated ghrelin increased (P<0.05), with no other modifications in metabolic hormones. In contrast, treatment with the leptin antagonist on PND 9 did not affect food intake but reduced body weight beginning around PND 60 (P<0.02). This was associated with a reduction in fat mass, insulin (P<0.01), and leptin (P<0.007) levels and an increase in testosterone levels (P<0.01). Hypothalamic neuropeptide Y (P<0.05) and leptin receptor (P<0.005) mRNA levels were reduced, whereas mRNA levels for uncoupling protein 2 (P<0.005) were increased in visceral fat, which may indicate an increase in energy expenditure. In conclusion, acute changes in neonatal leptin levels induce different metabolic profiles depending on how and when leptin levels are modified.

Early nutritional changes induce sexually dimorphic long-term effects on body weight gain and the response to sucrose intake in adult rats

Long-term metabolic effects induced by early nutritional changes are suspected to differ between males and females, but few studies have analyzed both sexes simultaneously. We analyzed the consequences of neonatal nutritional changes on body weight (BW) and the adult response to a sucrose-enriched diet in both male and female rats. Litter size was manipulated at birth to induce over- and undernutrition (4 pups: L4; 12 pups: L12; 20 pups: L20). From 50 to 65 days of age, half of each group received a 33% sucrose solution instead of water. Serum leptin, insulin, and ghrelin levels were analyzed at day 65. At weaning, rats from L4 weighed more and those from L20 weighed less than controls (L12). Body weight was greater in L4 rats throughout the study and increased further compared with controls in adult life. L20 males ate less and gained less weight throughout the study, but L20 females had a significant catch-up in BW. Sucrose intake increased total energy consumption in all groups, but not BW gain, with L4 males and L4 and L20 females reducing weight gain. Yet, sucrose intake increased serum leptin levels, with this increase being significant in L4 and L20 males. Our results suggest that females are more capable than males of recuperating and maintaining a normal BW after reduced neonatal nutrition. Furthermore, increased sucrose intake does not increase BW, but could alter body composition as reflected by leptin levels, with the percentage of calories consumed in the form of sucrose being affected by sex and neonatal nutrition.

IGF-I system, atrogenes and myogenic regulatory factors in arthritis induced muscle wasting

The aim of this work was to analyse the evolution of the ubiquitin-proteasome, the myogenic regulatory factors, and the IGF-I system during the development of experimental arthritis. Arthritis was induced by adjuvant injection and rats were killed 10, 15 and 22 days later. Gastrocnemius was progressively atrophied in arthritic rats. Arthritis induced a rapid increase in muscular IGFBP-3 and IGFBP-5 and, to a lesser extent, in IGF-I mRNA. An increased expression of the muscle-specific ubiquitin ligases atrogin-1/MAFbx and MuRF-1 was observed in the gastrocnemius from day 10, reaching its maximum value on day 15. Concomitantly, the proliferation marker PCNA and the early myogenic regulatory factor MyoD were also maximally increased on day 15. Myogenin, a late-acting myogenic regulatory factor, was maximally increased on days 15 and 22. These results suggest that muscle wasting in arthritis is secondary to an increase in muscle proteolysis, rather to a decrease in muscle regeneration.

Ghrelin treatment protects lactotrophs from apoptosis in the pituitary of diabetic rats.

Poorly controlled diabetes is associated with hormonal imbalances, including decreased prolactin production partially due to increased lactotroph apoptosis. In addition to its metabolic actions, ghrelin inhibits apoptosis in several cell types. Thus, we analyzed ghrelins effects on diabetes-induced pituitary cell death and hormonal changes. Six weeks after onset of diabetes in male Wistar rats (streptozotocin 70 mg/kg), minipumps infusing saline or 24 nmol ghrelin/day were implanted (jugular). Rats were killed two weeks later. Ghrelin did not modify body weight or serum glucose, leptin or adiponectin, but increased total ghrelin (P<0.05), IGF-I (P<0.01) and prolactin (P<0.01) levels. Ghrelin decreased cell death, iNOS and active caspase-8 (P<0.05) and increased prolactin (P<0.05), Bcl-2 (P<0.01) and Hsp70 (P<0.05) content in the pituitary. In conclusion, ghrelin prevents diabetes-induced death of lactotrophs, decreasing caspase-8 activation and iNOS content and increasing anti-apoptotic pathways such as pituitary Bcl-2 and Hsp70 and serum IGF-I concentrations.

Leptin in early life: A key factor for the development of the adult metabolic profile

Leptin levels during the perinatal period are important for the development of metabolic systems involved in energy homeostasis. In rodents, there is a postnatal leptin surge, with circulating leptin levels increasing around postnatal day (PND) 5 and peaking between PND 9 and PND 10. At this time circulating leptin acts as an important trophic factor for the development of hypothalamic circuits that control energy homeostasis and food seeking and reward behaviors. Blunting the postnatal leptin surge results in long-term leptin insensitivity and increased susceptibility to diet-induced obesity during adulthood. Pharmacologically increased leptin levels in the postnatal period also have long-term effects on metabolism. Nevertheless, this effect is controversial as postnatal hyperleptinemia is reported to both increase and decrease the predisposition to obesity in adulthood. The different effects reported in the literature could be explained by the different moments at which this hormone was administered, suggesting that modifications of the neonatal leptin surge at specific time points could selectively affect the development of central and peripheral systems that are undergoing modifications at this moment resulting in different metabolic and behavioral outcomes. In addition, maternal nutrition and the hormonal environment during pregnancy and lactation may also modulate the offspring’s response to postnatal modifications in leptin levels. This review highlights the importance of leptin levels during the perinatal period in the development of metabolic systems that control energy homeostasis and how modifications of these levels may induce long-lasting and potentially irreversible effects on metabolism.

Early postnatal overnutrition increases adipose tissue accrual in response to a sucrose-enriched diet

Both overnutrition and an incorrect nutrient balance have contributed to the rise in obesity. Moreover, it is now clear that poor nutrition during early life augments the possibility of excess weight gain in later years. Our aim was to determine how neonatal overnutrition affects later responses to a sucrose-enriched diet and whether this varies depending upon when the diet is introduced in postnatal life. Male Wistar rats raised in litters of four or 12 pups were given a 33% sucrose solution instead of water from weaning (day 21) or postnatal day (PND) 65. All rats received normal chow ad libitum until they were euthanized on PND 80. Body weight (BW) and food and liquid intake were monitored throughout the study. Fat mass, adipocyte morphology, serum biochemical and hormonal parameters, and hypothalamic neuropeptide mRNA levels were measured at study termination. Neonatal overnutrition increased food intake, BW, and leptin levels, induced adipocyte hypertrophy, and decreased total ghrelin levels. The sucrose-enriched diet increased total energy intake, adipose accrual, and leptin, adiponectin, and acylated ghrelin levels but decreased BW. Most of these responses were accentuated in neonatally overnourished rats, which also had increased insulin and triglyceride levels. However, long-term sucrose intake induced adipocyte hypertrophy in rats from normal-sized litters but not in neonatally overfed rats. The results reported here indicate that neonatal overnutrition increases the detrimental response to a diet rich in sucrose later in life. Moreover, the timing and duration of the exposure to a sucrose-enriched diet alter the adverse metabolic outcomes.