Kari Mäkelä

Are skeletal muscle FNDC5 gene expression and irisin release regulated by exercise and related to health

•  Contradictory findings have been reported concerning the function of irisin and its precursor gene, skeletal muscle FNDC5, in energy homeostasis and metabolic health, and the associated regulatory role of exercise and PGC‐1α. •  We analysed the effects of different short‐ and long‐term exercise regimens on muscle FNDC5 and PGC‐1α, and serum irisin, and studied the associations of irisin and FNDC5 with health parameters. •  FNDC5 and serum irisin did not change after acute aerobic, long‐term endurance training or endurance training combined with resistance exercise (RE) training, or associate with metabolic disturbances. A single RE bout increased FNDC5 mRNA in young, but not older men (27 vs. 62 years). Changes in PGC‐1α or serum irisin were not consistently accompanied by changes in FNDC5. •  Our data suggest that the effects of exercise on FNDC5 and irisin are not consistent, and that their role in health is questionable. Moreover, the regulatory mechanisms should be studied further.

Functions of orexins in peripheral tissues

Orexin A (OXA) and orexin B were originally isolated as hypothalamic peptides regulating sleep, wakefulness and feeding. However, growing evidence suggests that orexins have major functions also in the peripheral tissues. Central orexigenic pathways originating from medulla activate the hypothalamus–pituitary axis and can influence the sympathetic tone. Orexins and their receptors are widely dispersed throughout the intestine, where orexin receptors are regulated by the nutritional status, affect insulin secretion and intestinal motility. Although the primary source of the peptide has not been elucidated, OXA is detected in plasma and its level varies in response to the metabolic state. In this review, we focus on the current knowledge on peripheral functions of orexins and discuss possible endocrine, paracrine and neurocrine roles.

HIF Prolyl 4-Hydroxylase-2 Inhibition Improves Glucose and Lipid Metabolism and Protects Against Obesity and Metabolic Dysfunction

Obesity is a major public health problem, predisposing subjects to metabolic syndrome, type 2 diabetes, and cardiovascular diseases. Specific prolyl 4-hydroxylases (P4Hs) regulate the stability of the hypoxia-inducible factor (HIF), a potent governor of metabolism, with isoenzyme 2 being the main regulator. We investigated whether HIF-P4H-2 inhibition could be used to treat obesity and its consequences. Hif-p4h-2–deficient mice, whether fed normal chow or a high-fat diet, had less adipose tissue, smaller adipocytes, and less adipose tissue inflammation than their littermates. They also had improved glucose tolerance and insulin sensitivity. Furthermore, the mRNA levels of the HIF-1 targets glucose transporters, glycolytic enzymes, and pyruvate dehydrogenase kinase-1 were increased in their tissues, whereas acetyl-CoA concentration was decreased. The hepatic mRNA level of the HIF-2 target insulin receptor substrate-2 was higher, whereas that of two key enzymes of fatty acid synthesis was lower. Serum cholesterol levels and de novo lipid synthesis were decreased, and the mice were protected against hepatic steatosis. Oral administration of an HIF-P4H inhibitor, FG-4497, to wild-type mice with metabolic dysfunction phenocopied these beneficial effects. HIF-P4H-2 inhibition may be a novel therapy that not only protects against the development of obesity and its consequences but also reverses these conditions.

Apelin stimulation of duodenal bicarbonate secretion: feeding-dependent and mediated via apelin-induced release of enteric cholecystokinin

Aims:  Apelin peptides are the endogenous ligand of the G protein‐coupled receptor APJ. Proposed actions include involvement in control of cardiovascular functions, appetite and body metabolism. We have investigated the effects of apelin peptides on duodenal bicarbonate secretion in vivo and the release of cholecystokinin (CCK) from acutely isolated mucosal cells and the neuroendocrine cell line STC‐1.

Effects of short-term food deprivation on orexin-A-induced intestinal bicarbonate secretion in comparison with related secretagogues

Studies of gastrointestinal physiology in humans and intact animals are usually conducted after overnight fast. We compared the effects of orexin‐A, vasoactive intestinal polypeptide (VIP), melatonin, serotonin, uroguanylin, ghrelin and prostaglandin E2 (PGE2) on duodenal bicarbonate secretion in fed and overnight fasted animals. This review is a summary of our findings. Secretagogues were administered by intra‐arterial infusion or luminally (PGE2). Enterocyte intracellular calcium ([Ca2+]i) signalling was studied by fluorescence imaging. Total RNA was extracted, reverse transcripted to cDNA and expression of orexin receptors measured by quantitative real‐time PCR. Orexin‐A stimulates the duodenal secretion in continuously fed animals but not in food‐deprived animals. Similarly, short‐term fasting causes a 100‐fold decrease in the amount of the muscarinic agonist bethanechol required for stimulation of secretion. In contrast, fasting does not affect secretory responses to intra‐arterial VIP, melatonin, serotonin, uroguanylin and ghrelin, or that to luminal PGE2. Orexin‐A induces [Ca2+]i signalling in enterocytes from fed rats but no significant [Ca2+]i responses occur in enterocytes from fasted animals. In addition, overnight fasting decreases the expression of mucosal orexin receptors. Short‐term food deprivation thus decreases duodenal expression of orexin receptors and abolishes the secretory response to orexin‐A as well as orexin‐A‐induced [Ca2+]i signalling. Fasting, furthermore, decreases mucosal sensitivity to bethanechol. The absence of declines in secretory responses to other secretagogues tested strongly suggests that short‐term fasting does not affect the secretory capacity of the duodenal mucosa in general. Studies of intestinal secretion require particular evaluation with respect to feeding status.

The effects of group and single housing and automated animal monitoring on urinary corticosterone levels in male C57BL/6 mice

Mice are used extensively in physiological research. Automated home‐cage systems have been developed to study single‐housed animals. Increased stress by different housing conditions might affect greatly the results when investigating metabolic responses. Urinary corticosteroid concentration is considered as a stress marker. The aim of the study was to compare the effects of different housing conditions and an automated home‐cage system with indirect calorimetry located in an environmental chamber on corticosterone levels in mice. Male mice were housed in different conditions and in automated home‐cage system to evaluate the effects of housing and measuring conditions on urine corticosterone levels. Corticosterone levels in single‐housed mice in the laboratory animal center were consistently lower compared with the group‐housed mice. Single‐housed mice in a separate, small animal unit showed a rise in their corticosterone levels a day after they were separated to their individual cages, which decreased during the following 2 days. The corticosterone levels of group‐housed mice in the same unit were increased during the first 7 days and then decreased. On day 7, the corticosterone concentrations of group‐housed mice were significantly higher compared with that of single‐housed mice, including the metabolic measurement protocol. In conclusion, single housing caused less stress when compared with group‐housed mice. In addition, the urine corticosterone levels were decreased in single‐housed mice before the metabolic measurement started. Thus, stress does not affect the results when utilizing the automated system for measuring metabolic parameters like food and water intake and calorimetry.

Neuropeptide Y in the noradrenergic neurones induces obesity and inhibits sympathetic tone in mice

Neuropeptide Y (NPY) co‐localized with noradrenaline in central and sympathetic nervous systems seems to play a role in the control of energy metabolism. In this study, the aim was to elucidate the effects and pathophysiological mechanisms of increased NPY in catecholaminergic neurones on accumulation of body adiposity.

Fat mass- and obesity-associated gene Fto affects the dietary response in mouse white adipose tissue

Common variants of human fat mass- and obesity-associated gene Fto have been linked with higher body mass index, but the biological explanation for the link has remained obscure. Recent findings suggest that these variants affect the homeobox protein IRX3. Here we report that FTO has a role in white adipose tissue which modifies its response to high-fat feeding. Wild type and Fto-deficient mice were exposed to standard or high-fat diet for 16 weeks after which metabolism, behavior and white adipose tissue morphology were analyzed together with adipokine levels and relative expression of genes regulating white adipose tissue adipogenesis and Irx3. Our results indicate that Fto deficiency increases the expression of genes related to adipogenesis preventing adipocytes from becoming hypertrophic after high-fat diet. In addition, we report a novel finding of increased Irx3 expression in Fto-deficient mice after high-fat feeding indicating a complex link between FTO, IRX3 and fat metabolism.

Characterization of sleep–wake patterns in a novel transgenic mouse line overexpressing human prepro‐orexin/hypocretin

Aim:  Orexin/hypocretin peptides are expressed in the lateral hypothalamus and involved in the regulation of autonomic functions, energy homeostasis and arousal states. The sleep disorder narcolepsy, which is characterized by excessive daytime sleepiness and occurrence of sudden rapid eye movement (REM) sleep, is associated with a loss of orexin neurones. Our study investigated the effects of orexins on sleep–wake patterns in a novel transgenic mouse line overexpressing the human prepro‐orexin (hPPO) gene under the control of its endogenous promoter.

Short food deprivation inhibits orexin receptor 1 expression and orexin-A induced intracellular calcium signaling in acutely isolated duodenal enterocytes.

Close intra-arterial infusion of the appetite regulating peptide orexin-A stimulates bicarbonate secretion from the duodenal mucosa. The aim of the present study was to elucidate the ability of orexin-A to induce intracellular calcium signaling in acutely isolated duodenal enterocytes. Freshly isolated clusters of enterocytes, obtained from rat duodenal mucosa or human duodenal biopsies, were loaded with fura 2-AM and mounted in a perfusion chamber. Cryptlike enterocytes were selected (caged), and changes in intracellular calcium concentration ([Ca2+]i) were evaluated by fluorescence imaging. Total RNA was extracted from pellets of enterocytes and reverse transcribed to cDNA, and expression of orexin receptors 1 and 2 (OX1R and OX2R) was measured by quantitative real-time PCR. Orexin-A at all concentrations tested (1-100 nM) increased [Ca2+]i in enterocytes isolated from continuously fed rats, and the OX1R-antagonist SB-334867 (10 nM) attenuated the response. The primary [Ca2+]i response was a slow increase to a sustained plateau persisting after orexin-A removal, and a similar response was observed in enterocytes from human biopsies. In contrast to orexin-A, the OX2R agonist (Ala11,D-Leu15)-orexin-B (1-10 nM) did not induce calcium signaling. There were no significant [Ca2+]i responses in enterocytes from animals food deprived overnight, and overnight fasting decreased (P<0.01) enterocyte OX1R as well as OX2R mRNA. Induction of intracellular calcium signaling in isolated duodenal enterocytes is thus mediated primarily by OX1R receptors. Short (overnight) food deprivation markedly depresses receptor expression and inhibits orexin-A induced increases in [Ca2+]i. Studies of enterocyte signaling and intestinal secretion requires particular evaluation regarding feeding status.