Fasting induces a biphasic adaptive metabolic response in murine small intestine

The role of microRNA in the regulation of hepatic metabolism and energy-expensive processes in the hibernating dormouse

The garden dormouse (Eliomys quercinus) is a fat-storing mammal that undergoes annual periods of hibernation to mitigate the effects of food scarcity, low ambient temperatures, and reduced photoperiod that characterize winter. Like other hibernating species, this animal suppresses its metabolic rate by downregulating nonessential genes and processes in order to prolong available energy stores and limit waste accumulation throughout the season. MicroRNAs (miRNAs) are short, single-stranded, noncoding RNAs that bind to mRNA and mediate post-transcriptional suppression, making miRNA ideal for modulating widespread changes in gene expression, including global downregulation typified by metabolic rate depression. Using next-generation sequencing, we analyzed an RNA-seq dataset to determine which miRNAs are differentially regulated during hibernation in the dormouse liver. We found that the expression of 19 miRNAs was altered during hibernation; however, only one major miRNA (miR-34a-5p) remained significantly downregulated after correcting for false discovery rate. Gene Ontology, KEGG Pathway Analysis, and DIANA-miRPath predicted that energy metabolism, nuclear-related functions such as histone binding, chromatin- and chromosomal binding, and the cell cycle are processes that may be differentially regulated during hibernation due to miRNA regulation. Taken together, our data suggest that miRNA influence appears to be strongly directed toward suppressing energy-intensive processes in the nucleus hence contributing to extend the animal's endogenous fuel reserves for the duration of hibernation.

Comparison of human breast milk vs commercial formula-induced early trophic enteral nutrition during postoperative prolonged starvation in an animal model

The present study aimed to characterize the changes in macromolecular composition and structure in ileal tissue induced by postoperative prolonged starvation (PS), human breast milk feeding (HM) and commercial formula feeding (CF) for 48 and 72 h (h). Forty-two Wistar albino rats underwent an ileal transection and primary anastomosis and were then divided into six subgroups. Two groups of seven rats were food-deprived for 48 and 72 h with free access to water only in metabolic cages (48 h PS, 72 h PS). Then, two groups of seven rats received early enteral trophic nutrition (EEN) either using HM, and CF at 48 h post-operation (48 h HM, 48 h CF). The other two groups of seven rats received the same trophic enteral nutrition at 72 h post-operation (72 h HM, 72 h CF). An additional seven rats were fed normal rat chow (control), after which the ileal tissues were harvested and freeze-dried overnight. Then sample spectra were recorded by Fourier transform infrared (FTIR) spectroscopy. PS at 48 and 72 h resulted in an increase in the concentration of lipids and a decrease in the concentration of proteins. CF and HM trophic feeding induced a decrease in membrane fluidity and an increase in lipid order. Ileal tissues showed similar compositional and structural changes in lipids and proteins in the PS and CF groups after 48 and 72 h. A marked decrease in nucleic acid concentration was seen in CF at 48 h compared to HM. The human milk feeding groups did not induce any significant alterations and showed compositional and structural data similar to the controls. In conclusion, EEN application seems to be safer when introduced at 48 h rather than 72 h and time of this nutrition is crucial to maintain ileum structure and therefore immunity and well-being. HM-induced trophic nutrition is seen to protect the ileal tissue from significant alterations within lipid and protein compositions, whereas CF caused notable changes. HM is absolutely the best nutritional source for gut health in this animal model.

Intermittent fasting modulates human gut microbiota diversity in a phenotype-dependent manner: a systematic review

Cumulative evidence suggests that intermittent fasting (IF) has beneficial effects on human metabolic health. It has been indicated that its impact on the gut microbiota may mediate these beneficial effects. As a result, we hypothesized that IF may impact the human gut microbiota. A systematic review was carried out according to the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) protocol using the PubMed, Scopus, and CINAHL databases. We registered our systematic review protocol in PROSPERO under registration number CRD42021270050. Human intervention studies published until April 30, 2023, were included. The quality of the included studies was assessed using National Institutes of Health (NIH) quality assessment study tools for intervention studies. The search in the database returned 166 studies, of which 13 matched all criteria for the final qualitative analysis. The body of evidence suggests that IF modulates human gut microbiota alpha and beta diversity in lean (relatively healthy) and relatively healthy overweight/obese individuals but not in individuals with metabolic syndrome. Furthermore, IF also alters human gut microbiota composition in all phenotypes. Of interest, the gut microbiota taxa or microbial metabolites after an IF intervention are associated with metabolic markers. According to this review, IF influences the diversity and taxonomic levels of the human gut microbiota. Individual metabolic phenotypes may alter the effect of IF on the diversity and taxonomic levels of the gut microbiota.

Plastic food? Energy compensation of zebrafish (Danio rerio) after long-term exposure to polylactic acid biomicroplastics

The extensive use of bio-based plastics has led to their widespread distribution in the environment. However, their long-term ecological impact on aquatic animals is not well understood. In this study, adult zebrafish (Danio rerio) were exposed to 1000 items·L-1 of either polylactic acid (PLA) or polyethylene terephthalate (PET) microplastics (MPs), for 90 days. PLA is a typical bio-based plastic, while PET is a typical petroleum-derived plastic. The abundances of PLA and PET MPs in fish intestines were 981 ± 66 and 671 ± 151 items per fish, respectively, indicating a greater amount of PLA MP residues than PET MPs. However, the inhibitory effect of PET on fish weight was 1.8 times higher than that of PLA, suggesting energy compensation in PLA-treated zebrafish. Proliferation of Lactobacillus was observed in the fish intestines of the PLA group, indicating increased utilization capacity of intestinal flora for lactic acid production during PLA degradation. Metabolomics showed that the tricarboxylic acid pathway was up-regulated in the PLA group compared with that in the PET group, providing evidence of energy compensation. However, more ingested PLA MPs caused more significant histological damage to fish intestines than PET MPs. Therefore, the ecological risks of bio-based plastics still require attention.

Effects of intermittent fasting on the histology and mRNA expression of major drug-metabolizing cyp450s in the liver of diabetic mice

Introduction:There is a variation in drug response among patients who practice intermittent fasting. Alteration in the expression of drug-metabolizing enzymes (DMEs) can affect the pharmacokinetics and drug response.Aims: This research aimed to determine the effect of intermittent fasting on the mRNA expression of major drug-metabolizing cyp450s in the liver of diabetic mice.Methods: Thirty-two male Balb/c mice were divided into four groups; control, nonfasting diabetic, non-diabetic fasting, and diabetic fasting mice. Insulin-dependent diabetes was induced in mice by a single high-dose (250 mg/kg) streptozocin. Mice of non-diabetic and diabetic fasting groups were subjected to 10-day intermittent fasting for 17 hours daily. Then, the mRNA expression of mouse phase I DMEs cyp1a1, cyp2c29, cyp2d9, and cyp3a11 was analyzed using real-time polymerase chain reaction. In addition, the liver of mice in all groups was examined for pathohistological alterations.Results: Diabetes downregulated the mRNA expression of hepatic drug-metabolizing cyp450s in diabetic mice, while intermittent fasting significantly (P < 0.05) increased it. Also, cyp2d9 and cyp3a11 were upregulated in the liver of diabetic fasting mice. These alterations in the gene expression were correlated with the pathohistological alterations, where livers of diabetic mice showed dilatation in the blood sinusoids and inflammatory cells leukocyte infiltrations. Whereas livers of diabetic fasting mice showed almost comparable histological findings to control mice.Conclusions: Intermittent fasting can protect the liver against diabetes-induced hepatotoxicity and the down-regulation of DME genes in the diabetic liver. These results can explain, at least partly, the inter-individual variation in the drug response during practicing fasting.

Functional and molecular profiling of fasted piglets reveals decreased energy metabolic function and cell proliferation in the small intestine

The small intestine requires energy to exert its important role in nutrient uptake and barrier function. Pigs are an important source of food and a model for humans. Young piglets and infants can suffer from periods of insufficient food intake. Whether this functionally affects the small intestinal epithelial cell (IEC) metabolic capacity and how this may be associated with an increased vulnerability to intestinal disease is unknown. We therefore performed a 48-h fasting intervention in young piglets. After feeding a standard weaning diet for 2 wk, 6-wk-old piglets (n = 16/group) were fasted for 48 h, and midjejunal IECs were collected upon euthanasia. Functional metabolism of isolated IECs was analyzed with the Seahorse XF analyzer and gene expression was assessed using RNA-sequencing. Fasting decreased the mitochondrial and glycolytic function of the IECs by 50% and 45%, respectively (P < 0.0001), signifying that overall metabolic function was decreased. The RNA-sequencing results corroborated our functional metabolic measurements, showing that particularly pathways related to mitochondrial energy production were decreased. Besides oxidative metabolic pathways, decreased cell-cycle progression pathways were most regulated in the fasted piglets, which were confirmed by 43% reduction of Ki67-stained cells (P < 0.05). Finally, the expression of barrier function genes was reduced upon fasting. In conclusion, we found that the decreased IEC energy metabolic function in response to fasting is supported by a decreased gene expression of mitochondrial pathways and is likely linked to the observed decreased intestinal cell proliferation and barrier function, providing insight into the vulnerability of piglets, and infants, to decreased food intake.NEW & NOTEWORTHY Fasting is identified as one of the underlying causes potentiating diarrhea development, both in piglets and humans. With this study, we demonstrate that fasting decreases the metabolism of intestinal epithelial cells, on a functional and transcriptional level. Transcriptional and histological data also show decreased intestinal cell proliferation. As such, fasting-induced intestinal energy shortage could contribute to intestinal dysfunction upon fasting.

Lepr+ mesenchymal cells sense diet to modulate intestinal stem/progenitor cells via Leptin-Igf1 axis

Diet can impact on gut health and disease by modulating intestinal stem cells (ISCs). However, it is largely unknown if and how the ISC niche responds to diet and influences ISC function. Here, we demonstrate that Lepr⁺ mesenchymal cells (MCs) surrounding intestinal crypts sense diet change and provide a novel niche signal to maintain ISC and progenitor cell proliferation. The abundance of these MCs increases upon administration of a high-fat diet (HFD) but dramatically decreases upon fasting. Depletion of Lepr⁺ MCs resulted in fewer intestinal stem/progenitor cells, compromised the architecture of crypt-villus axis and impaired intestinal regeneration. Furthermore, we showed that IGF1 secreted by Lepr⁺ MCs is an important effector that promotes proliferation of ISCs and progenitor cells in the intestinal crypt. We conclude that Lepr⁺ MCs sense diet alterations and, in turn, modulate intestinal stem/progenitor cell function via a stromal IGF1-epithelial IGF1R axis. These findings reveal that Lepr⁺ MCs are important mediators linking systemic diet changes to local ISC function and might serve as a novel therapeutic target for gut diseases.

Lipid oxidation during thermogenesis in high-altitude deer mice (Peromyscus maniculatus)

When at their maximum thermogenic capacity (cold-induced V̇o_2max), small endotherms reach levels of aerobic metabolism as high, or even higher, than running V̇o_2max. How these high rates of thermogenesis are supported by substrate oxidation is currently unclear. The appropriate utilization of metabolic fuels that could sustain thermogenesis over extended periods may be important for survival in cold environments, like high altitude. Previous studies show that high capacities for lipid use in high-altitude deer mice may have evolved in concert with greater thermogenic capacities. The purpose of this study was to determine how lipid utilization at both moderate and maximal thermogenic intensities may differ in high- and low-altitude deer mice, and strictly low-altitude white-footed mice. We also examined the phenotypic plasticity of lipid use after acclimation to cold hypoxia (CH), conditions simulating high altitude. We found that lipids were the primary fuel supporting both moderate and maximal rates of thermogenesis in both species of mice. Lipid oxidation increased threefold in mice from 30°C to 0°C, consistent with increases in oxidation of [¹³C]palmitic acid. CH acclimation led to an increase in [¹³C]palmitic acid oxidation at 30°C but did not affect total lipid oxidation. Lipid oxidation rates at cold-induced V̇o_2max were two- to fourfold those at 0°C and increased further after CH acclimation, especially in high-altitude deer mice. These are the highest mass-specific lipid oxidation rates observed in any land mammal. Uncovering the mechanisms that allow for these high rates of oxidation will aid our understanding of the regulation of lipid metabolism.

Reappraisal of the optimal fasting time for insulin tolerance tests in mice

Objective

Most studies routinely use overnight or 6 h of fasting before testing metabolic glucose homeostasis in mice. Other studies used empirically shorter fasting times (<6 h). We attempted to determine the shortest fasting time required for optimal insulin responsiveness while minimizing metabolic stress.

Methods

A course of fasting for up to 24 h (0, 2, 4, 6, 12, and 24 h) was conducted in C57Bl/6J male mice. Body weight, metabolic parameters, and insulin tolerance were measured in each experimental group. The organs were collected at the same time on separate occasions and glycogen and metabolic gene expression were measured in the liver and skeletal muscle.

Results

Our data show that blood glucose levels do not significantly change during a 6 h fast, while plasma insulin levels decrease to similar levels between 2 h and 6 h of fasting. During overnight (12 h) and 24 h fasts, a robust decrease in blood glucose and plasma insulin was observed along with a profound depletion in liver glycogen content. Insulin tolerance was comparable between baseline and 6 h fasts while 4 h and 6 h fasts were associated with a greater depletion of liver glycogen than 2 h fasts, impacting the glucose counter-regulatory response. Fasting induced progressive weight loss that was attenuated at thermoneutrality. Fasting longer than 4 h induced major body weight loss (>5%) and significant changes in catabolic gene expression in the liver and skeletal muscle.

Conclusion

Collectively, these data suggest that 2 h of fasting appears optimal for the assessment of insulin tolerance in mice as this duration minimizes major metabolic stress and weight loss.

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Fasting in mice induces time-dependent metabolic stress and weight loss.

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Fasting promotes profound changes in catabolic gene expression in liver and muscles.

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Fasting-induced weight loss is attenuated at thermoneutrality.

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Two hours fasting appears to be optimal prior to assessing insulin tolerance in mice.

Fasting of male mice - Effects of time point of initiation and duration on clinical chemistry parameters and animal welfare

Fasting of mice is a common procedure, which can affect the outcome of the study as well as animal welfare. In this study, we assess the effects of fasting, fasting duration and fasting initiation time in relation to light schedule and present suggestions for optimization of fasting. Male C57BL/6NCrl mice were fasted for 0, 3, 6, 12, 18 and 24 hours initiated either in the light period (photophase) or the dark period (scotophase). Body weight, gastric content, body temperature, corticosterone and 19 routine clinical chemistry parameters were evaluated. Fasting caused significant changes in most of the measured parameters. Increasing duration of fasting resulted in increasing physiological changes. Fasting initiated in the scotophase caused more significant changes than fasting initiated in the photophase. To cause the least physiological changes in mice and increase animal welfare, mice should preferably be fasted in the photophase and for the shortest possible period allowed by the experimental purpose of fasting.

Systemic oscillator-driven and nutrient-responsive hormonal regulation of daily expression rhythms for gluconeogenic enzyme genes in the mouse liver

Gluconeogenesis is de novo glucose synthesis from substrates such as amino acids and is vital when glucose is lacking in the diurnal nutritional fluctuation. Accordingly, genes for hepatic gluconeogenic enzymes exhibit daily expression rhythms, whose detailed regulations under nutritional variations remain elusive. As a first step, we performed general systematic characterization of daily expression profiles of gluconeogenic enzyme genes for phosphoenolpyruvate carboxykinase (PEPCK), cytosolic form (Pck1), glucose-6-phosphatase (G6Pase), catalytic subunit (G6pc), and tyrosine aminotransferase (TAT) (Tat) in the mouse liver. On a standard diet fed ad libitum, mRNA levels of these genes showed robust daily rhythms with a peak or an elevation phase during the late sleep-fasting period in the diurnal feeding/fasting (wake/sleep) cycle. The rhythmicity was preserved in constant darkness, modulated with prolonged fasting, attenuated by Clock mutation, and entrained to varied photoperiods and time-restricted feedings. These results are concordant with the notion that gluconeogenic enzyme genes are under the control of the intrinsic circadian oscillator, which is entrained by the light/dark cycle, and which in turn entrains the feeding/fasting cycle and also drives systemic signaling pathways such as the hypothalamic-pituitary-adrenal axis. On the other hand, time-restricted feedings also showed that the ingestion schedule, when separated from the light/dark cycle, can serve as an independent entrainer to daily expression rhythms of gluconeogenic enzyme genes. Moreover, nutritional changes dramatically modified expression profiles of the genes. In addition to prolonged fasting, a high-fat diet and a high-carbohydrate (no-protein) diet caused modification of daily expression rhythms of the genes, with characteristic changes in profiles of glucoregulatory hormones such as corticosterone, glucagon, and insulin, as well as their modulators including ghrelin, leptin, resistin, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide-1 (GLP-1). Remarkably, high-protein (60% casein or soy-protein) diets activated the gluconeogenic enzyme genes atypically during the wake-feeding period, with paradoxical up-regulation of glucagon, which frequently formed correlation networks with other humoral factors. Based on these results, we propose that daily expression rhythms of gluconeogenic enzyme genes are under the control of systemic oscillator-driven and nutrient-responsive hormones.

Food deprivation increases hepatic hepcidin expression and can overcome the effect of Hfe deletion in male mice

Iron-loading disorders, such as hereditary hemochromatosis, are associated with inappropriately low expression of the iron regulatory hormone, hepcidin. A recent study has demonstrated that food deprivation can increase hepcidin production in mice. We have examined this effect in more detail to determine whether the pathway(s) that are responsible might provide novel targets for pharmaceutical intervention in disorders of iron homeostasis. C57BL/6 mice were deprived of food for 5, 10, 16, or 24 h before euthanasia, then blood and tissue samples were collected for analysis. The effect of food deprivation was also examined in Hfe-/- mice, a model of hereditary hemochromatosis, as well as mice that were maintained on an iron-deficient diet or injected with erythropoietin. Food deprivation increased the hepatic expression of the gene that encodes hepcidin, hepcidin antimicrobial peptide 1 ( Hamp1), with maximal expression observed after 16 h, and was able to overcome the reduction in Hamp1 expression associated with Hfe deficiency. Food deprivation also increased Hamp1 expression in response to stimuli that more strongly suppress the gene, such as iron deficiency and erythropoietin treatment, but the effects were not significant. These results indicate that Hamp1 induction by food deprivation is independent of HFE and suggest that targeting the pathway regulated by food deprivation could have clinical benefit in iron-loading conditions.-Mirciov, C. S. G., Wilkins, S. J., Anderson, G. J., Frazer, D. M. Food deprivation increases hepatic hepcidin expression and can overcome the effect of Hfe deletion in male mice.

Transcriptomic analysis of Bama pig's liver in various nutritional states reveals a metabolic difference of fatty acids

Both fasting and treatment with a high-fat diet (HFD) can dramatically change fat metabolism in the liver and, thus, are commonly used methods to investigate hepatic fat metabolism and related diseases. Here, the gene expression profiles of pig liver under both conditions were investigated and changes in hepatic triacylglycerol (TG) levels under different diet conditions were determined. In this study, both fasting and HFD conditions significantly increased hepatic TG levels and serum levels of cholesterol, TG, high-density lipoprotein, low-density lipoprotein, and apolipoprotein B. Transcriptome sequencing analysis identified 580 differentially expressed genes (DEGs) between the fasting group and the control group (F/C group) and 613 between the HFD group and the control group (H/C group). Kyoto Encyclopedia of Genes and Genomes enrichment analysis found that the DEGs of the F/C group were mainly enriched in the synthesis pathways of fatty acids (FAs) with less than 16 carbons, while the DEGs of the H/C group were mainly enriched in the synthesis pathways of FAs with more than 16 carbons. In order to verify whether changes in the expression levels of the DEGs caused changes in FA metabolism, the composition and saturation of the FAs in liver TG were analyzed, which showed that under fasting conditions, the contents of monounsaturated and polyunsaturated FAs had increased, while the proportion of saturated FAs had decreased. However, the content of polyunsaturated FAs decreased, while the contents of monounsaturated and saturated FAs increased under HFD conditions. These results will help clarify the differences in FA metabolism in the liver under different nutritional states and indicate that the proportion of unsaturated FAs had increased in hepatic fat under fasting conditions.

The effect of endurance exercise on intestinal integrity in well-trained healthy men

Exercise is one of the external factors associated with impairment of intestinal integrity, possibly leading to increased permeability and altered absorption. Here, we aimed to examine to what extent endurance exercise in the glycogen‐depleted state can affect intestinal permeability toward small molecules and protein‐derived peptides in relation to markers of intestinal function. Eleven well‐trained male volunteers (27 ± 4 years) ingested 40 g of casein protein and a lactulose/rhamnose (L/R) solution after an overnight fast in resting conditions (control) and after completing a dual – glycogen depletion and endurance – exercise protocol (first protocol execution). The entire procedure was repeated 1 week later (second protocol execution). Intestinal permeability was measured as L/R ratio in 5 h urine and 1 h plasma. Five‐hour urine excretion of betacasomorphin‐7 (BCM7), postprandial plasma amino acid levels, plasma fatty acid binding protein 2 (FABP‐2), serum pre‐haptoglobin 2 (preHP2), plasma glucagon‐like peptide 2 (GLP2), serum calprotectin, and dipeptidylpeptidase‐4 (DPP4) activity were studied as markers for excretion, intestinal functioning and recovery, inflammation, and BCM7 breakdown activity, respectively. BCM7 levels in urine were increased following the dual exercise protocol, in the first as well as the second protocol execution, whereas 1 h‐plasma L/R ratio was increased only following the first exercise protocol execution. FABP2, preHP2, and GLP2 were not changed after exercise, whereas calprotectin increased. Plasma citrulline levels following casein ingestion (iAUC) did not increase after exercise, as opposed to resting conditions. Endurance exercise in the glycogen depleted state resulted in a clear increase of BCM7 accumulation in urine, independent of DPP4 activity and intestinal permeability. Therefore, strenuous exercise could have an effect on the amount of food‐derived bioactive peptides crossing the epithelial barrier. The health consequence of increased passage needs more in depth studies.

Investigational new drugs for brain cancer

INTRODUCTION

Despite substantial improvements in standards of care, the most common aggressive pediatric and adult high-grade gliomas (HGG) carry uniformly fatal diagnoses due to unique treatment limitations, high recurrence rates and the absence of effective treatments following recurrence. Recent advancements in our understanding of the pathophysiology, genetics and epigenetics as well as mechanisms of immune surveillance during gliomagenesis have created new knowledge to design more effective and target-directed therapies to improve patient outcomes.

AREAS COVERED

In this review, the authors discuss the critical genetic, epigenetic and immunologic aberrations found in gliomas that appear rational and promising for therapeutic developments in the presence and future. The current state of the latest therapeutic developments including tumor-specific targeted drug therapies, metabolic targeting, epigenetic modulation and immunotherapy are summarized and suggestions for future directions are offered. Furthermore, they highlight contemporary issues related to the clinical development, such as challenges in clinical trials and toxicities.

EXPERT OPINION

The commitment to understanding the process of gliomagenesis has created a catalogue of aberrations that depict multiple mechanisms underlying this disease, many of which are suitable to therapeutic inhibition and are currently tested in clinical trials. Thus, future treatment endeavors will employ multiple treatment modalities that target disparate tumor characteristics personalized to the patient's individual tumor.

Morphological and metabolic adjustments in the small intestine to energy demands of growth, storage, and fasting in the first annual cycle of a hibernating lizard (Tupinambis merianae)

Seasonal plasticity in the small intestine of neonatal tegu lizards was investigated using morphometry and analysis of enzymes involved in supplying energy to the intestinal tissue. In the autumn, the intestinal mass (Mi) was 1.0% of body mass and the scaling exponent b=0.92 indicated that Mi was larger in smaller neonates. During arousal from dormancy Mi was 23% smaller; later in spring, Mi increased 60% in relation to the autumn and the exponent b=0.14 indicated that the recovery was disproportionate in smaller tegus. During the autumn, the intestinal villi were greatly elongated; by midwinter, the Hv, SvEp, and VvEp were smaller than during the autumn (59%, 54%, 29%) and were restored to autumn levels during spring. In the active tegus, the maximum activity (Vmax) of enzymes indicated that the enterocytes can obtain energy from different sources, and possess gluconeogenic capacity. During winter, the Vmax of CS, HOAD, GDH, PEPCK was 40-50% lower in relation to the autumn and spring, while the Vmax of HK, PK, LDH, AST was unchanged. The hypoglycemia and the mucosal atrophy/ischemia during winter would prevent the enterocytes from using glucose, whereas they could slowly oxidize fatty acids released from body stores and amino acids from the tissue proteolysis to satisfy their needs of energy. Contrastingly, starvation during spring caused severe mass loss (50%); the tissue protein and the VvEp and VvLP did not change while the thickness of the muscular layer increased 51%, which suggested different effects along the length of the organ. In addition, the Vmax of the glycolytic enzymes was lower, indicating that a regulatory mechanism would spare blood glucose for vital organs during unanticipated food restriction.

Morphological, biochemical, transcriptional and epigenetic responses to fasting and refeeding in intestine of Xenopus laevis

Background

Amphibians are able to survive for several months without food. However, it is unclear what molecular mechanisms underlie their survival. To characterize the intestinal responses to fasting and refeeding, we investigated morphological, biochemical, transcriptional and epigenetic changes in the intestine from adult male Xenopus laevis.

Results

Frogs were fed for 22 days, fasted for 22 days, or fasted for 21 days and refed for 1 day. Fasting reduced, and refeeding recovered partially or fully, morphological parameters (wet weight of the intestine, circumference of the epithelial layer and number of troughs in a villus-trough unit), activities of digestive enzymes and plasma biochemical parameters (glucose, triglycerides, cholesterol and free fatty acids). Reverse transcription-quantitative polymerase chain reaction analysis revealed overall suppression of the transcript levels by fasting, with various recovery rates on refeeding. Chromatin immunoprecipitation assays on the selected genes whose transcript levels declined with fasting and recovered quickly with refeeding, showed several euchromatin marks in histone (acetylation and methylation) and RNA polymerase II modifications (phosphorylation) with fasting, and returned to the feeding levels by refeeding. The mRNA levels of these genes responded to fasting and refeeding to greater extents than did the pre-mRNA levels, suggesting the involvement of post-transcriptional regulation.

Conclusions

Our results demonstrate that the X. laevis intestine may undergo overall metabolic suppression at least at the transcriptional level to save energy during fasting and quickly recovered to moderate nutritional deficiency by refeeding, and suggest that these dietary responses of the intestine are epigenetically and post-transcriptionally regulated.

Electronic supplementary material

The online version of this article (doi:10.1186/s13578-016-0067-9) contains supplementary material, which is available to authorized users.

Proteome changes in the small intestinal mucosa of broilers (Gallus gallus) induced by high concentrations of atmospheric ammonia

Background

Ammonia is a well-known toxicant both existing in atmospheric and aquatic system. So far, most studies of ammonia toxicity focused on mammals or aquatic animals. With the development of poultry industry, ammonia as a main source of contaminant in the air is causing more and more problems on broiler production, especially lower growth rate. The molecular mechanisms that underlie the negative effects of ammonia on the growth and intestine of broilers are yet unclear. We investigated the growth, gut morphology, and mucosal proteome of Arbor Acres broilers (Gallus gallus) exposed to high concentrations of atmospheric ammonia by performing a proteomics approach integrated with traditional methods.

Results

Exposure to ammonia interfered with the development of immune organ and gut villi. Meanwhile, it greatly reduced daily weight gain and feed intake, and enhanced feed conversion ratio. A total of 43 intestinal mucosal proteins were found to be differentially abundant. Up-regulated proteins are related to oxidative phosphorylation and apoptosis. Down-regulated proteins are related to cell structure and growth, transcriptional and translational regulation, immune response, oxidative stress and nutrient metabolism. These results indicated that exposure to ammonia triggered oxidative stress, and interfered with nutrient absorption and immune function in the small intestinal mucosa of broilers.

Conclusions

These findings have important implications for understanding the toxic mechanisms of ammonia on intestine of broilers, which provides new information that can be used for intervention using nutritional strategies in the future.

Electronic supplementary material

The online version of this article (doi:10.1186/s12953-015-0067-4) contains supplementary material, which is available to authorized users.

Alves LA, Tong Y, Wu D, Joyner LT, Oriá RB, Guerrant RL, Fu Z. Intestinal cell kinase is a novel participant in intestinal cell signaling responses to protein malnutrition

Nutritional deficiency and stress can severely impair intestinal architecture, integrity and host immune defense, leading to increased susceptibility to infection and cancer. Although the intestine has an inherent capability to adapt to environmental stress, the molecular mechanisms by which the intestine senses and responds to malnutrition are not completely understood. We hereby report that intestinal cell kinase (ICK), a highly conserved serine/threonine protein kinase, is a novel component of the adaptive cell signaling responses to protein malnutrition in murine small intestine. Using an experimental mouse model, we demonstrated that intestinal ICK protein level was markedly and transiently elevated upon protein deprivation, concomitant with activation of prominent pro-proliferation and pro-survival pathways of Wnt/β-catenin, mammalian target of rapamycin (mTOR), mitogen-activated protein kinase (MAPK), and protein kinase B (PKB/Akt) as well as increased expression of intestinal stem cell markers. Using the human ileocecal epithelial cell line HCT-8 as an invitro model, we further demonstrated that serum starvation was able to induce up-regulation of ICK protein in intestinal epithelial cells in a reversible manner, and that serum albumin partially contributed to this effect. Knockdown of ICK expression in HCT-8 cells significantly impaired cell proliferation and down-regulated active β-catenin signal. Furthermore, reduced ICK expression in HCT-8 cells induced apoptosis through a caspase-dependent mechanism. Taken together, our findings suggest that increased ICK expression/activity in response to protein deprivation likely provides a novel protective mechanism to limit apoptosis and support compensatory mucosal growth under nutritional stress.

Effect of fasting in the digestive system: histological study of the small intestine in house sparrows

In birds and mammals the metabolic response to fasting has been studied and can be characterized by three consecutive phases reflecting metabolic and physiological adjustments. An effective way to minimize energy expenditure during food scarcity is to decrease the mass of the organs. As the digestive system is metabolically expensive to maintain, the small intestine and the liver are the most affected organs. We evaluated the effects of phase III starvation on the mass of the different organs and histological parameters on house sparrows, a small non-migrant bird. In a short period of time (34 h) we observed a larger reduction in the digestive organ mass when compared to the mass of the body and non-alimentary tissues. Furthermore, the intestinal mass was proportionally more reduced than its length and nominal surface area. A reduction on the intestinal mucosal layer also resulted in a shortening of villus (length and thickness) and crypt depth. Moreover, the morphology of the enterocytes changed from cylindrical to cubical, suggesting that the surface exposed to the lumen was conserved. This may indicate an adaptive response to the moment of refeeding. The nominal surface area/body mass remained constant in both groups and several histological parameters were reduced, suggesting that starving induces the atrophy of the small intestine. However, the goblet cells were conserved after fasting indicating a protective tendency.

Fasting of mice: a review

Fasting of mice is a common procedure performed in association with many different types of experiments mainly in order to reduce variability in investigatory parameters or to facilitate surgical procedures. However, the effects of fasting not directly related to the investigatory parameters are often ignored. The aim of this review is to present and summarize knowledge about the effects of fasting of mice to facilitate optimization of the fasting procedure for any given study and thereby maximize the scientific outcome and minimize the discomfort for the mice and hence ensure high animal welfare. The results are presented from a number of experimental studies, providing evidence for fasting-induced changes in hormone balance, body weight, metabolism, hepatic enzymes, cardiovascular parameters, body temperature and toxicological responses. A description of relevant normal behaviour and standard physiological parameters is given, concluding that mice are primarily nocturnal and consume two-thirds of their total food intake during the night. It is argued that overnight fasting of mice is not comparable with overnight fasting of humans because the mouse has a nocturnal circadian rhythm and a higher metabolic rate. It is suggested that because many physiological parameters are regulated by circadian rhythms, fasting initiated at different points in the circadian rhythm has different impacts and produces different results.

Insulin-like growth factor 1 enhances bile-duct proliferation and fibrosis in Abcb4(-/-) mice

Adamant progression of chronic cholangiopathies towards cirrhosis and limited therapeutic options leave a liver transplantation the only effective treatment. Insulin-like growth factor 1 (IGF1) effectively blocks fibrosis in acute models of liver damage in mice, and a phase I clinical trial suggested an improved liver function. IGF1 targets the biliary epithelium, but its potential benefit in chronic cholangiopathies has not been studied. To investigate the possible therapeutic effect of increased IGF1 expression, we crossed Abcb4(-/-) mice (a model for chronic cholangiopathy), with transgenic animals that overexpress IGF1. The effect on disease progression was studied in the resulting IGF1-overexpressing Abcb4(-/-) mice, and compared to that of Abcb4(-/-) littermates. The specificity of this effect was further studied in an acute model of fibrosis. The overexpression of IGF1 in transgenic Abcb4(-/-) mice resulted in stimulation of fibrogenic processes - as shown by increased expression of Tgfß, and collagens 1, 3 and 4, and confirmed by Sirius red staining and hydroxyproline measurements. Excessive extracellular matrix deposition was favored by raise in Timp1 and Timp2, while a reduction of tPA expression indicated lower tissue remodeling. These effects were accompanied by an increase in expression of inflammation markers like Tnfα, and higher presence of infiltrating macrophages. Finally, increased number of Ck19-expressing cells indicated proliferation of biliary epithelium. In contrast to liver fibrosis associated with hepatocellular damage, IGF1 overexpression does not inhibit liver fibrogenesis in chronic cholangiopathy.

Mice deficient in the respiratory chain gene Cox6a2 are protected against high-fat diet-induced obesity and insulin resistance

Oxidative phosphorylation in mitochondria is responsible for 90% of ATP synthesis in most cells. This essential housekeeping function is mediated by nuclear and mitochondrial genes encoding subunits of complex I to V of the respiratory chain. Although complex IV is the best studied of these complexes, the exact function of the striated muscle-specific subunit COX6A2 is still poorly understood. In this study, we show that Cox6a2-deficient mice are protected against high-fat diet-induced obesity, insulin resistance and glucose intolerance. This phenotype results from elevated energy expenditure and a skeletal muscle fiber type switch towards more oxidative fibers. At the molecular level we observe increased formation of reactive oxygen species, constitutive activation of AMP-activated protein kinase, and enhanced expression of uncoupling proteins. Our data indicate that COX6A2 is a regulator of respiratory uncoupling in muscle and we demonstrate that a novel and direct link exists between muscle respiratory chain activity and diet-induced obesity/insulin resistance.

Trust your gut: galvanizing nutritional interest in intestinal cholesterol metabolism for protection against cardiovascular diseases

Recent studies have demonstrated that the intestine is a key target organ for overall health and longevity. Complementing these studies is the discovery of the trans-intestinal cholesterol efflux pathway and the emerging role of the intestine in reverse cholesterol transport. The surfacing dynamics of the regulation of cholesterol metabolism in the intestine provides an attractive platform for intestine-specific nutritional intervention strategies to lower blood cholesterol levels for protection against cardiovascular diseases. Notably, there is mounting evidence that stimulation of pathways associated with calorie restriction may have a large effect on the regulation of cholesterol removal by the intestine. However, intestinal energy metabolism, specifically the idiosyncrasies surrounding intestinal responses to energy deprivation, is poorly understood. The goal of this paper is to review recent insights into cholesterol regulation by the intestine and to discuss the potential for positive regulation of intestine-driven cholesterol removal through the nutritional induction of pathways associated with calorie restriction.

Overexpression of insulin like growth factor binding protein 5 reduces liver fibrosis in chronic cholangiopathy

The ATP-binding cassette, sub-family B member 4 knock-out mouse (Abcb4(-/-)) is a relevant model for chronic cholangiopathy in man. Due to the lack of this P-glycoprotein in the canalicular membrane of hepatocytes, the secretion of phospholipids into bile is absent, resulting in increased bile toxicity. Expression of insulin like growth factor binding protein 5 (Igfbp5) increases in time in the livers of these mice. It is unclear whether this induction is a consequence of or plays a role in the progression of liver pathology. The aim of this study was therefore to investigate the effect of IGFBP5 induction on the progression of liver fibrosis caused by chronic cholangiopathy. IGFBP5 and, as a control, green fluorescent protein were overexpressed in the hepatocytes of Abcb4(-/-) mice, using an adeno-associated viral vector (AAV). Progression of liver fibrosis was studied 3, 6, and 12 weeks after vector injection by analyzing serum parameters, collagen deposition, expression of pro-fibrotic genes, inflammation and oxidative stress. A single administration of the AAV vectors provided prolonged expression of IGFBP5 and GFP in the livers of Abcb4(-/-) mice. Compared to GFP control, fractional liver weight, extracellular matrix deposition and amount of activated hepatic stellate cells significantly decreased in IGFBP5 overexpressing mice even 12 weeks after treatment. This effect was not due to a change in bile composition, but driven by reduced inflammation, oxidative stress, and proliferation. Overexpression of IGFBP5 seems to have a protective effect on liver pathology in this model for chronic cholangiopathy.

Loss of glucagon-like peptide-2-induced proliferation following intestinal epithelial insulin-like growth factor-1-receptor deletion

BACKGROUND & AIMS

Glucagon-like peptide-2 (GLP-2) is an intestinal hormone that promotes growth of the gastrointestinal tract. Although insulin-like growth factor (IGF)-1 and the IGF-1 receptor (IGF-1R) are required for GLP-2-induced proliferation of crypt cells, little is known about localization of the IGF-1R which mediates the intestinotropic actions of GLP-2.

METHODS

We examined intestinal growth and proliferative responses in mice with conditional deletion of IGF-1R from intestinal epithelial cells (IE-igf1rKO) after acute administration (30-90 min) of GLP-2, in response to 24-hour fasting and re-feeding (to induce GLP-2-dependent adaptation), and after chronic exposure (10 days) to GLP-2.

RESULTS

IE-igf1rKO mice had normal small intestinal weight, morphometric parameters, proliferative indices, and distribution of differentiated epithelial cell lineages. Acute administration of GLP-2 increased nuclear translocation of β-catenin in non-Paneth crypt cells and stimulated the crypt-cell proliferative marker c-Myc in control but not IE-igf1rKO mice. Small intestinal weight, crypt depth, villus height, and crypt-cell proliferation were decreased in control and IE-igf1rKO mice after 24-hour fasting. Although re-feeding control mice restored all of these parameters, re-fed IE-igf1rKO mice had reductions in adaptive regrowth of the villi and crypt-cell proliferation. Control mice that were given chronic GLP-2 had increases in small intestinal weight, mucosal cross-sectional area, crypt depth, villus height, and crypt-cell proliferation. However, the GLP-2-induced increase in crypt-cell proliferation was not observed in IE-igf1rKO mice, and growth of the crypt-villus axis was reduced.

CONCLUSIONS

The proliferative responses of the intestinal epithelium to exogenous GLP-2 administration and conditions of GLP-2-dependent adaptive re-growth require the intestinal epithelial IGF-1R.

Starvation compromises Paneth cells

Lack of enteral feeding, with or without parenteral nutritional support, is associated with increased intestinal permeability and translocation of bacteria. Such translocation is thought to be important in the high morbidity and mortality rates of patients who receive nothing by mouth. Recently, Paneth cells, important constituents of innate intestinal immunity, were found to be crucial in host protection against invasion of both commensal and pathogenic bacteria. This study investigates the influence of food deprivation on Paneth cell function in a mouse starvation model. Quantitative PCR showed significant decreases in mRNA expression of typical Paneth cell antimicrobials, lysozyme, cryptdin, and RegIIIγ, in ileal tissue after 48 hours of food deprivation. Protein expression levels of lysozyme and RegIIIγ precursor were also significantly diminished, as shown by Western blot analysis and IHC. Late degenerative autophagolysosomes and aberrant Paneth cell granules in starved mice were evident by electron microscopy, Western blot analysis, and quantitative PCR. Furthermore, increased bacterial translocation to mesenteric lymph nodes coincided with Paneth cell abnormalities. The current study demonstrates the occurrence of Paneth cell abnormalities during enteral starvation. Such changes may contribute to loss of epithelial barrier function, causing the apparent bacterial translocation in enteral starvation.

The "cryptic" mechanism of action of glucagon-like peptide-2

Glucagon-like peptide-2 (GLP-2) is a peptide hormone with multiple beneficial effects on the intestine, including expansion of the mucosal surface area through stimulation of crypt cell proliferation, as well as enhancement of nutrient digestion and absorption. Recent advances in clinical trials involving GLP-2 necessitate elucidation of the exact signaling pathways by which GLP-2 acts. In particular, the GLP-2 receptor has been localized to several intestinal cell types that do not include the proliferating crypt cells, and the actions of GLP-2 have thus been linked to a complex network of indirect mediators that induce diverse signaling pathways. The intestinotropic actions of GLP-2 on the colon have been shown to be mediated through the actions of keratinocyte growth factor and insulin-like growth factor (IGF)-2, whereas small intestinal growth has been linked to IGF-1, IGF-2, and ErbB ligands, as well as the IGF-1 receptor and ErbB. The cellular source of these mediators remains unclear, but it likely includes the intestinal subepithelial myofibroblasts. Conversely, the anti-inflammatory and blood flow effects of GLP-2 are dependent on vasoactive intestinal polypeptide released from submucosal enteric neurons and nitric oxide, respectively. Finally, recent studies have suggested that GLP-2 not only modulates intestinal stem cell behavior but may also promote carcinogenesis in models of sporadic colon cancer. Further consideration of the molecular cross-talk and downstream signaling pathways mediating the intestinotropic effects of GLP-2 is clearly warranted.

Interorgan coordination of the murine adaptive response to fasting

Starvation elicits a complex adaptive response in an organism. No information on transcriptional regulation of metabolic adaptations is available. We, therefore, studied the gene expression profiles of brain, small intestine, kidney, liver, and skeletal muscle in mice that were subjected to 0-72 h of fasting. Functional-category enrichment, text mining, and network analyses were employed to scrutinize the overall adaptation, aiming to identify responsive pathways, processes, and networks, and their regulation. The observed transcriptomics response did not follow the accepted "carbohydrate-lipid-protein" succession of expenditure of energy substrates. Instead, these processes were activated simultaneously in different organs during the entire period. The most prominent changes occurred in lipid and steroid metabolism, especially in the liver and kidney. They were accompanied by suppression of the immune response and cell turnover, particularly in the small intestine, and by increased proteolysis in the muscle. The brain was extremely well protected from the sequels of starvation. 60% of the identified overconnected transcription factors were organ-specific, 6% were common for 4 organs, with nuclear receptors as protagonists, accounting for almost 40% of all transcriptional regulators during fasting. The common transcription factors were PPARα, HNF4α, GCRα, AR (androgen receptor), SREBP1 and -2, FOXOs, EGR1, c-JUN, c-MYC, SP1, YY1, and ETS1. Our data strongly suggest that the control of metabolism in four metabolically active organs is exerted by transcription factors that are activated by nutrient signals and serves, at least partly, to prevent irreversible brain damage.

ErbB activity links the glucagon-like peptide-2 receptor to refeeding-induced adaptation in the murine small bowel

BACKGROUND & AIMS

The small bowel mucosa is sensitive to nutrients and undergoes rapid adaptation to nutrient deprivation and refeeding through changes in apoptosis and cell proliferation, respectively. Although glucagon-like peptide-2 (GLP-2) exerts trophic effects on the gut and levels increase with refeeding, mechanisms linking GLP-2 to mucosal adaptation to refeeding remain unclear.

METHODS

Fasting and refeeding were studied in wild-type (WT) and Glp2r(-/-) mice and in WT mice treated with the pan ErbB inhibitor CI-1033. Experimental end points included intestinal weights, histomorphometry, gene and protein expression, and crypt cell proliferation.

RESULTS

Fasting was associated with significant reductions in small bowel mass, decreased crypt plus villus height, and reduced crypt cell proliferation. Refeeding increased plasma levels of GLP-2, reversed small bowel atrophy, increased villus height and cell number, and stimulated jejunal crypt cell proliferation. In contrast, refeeding failed to increase small bowel weight, crypt cell proliferation, or villus cell number in Glp2r(-/-) mice. Levels of mRNA transcripts for egf, kgf, and igfr were lower in fasted Glp2r(-/-) mice. Epidermal growth factor but not insulin-like growth factor-1 restored the intestinal adaptive response to refeeding in Glp2r(-/-) mice. Furthermore, CI-1033 prevented adaptive crypt cell proliferation, Akt activation, and induction of ErbB ligand gene expression after refeeding. Up-regulation of ErbB ligand expression and intestinal Akt phosphorylation were significantly diminished in refed Glp2r(-/-) mice.

CONCLUSIONS

These findings identify Glp2r and ErbB pathways as essential components of the signaling network regulating the adaptive mucosal response to refeeding in the mouse intestine.

Unexpected effects of fasting on murine lipid homeostasis--transcriptomic and lipid profiling

BACKGROUND & AIMS

Starvation induces massive perturbations in metabolic pathways involved in energy metabolism, but its effect on the metabolism of lipids, particularly cholesterol, is little understood.

METHODS

A comparative genomic analysis of the gut and the liver in response to fasting was performed, with intestinal perfusion and lipid profiling of the plasma, bile, liver, intestinal tissue, perfusate, and faeces in FVB mice.

RESULTS

The expression profiles suggested increased cholesterol trafficking in the liver and decreased trafficking in the small intestine. Plasma cholesterol concentrations significantly increased, and triglycerides decreased in fasting. Surprisingly, in prolonged fasting, the biliary bile salt and lipid output rates increased, with increased hepatic and intestinal lipid turnover, and enhanced trans-intestinal cholesterol excretion. In contrast, faecal sterol loss declined sharply. To investigate whether the increased biliary phospholipid secretion could nourish the intestinal epithelium, we studied the histology of the small intestines upon fasting in multidrug resistant protein 2 deficient mice with scarce biliary phospholipids. Their adaptive biliary response to fasting was lost, while the shortage of biliary phospholipids strongly induced apoptosis and proliferation in the small intestine and increased the number of mucin-producing cells.

CONCLUSION

Even with no dietary fat, lipid levels remain remarkably constant in the murine liver and intestines during prolonged fasting. The biliary system, always assumed to be coupled to the postprandial response, shows a paradoxical increase in activity. We hypothesise that biliary lipids are mobilised to supply the enterocytes with luminal fuel and to stabilise transport systems in the intestine for ensuring a rapid recovery when the food supply resumes.

The transcriptomic signature of fasting murine liver

Background

The contribution of individual organs to the whole-body adaptive response to fasting has not been established. Hence, gene-expression profiling, pathway, network and gene-set enrichment analysis and immunohistochemistry were carried out on mouse liver after 0, 12, 24 and 72 hours of fasting.

Results

Liver wet weight had declined ~44, ~5, ~11 and ~10% per day after 12, 24, 48 and 72 hours of fasting, respectively. Liver structure and metabolic zonation were preserved. Supervised hierarchical clustering showed separation between the fed, 12–24 h-fasted and 72 h-fasted conditions. Expression profiling and pathway analysis revealed that genes involved in amino-acid, lipid, carbohydrate and energy metabolism responded most significantly to fasting, that the response peaked at 24 hours, and had largely abated by 72 hours. The strong induction of the urea cycle, in combination with increased expression of enzymes of the tricarboxylic-acid cycle and oxidative phosphorylation, indicated a strong stimulation of amino-acid oxidation peaking at 24 hours. At this time point, fatty-acid oxidation and ketone-body formation were also induced. The induction of genes involved in the unfolded-protein response underscored the cell stress due to enhanced energy metabolism. The continuous high expression of enzymes of the urea cycle, malate-aspartate shuttle, and the gluconeogenic enzyme Pepck and the re-appearance of glycogen in the pericentral hepatocytes indicate that amino-acid oxidation yields to glucose and glycogen synthesis during prolonged fasting.

Conclusion

The changes in liver gene expression during fasting indicate that, in the mouse, energy production predominates during early fasting and that glucose production and glycogen synthesis become predominant during prolonged fasting.

Proteomic analysis of the winter-protected phenotype of hibernating ground squirrel intestine

The intestine of hibernating ground squirrels is protected against damage by ischemia-reperfusion (I/R) injury. This resistance does not depend on the low body temperature of torpor; rather, it is exhibited during natural interbout arousals that periodically return hibernating animals to euthermia. Here we use fluorescence two-dimensional difference gel electrophoresis (DIGE) to identify protein spot differences in intestines of 13-lined ground squirrels in the sensitive and protected phases of the circannual hibernation cycle, comparing sham-treated control animals with those exposed to I/R. Protein spot differences distinguished the sham-treated summer and hibernating samples, as well as the response to I/R between summer and hibernating intestines. The majority of protein changes among these groups were attributed to a seasonal difference between summer and winter hibernators. Many of the protein spots that differed were unambiguously identified by high-pressure liquid chromatography followed by tandem mass spectrometry of their constituent peptides. Western blot analysis confirmed significant upregulation for three of the proteins, albumin, apolipoprotein A-I, and ubiquitin hydrolase L1, that were identified in the DIGE analysis as increased in sham-treated hibernating squirrels compared with sham-treated summer squirrels. This study identifies several candidate proteins that may contribute to hibernation-induced protection of the gut during natural torpor-arousal cycles and experimental I/R injury. It also reveals the importance of enterocyte maturation in defining the hibernating gut proteome and the role of changing cell populations for the differences between sham and I/R-treated summer animals.