Reversible brain response to an intragastric load of l -lysine under l -lysine depletion in conscious rats

Lysine Deprivation Suppresses Adipogenesis in 3T3-L1 Cells: A Transcriptome Analysis

Growing evidence proves that amino acid restriction can reverse obesity by reducing adipose tissue mass. Amino acids are not only the building blocks of proteins but also serve as signaling molecules in multiple biological pathways. The study of adipocytes' response to amino acid level changes is crucial. It has been reported that a low concentration of lysine suppresses lipid accumulation and transcription of several adipogenic genes in 3T3-L1 preadipocytes. However, the detailed lysine-deprivation-induced cellular transcriptomic changes and the altered pathways have yet to be fully studied. Here, using 3T3-L1 cells, we performed RNA sequencing on undifferentiated and differentiated cells, and differentiated cells under a lysine-free environment, and the data were subjected to KEGG enrichment. We found that the differentiation process of 3T3-L1 cells to adipocytes required the large-scale upregulation of metabolic pathways, mainly on the mitochondrial TCA cycle, oxidative phosphorylation, and downregulation of the lysosomal pathway. Single amino acid lysine depletion suppressed differentiation dose dependently. It disrupted the metabolism of cellular amino acids, which could be partially reflected in the changes in amino acid levels in the culture medium. It inhibited the mitochondria respiratory chain and upregulated the lysosomal pathway, which are essential for adipocyte differentiation. We also noticed that cellular interleukin 6 (IL6) expression and medium IL6 level were dramatically increased, which was one of the targets for suppressing adipogenesis induced by lysine depletion. Moreover, we showed that the depletion of some essential amino acids such as methionine and cystine could induce similar phenomena. This suggests that individual amino acid deprivation may share some common pathways. This descriptive study dissects the pathways for adipogenesis and how the cellular transcriptome was altered under lysine depletion.

Lysine or Threonine Deficiency Decreases Body Weight Gain in Growing Rats despite an Increase in Food Intake without Increasing Energy Expenditure in Response to FGF21

The objective of this study is to evaluate the effects of a strictly essential amino acid (lysine or threonine; EAA) deficiency on energy metabolism in growing rats. Rats were fed for three weeks severely (15% and 25% of recommendation), moderately (40% and 60%), and adequate (75% and 100%) lysine or threonine-deficient diets. Food intake and body weight were measured daily and indirect calorimetry was performed the week three. At the end of the experimentation, body composition, gene expression, and biochemical analysis were performed. Lysine and threonine deficiency induced a lower body weight gain and an increase in relative food intake. Lysine or threonine deficiency induced liver FGF21 synthesis and plasma release. However, no changes in energy expenditure were observed for lysine deficiency, unlike threonine deficiency, which leads to a decrease in total and resting energy expenditure. Interestingly, threonine severe deficiency, but not lysine deficiency, increase orexigenic and decreases anorexigenic hypothalamic neuropeptides expression, which could explain the higher food intake. Our results show that the deficiency in one EAA, induces a decrease in body weight gain, despite an increased relative food intake, without any increase in energy expenditure despite an induction of FGF21.

Integrating RNA-sequencing and untargeted LC-MS metabolomics to evaluate the effect of lysine deficiency on hepatic functions in Holstein calves

Lysine (Lys) is majorly metabolized in the liver. The liver functional consequences of a dietary Lys deficiency in young Holstein calves are unknown. This study aimed to investigate the effects of Lys deficiency in Holstein calf livers using RNA-sequencing and untargeted LC-MS metabolomics. Calves (n = 36; initial body weight 101.2 ± 10.8 kg; 90-day-old) were fed restricted diets, for 90 days, containing 19.2% crude protein that varied in Lys content (PC group 1.21%; PC-Lys group 0.85%; dry matter basis) for 90 days. Body weight, average daily gain, gain/feed, and Lys intake were significantly decreased in response to Lys deficiency (P < 0.05). Dry matter intake was not altered (P > 0.05). Network and pathway analyses revealed that noradrenaline, adenosine 5'-monophosphate, acetyl-CoA, and coenzyme A were significantly decreased. Regulating of lipolysis in adipocytes pathway and fatty acid degradation pathway were downregulated. We also identified eight significantly differentially expressed genes (SDEGs), among which adrenoceptor beta 2 (ADRB2), WAP four-disulfide core domain 2 (WFDC2), and claudin-4 (CLDN4) were associated with inhibition of lipolysis, and carbon catabolite repression 4-like (CCRN4L), FOS like 2 (FOSL2), and arginase 2 (ARG2) were associated with inhibiting lipid synthesis. Correlation tests showed that coenzyme A was strongly correlated with SDEGs (0.82 ≤|r|≤ 0.96). Acetyl-CoA and adenosine 5'-monophosphate were strongly correlated with CCRN4L (0.90 ≤|r|≤ 0.92), indicating a strong correlation between the changes in SDEGs and these metabolites. In conclusion, Lys deficiency caused dysplasia and affected lipid metabolism in the liver by inhibiting lipolysis and lipid synthesis in calves.

Differences in BOLD responses to intragastrically infused glucose and saccharin in rats

The postingestive effect is different between caloric and noncaloric sweeteners. The gut administration of glucose induces a preference for flavored water which is paired with the intragastric infusion of glucose. However, a comparison of the brain response to the gut glucose and saccharin stimuli still remains to be demonstrated. Here, using functional magnetic resonance imaging, we investigated the blood oxygenation level-dependent signal response to gut glucose and saccharin in the brain of conscious rats. Glucose induced a positive signal increase in the amygdala and nucleus accumben, both of which receive dopaminergic input from the ventral tegmental area. In contrast, saccharin administration did not activate these areas. Both glucose and saccharin increased the blood oxygenation level-dependent signal intensity in the insular cortex and the nucleus of the solitary tract. These results show that there were significant differences between postingestive glucose and saccharin-induced increases in the blood oxygenation level-dependent signal in rats. Together with previous findings, these results suggest distinct activation patterns in the brain for both glucose and saccharin, which is partially due to different changes of internal signals, including the blood glucose and insulin levels.

Different TRPV1-mediated brain responses to intragastric infusion of capsaicin and capsiate

Capsaicin and capsiate, which is an analogue of capsaicin, are agonists of capsaicin-binding transient potential vanilloid 1 (TRPV1) receptors. However, their physiological effects are different. Capsaicin induces thermogenesis and nociception, while the different kinetics of capsiate result in thermogenesis without nociception in the oral cavity. In the present study, using functional magnetic resonance imaging, we compared the brain activation after intragastric infusion of non-nociceptive levels of capsaicin and capsiate in wild-type and TRPV1-knockout (KO) mice. Capsaicin activated several brain regions, such as the periaqueductal grey (PAG), thalamic nuclei and hypothalamus, including the medial preoptic area (mPOA) and ventromedial hypothalamus (VMH). Most of these areas were not activated in TRPV1-KO mice. Capsiate activated several regions, including the thalamic nuclei, mPOA and VMH but not PAG in wild-type mice. Most of the activated areas were not activated by intragastric capsiate infusion in TRPV1-KO mice. These results demonstrate that TRPV1 is critical for the induction of activation in the hypothalamus by capsaicin and capsiate, and these distinct brain activations could help to explain the individual physiological reactions of capsaicin and capsiate.