Restriction of essential amino acids dictates the systemic metabolic response to dietary protein dilution.

Dietary protein dilution (DPD) promotes metabolic-remodelling and -health but the precise nutritional components driving this response remain elusive. Here, by mimicking amino acid (AA) supply from a casein-based diet, we demonstrate that restriction of dietary essential AA (EAA), but not non-EAA, drives the systemic metabolic response to total AA deprivation; independent from dietary carbohydrate supply. Furthermore, systemic deprivation of threonine and tryptophan, independent of total AA supply, are both adequate and necessary to confer the systemic metabolic response to both diet, and genetic AA-transport loss, driven AA restriction. Dietary threonine restriction (DTR) retards the development of obesity-associated metabolic dysfunction. Liver-derived fibroblast growth factor 21 is required for the metabolic remodelling with DTR. Strikingly, hepatocyte-selective establishment of threonine biosynthetic capacity reverses the systemic metabolic response to DTR. Taken together, our studies of mice demonstrate that the restriction of EAA are sufficient and necessary to confer the systemic metabolic effects of DPD.

Indispensable Amino Acid-Deficient Diets Induce Seizures in Ketogenic Diet-Fed Rodents, Demonstrating a Role for Amino Acid Balance in Dietary Treatments for Epilepsy.

Background: Low protein amounts are used in ketogenic diets (KDs), where an essential (indispensable) amino acid (IAA) can become limiting. Because the chemically sensitive, seizurogenic, anterior piriform cortex (APC) is excited by IAA limitation, an imbalanced KD could exacerbate seizure activity. Objective: We questioned whether dietary IAA depletion worsens seizure activity in rodents fed KDs. Methods: In a series of 6 trials, male rats or gerbils of both sexes (6-8/group) were given either control diets (CDs) appropriate for each trial, a KD, or a threonine-devoid (ThrDev) diet for ≥7 d, and tested for seizures using various stimuli. Microchip analysis of rat APCs was also used to determine if changes in transcripts for structures relevant to seizurogenesis are affected by a ThrDev diet. Glutamate release was measured in microdialysis samples from APCs during the first meal after 7 d on a CD or a ThrDev diet. Results: Adult rats showed increased susceptibility to seizures in both chemical (58%) and electroshock (doubled) testing after 7 d on a ThrDev diet compared with CD (each trial, P ≤ 0.05). Seizure-prone Mongolian gerbils had fewer seizures after receiving a KD, but exacerbated seizures (68%) after 1 meal of KD minus Thr (KD-T compared with CD, P < 0.05). In kindled rats fed KD-T, both counts (19%) and severities (77%) of seizures were significantly elevated (KD-T compared with CD, P < 0.05). Gene transcript changes were consistent with enhanced seizure susceptibility (7-21 net-fold increases, P = 0.045-0.001) and glutamate release into the APC was increased acutely (4-fold at 20 min, 2.6-fold at 60 min, P < 0.05) after 7 d on a ThrDev diet. Conclusion: Seizure severity in rats and gerbils was reduced after KDs and exacerbated by ThrDev, both in KD- and CD-fed animals, consistent with the mechanistic studies. We suggest that a complete protein profile in KDs may improve IAA balance in the APC, thereby lowering the risk of seizures.

Dietary threonine deficiency depressed the disease resistance, immune and physical barriers in the gills of juvenile grass carp (Ctenopharyngodon idella) under infection of Flavobacterium columnare.

This study was conducted to investigate the effects of dietary threonine on the disease resistance, gill immune and physical barriers function of juvenile grass carp (Ctenopharyngodon idella). A total of 1080 juveniles were fed six iso-nitrogenous diets containing graded levels of threonine (3.99-21.66 g kg-1 diet) for 8 weeks, and then challenged with Flavobacterium columnare. Results showed that threonine deficiency (3.99 g kg-1 diet): (1) increased the gill rot morbidity after exposure to F. columnare; (2) attenuated the gill immune barrier function by decreasing antimicrobial substances production, up-regulating the mRNA levels of pro-inflammatory cytokines (except IL-12p40), and down-regulating the anti-inflammatory cytokines partly due to the modulation of NF-κB and TOR signaling. (3) disrupt the gill tight junction complexes by down-regulating TJs (claudin-3, -b, -c, 12, occludin, ZO-1 and ZO-2) and up-regulating TJs (claudin-7a, -7b) as well as related signaling molecule myosin light chain kinase mRNA levels (P < 0.05). (4) exacerbated the gill apoptosis by up-regulating cysteinyl aspartic acid-protease-3, 8, 9, c-Jun N-terminal kinases and mediating apoptosis related factors mRNA levels (P < 0.05); (5) exacerbated oxidative injury with increased reactive oxygen species, malondialdehyde and protein carbonyl contents (P < 0.05), decreased the antioxidant related enzymes activities and corresponding mRNA levels (except glutathione peroxidase-1b and glutathione-S-transferase-omega 2) as well as glutathione contents (P < 0.05) partly ascribe to the abridgement of NF-E2-related factor 2 signaling [Nrf2/Keap1a (not Keap1b)] in fish gill. Overall, threonine deficiency depressed the disease resistance, and impaired immune and physical barriers in fish gill. Finally, based on the gill rot morbidity and biochemical indices (immune indices LA activity and antioxidant indices MDA content), threonine requirements for juvenile grass carp (9.53-53.43 g) were estimated to be 15.32 g kg-1 diet (4.73 g 100 g-1 protein), 15.52 g kg-1 diet (4.79 g 100 g-1 protein), 15.46 g kg-1 diet (4.77 g 100 g-1 protein), respectively.

Threonine deficiency decreased intestinal immunity and aggravated inflammation associated with NF-κB and target of rapamycin signalling pathways in juvenile grass carp (Ctenopharyngodon idella) after infection with Aeromonas hydrophila.

This study aimed to investigate the impacts of dietary threonine on intestinal immunity and inflammation in juvenile grass carp. Six iso-nitrogenous semi-purified diets containing graded levels of threonine (3·99-21·66 g threonine/kg) were formulated and fed to fishes for 8 weeks, and then challenged with Aeromonas hydrophila for 14 d. Results showed that, compared with optimum threonine supplementation, threonine deficiency (1) decreased the ability of fish against enteritis, intestinal lysozyme activities (except in the distal intestine), acid phosphatase activities, complement 3 (C3) and C4 contents and IgM contents (except in the proximal intestine (PI)), and it down-regulated the transcript abundances of liver-expressed antimicrobial peptide (LEAP)-2A, LEAP-2B, hepcidin, IgZ, IgM and ß-defensin1 (except in the PI) (P<0·05); (2) could up-regulate intestinal pro-inflammatory cytokines TNF-α, IL-1ß, IL-6, IL-8 and IL-17D mRNA levels partly related to NF-κB signalling; (3) could down-regulate intestinal anti-inflammatory cytokine transforming growth factor (TGF)-ß1, TGF-ß2, IL-4/13A (not IL-4/13B) and IL-10 mRNA levels partly by target of rapamycin signalling. Finally, on the basis of the specific growth rate, against the enteritis morbidity and IgM contents, the optimum threonine requirements were estimated to be 14·53 g threonine/kg diet (4·48 g threonine/100 g protein), 15.05 g threonine/kg diet (4·64 g threonine/100 g protein) and 15·17 g threonine/kg diet (4·68 g threonine/100 g protein), respectively.

Effect of threonine deficiency on intestinal integrity and immune response to feed withdrawal combined with coccidial vaccine challenge in broiler chicks.

For this study, threonine (Thr) deficiency was hypothesised to exacerbate the intestinal damage induced by feed withdrawal with coccidial infection because of its high obligatory requirement by the gut; two dietary Thr treatments (0·49 and 0·90 %) were applied to chicks from 0 to 21 d of age. At 13 d of age, feed was withdrawn for 24 h from one-half of birds of each dietary treatment with subsequent gavage of a 25× dose of coccidial vaccine. Overall, there were four treatments with eight replicate cages per treatment. Under combined challenge, birds fed the Thr-deficient diet had 38 % lower 13-21-d body weight gain (P≤0·05) compared with birds fed the Thr-control diet. At 21 d, the challenged group fed low Thr had higher number of oocysts (+40 %, P=0·03) and lower crypt depth (-31 %, P0·05). Overall, Thr deficiency worsened the detrimental effects of combined feed withdrawal and coccidial infection on growth performance and oocyst shedding by impairing intestinal morphology, barrier function, lymphocyte profiles and their cytokine expressions.

Milk protein yield and mammary metabolism are affected by phenylalanine deficiency but not by threonine or tryptophan deficiency.

Efficient milk protein synthesis requires that the essential AA be presented to the mammary gland in the right amount and proportion to maximize protein synthesis and minimize losses. This study investigated the effects of individual AA deficiencies on cow productivity, mammary metabolism, and glucose whole-body rate of appearance. Five Holstein cows were used in a 5 × 5 Latin square design trial with 10-d periods. Treatments were abomasal infusions of (1) water (CTL); (2) complete AA mixture (TAA); (3) TAA without Phe (No-Phe); (4) TAA without Thr (No-Thr); and (5) TAA without Trp (No-Trp). Each treatment was compared with TAA. Treatment did not affect milk, fat, or lactose yields. Arterial concentrations of Phe, Thr, and Trp decreased with their respective deletions by 60, 76, and 69%. In response to the decreased arterial supply of the deleted AA, mammary plasma flow significantly increased by 55% with No-Thr but did not increase with No-Phe or No-Trp. Mammary uptake of Phe was reduced by No-Phe, accompanied by a reduced milk protein yield; uptakes of Thr and Trp were not affected by their respective deletions, and milk protein yield did not decrease with these treatments. Deletion of Phe tended to reduce its mammary uptake relative to milk output (U:O), accompanied by an increased U:O of Tyr, but deletion of Thr and Trp did not affect the U:O of the corresponding AA. Plasma urea-N concentration was lower with CTL and tended to be higher with No-Phe. Arterial concentrations and mammary uptake of acetate, ß-hydroxybutyrate, glucose, and lactate were unaffected by treatment. Treatment had no effect on glucose rate of appearance at the whole-body level. Lactose output as a percentage of glucose whole-body rate of appearance was not affected by treatment. Overall, the study indicated that a deficiency of Phe negatively affected productivity and mammary metabolism but that a deficiency of Thr or Trp did not.

A deficiency or an excess of dietary threonine level affects weight gain, enzyme activity, immune response and immune-related gene expression in juvenile blunt snout bream (Megalobrama amblycephala).

A feeding trial was conducted to investigate the impacts of deficient and excess dietary threonine levels on weight gain, plasma enzymes activities, immune responses and expressions of immune-related genes in the intestine of juvenile blunt snout bream. Triplicate groups of fish (initial weight 3.01 ± 0.01 g, 30 fish per tank) were fed with deficient (0.58%), optimum (1.58%) and excess (2.58%) threonine level diets to near satiation four times a day for 9 weeks. A mixture of l-amino acids was supplemented to simulate the whole body amino acid pattern of blunt snout bream, except for threonine. The results showed that both deficiency and excess threonine level diets significantly (P < 0.05) reduced the weight gain of blunt snout bream. Excess dietary threonine level triggered plasma aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities (P < 0.05); whereas superoxide dismutase (SOD) activity was not significantly influenced by imbalanced-dietary threonine level (P > 0.05). Plasma complement component 3 (C3) and component 4 (C4) concentrations were significantly depressed by the deficiency of dietary threonine (P < 0.05). Dietary threonine regulated the target of rapamycin (TOR), eukaryotic translation initiation factor 4E-binding protein 2 (4E-BP2), tumour necrosis factor alpha (TNF-α) and copper-zinc superoxide dismutase (Cu/Zn-SOD) gene expressions in the intestine of blunt snout bream, which may go further to explain the adverse effects of a deficient and/or an excess dietary threonine level on growth, immunity and health of fish. Furthermore, the present study also suggests that an optimum dietary threonine could play an important role in improving growth, enhancing immune function and maintaining health of fish.

Effect of xylanase and glucanase supplementation to a cereal-based, threonine-limited diet on the nitrogen balance of growing pigs.

In cereal-based diets, non-starch polysaccharides (NSP) lower precaecal nutrient absorption and increase endogenous protein and amino acid (AA) losses. Adding exogenous NSP-degrading enzymes aims amongst others to reduce these negative effects and to thereby improve protein and AA supply. However, biased results exist in the literature on their efficacy in growing pigs. Hence, the objective of this study was to analyse the effects of different levels of xylanase and beta-glucanase supplementation. Nitrogen (N) retention from a threonine-limited diet was chosen as an indirect indicator for differences in praecaecal threonine absorption and endogenous protein and AA losses. During three balance periods, 12 male pigs with a bodyweight of 31-66 kg were used in a cross-over design. They received three different diets based on wheat, barley, rye, and soybean meal containing 0, 40 or 80 mg/kg of an enzyme preparation containing endo-1,4,-beta-xylanase and endo-1,4-beta-glucanase. N excretion and retention were identical in animals of the different treatment groups, stressing that enzyme supplementation did not affect threonine absorption and/or endogenous protein and AA losses neither at medium nor at high supplementation level. Hence, in the present trial, beta-glucanase and xylanase addition to cereal diets did not improve protein and AA availability in growing pigs of a body weight > 30 kg.

The anterior piriform cortex is sufficient for detecting depletion of an indispensable amino acid, showing independent cortical sensory function.

Protein synthesis requires a continuous supply of all of the indispensable (essential) amino acids (IAAs). If any IAA is deficient, animals must obtain the limiting amino acid by diet selection. Sensing of IAA deficiency requires an intact anterior piriform cortex (APC), but does it act alone? Shortly after rats begin eating an IAA-deficient diet, the meal ends and EPSPs are activated in the APC; from there, neurons project to feeding circuits; the meal ends within 20 min. Within the APC in vivo, uncharged tRNA activates the general amino acid control non-derepressing 2 (GCN2) enzyme system increasing phosphorylation of eukaryotic initiation factor (P-eIF2α), which blocks general protein synthesis. If this paleocortex is sufficient for sensing IAA depletion, both neuronal activation and P-eIF2α should occur in an isolated APC slice. We used standard techniques for electrophysiology and immunohistochemistry. After rats ate IAA-devoid or -imbalanced diets, their depleted slices responded to different stimuli with increased EPSP amplitudes. Slices from rats fed a control diet were bathed in artificial CSF replete with all amino acids with or without the IAA, threonine, or a tRNA synthetase blocker, l-threoninol, or its inactive isomer, d-threoninol. Thr depletion in vitro increased both EPSP amplitudes and P-eIF2α. l (but not d)-threoninol also increased EPSP amplitudes relative to control. Thus, we show independent excitation of the APC with responses parallel to those known in vivo. These data suggest a novel idea: in addition to classical processing of peripheral sensory input, direct primary sensing may occur in mammalian cortex.

The effects of supplementing a low-protein threonine-deficient diet with different sources of non-essential amino acids on nitrogen retention and gut structure in young pigs.

A threonine-adequate control diet and four Thr-deficient (80% of requirement) diets were formulated to contain similar amounts of digestible lysine and the recommended pattern of other standardised ileal digestible essential amino acids (except Thr). Threonine-deficient diets were supplemented with different amounts and sources of non-essential amino acids, namely 0, 20, and 40 g wheat gluten (WG) protein per kg diet or 17.6 g monosodium glutamate (MSG) per kg diet. Each diet was fed for 28 days to six piglets (initial BW 15 kg). Supplementation of the Thr-deficient diet with WG or MSG had a positive effect on N retention (5.9-8.5%) in younger, but not in older, pigs. Furthermore, it reduced the plasma Thr level and increased threonine dehydrogenase activity in the liver and pancreas. The treatment effects on intestinal morphology differed according to the histological criteria used. It may be concluded that non-essential amino acids added to the low-protein diet deficient in Thr seem to improve utilisation of these amino acids for protein deposition in very young pigs, whereas their effects on the structure of the small intestine are equivocal.

Fungal homoserine kinase (thr1Delta) mutants are attenuated in virulence and die rapidly upon threonine starvation and serum incubation.

The fungally conserved subset of amino acid biosynthetic enzymes not present in humans offer exciting potential as an unexploited class of antifungal drug targets. Since threonine biosynthesis is essential in Cryptococcus neoformans, we further explored the potential of threonine biosynthetic enzymes as antifungal drug targets by determining the survival in mice of Saccharomyces cerevisiae homoserine kinase (thr1Delta) and threonine synthase (thr4Delta) mutants. In striking contrast to aspartate kinase (hom3Delta) mutants, S. cerevisiae thr1Delta and thr4Delta mutants were severely depleted after only 4 h in vivo. Similarly, Candida albicans thr1Delta mutants, but not hom3Delta mutants, were significantly attenuated in virulence. Consistent with the in vivo phenotypes, S. cerevisiae thr1Delta and thr4Delta mutants as well as C. albicans thr1Delta mutants were extremely serum sensitive. In both species, serum sensitivity was suppressed by the addition of threonine, a feedback inhibitor of Hom3p. Because mutation of the HOM3 and HOM6 genes, required for the production of the toxic pathway intermediate homoserine, also suppressed serum sensitivity, we hypothesize that serum sensitivity is a consequence of homoserine accumulation. Serum survival is critical for dissemination, an important virulence determinant: thus, together with the essential nature of C. neoformans threonine synthesis, the cross-species serum sensitivity of thr1Delta mutants makes the fungus-specific Thr1p, and likely Thr4p, ideal antifungal drug targets.

A moderate threonine deficiency affects gene expression profile, paracellular permeability and glucose absorption capacity in the ileum of piglets.

High dietary threonine extraction by the digestive tract suggests that threonine contributes to maintain gut physiology. In the present study, we evaluated the impact of a low (6.5 g of threonine/kg diet; LT group) or a control well-balanced threonine diet (9.3 g of threonine/kg diet; C group) given to piglets for 2 weeks on ileal permeability and Na+-dependant glucose absorption capacity in Ussing chambers. The paracellular permeability was significantly increased in the ileum of LT compared to C piglets (P=.017). The Na+-dependent glucose absorption capacity showed a nonsignificant increase in the LT piglets. In addition, we analysed ileal gene expression profiles in the LT and C groups using porcine multitissue cDNA microarrays. Compared to the C piglets, the expression of 324 genes was significantly modified in the ileum of the LT piglets: 214 genes were overexpressed (145 annotated) and 110 were down-expressed (79 annotated). Among them, some are involved in immune and defense responses, energy metabolism and protein synthesis. Furthermore, microarray analysis highlights changes in the expression of the gene encoding for the sodium/glucose cotransporter (SGLT1) and of genes involved in the regulation of paracellular permeability (ZO-1, cingulin and myosin light chain kinase). In conclusion, our results indicate that a moderate threonine deficiency affects intestinal functionality.

Dietary L-homoserine spares threonine in chicks.

Four chick bioassays were conducted to evaluate the threonine (Thr) replacement value of l-homoserine (HS). Growth rate was increased (P < 0.05) by dietary addition of 800 mg l-HS/kg diet to a purified diet severely deficient in Thr or by the addition of 800 or 1000 mg of l-HS/kg diet to a corn-peanut meal diet distinctly deficient in Thr. The addition of an isomolar level of alpha-ketobutyrate, a catabolic product of both Thr and HS, did not elicit a response. Standard-curve methodology predicted a Thr replacement value of 38 +/- 9% for HS. Interactions (P < 0.01) were observed in assays 2 and 4 between dietary Thr adequacy and 800 or 1000 mg l-HS/kg supplementation. Thus, HS improved growth performance when added to a Thr-deficient diet (0.46 g Thr/100 g diet), but it decreased growth performance when added to the same diet containing surfeit Thr (0.80 g Thr/100 g diet). The results indicate that low levels of HS elicit a growth response in young chicks fed Thr-deficient diets.

Threonine-deficient diets induced changes in hepatic bioenergetics.

Diets deficient in an indispensable amino acid are known to suppress food intake in rats. Few studies were focused at understanding how amino acid-deficient diets may elicit biochemical changes at the mitochondrial level. The goal of this study was to evaluate mitochondrial function in rats fed diets with 0.00, 0.18, 0.36, and 0.88% threonine (Thr) (set at 0, 30, 60, and 140% of Thr requirement for growth). Here, it is described for the first time that Thr-deficient diets induce a specific uncoupling of mitochondria in liver, especially with NADH-linked substrates, not observed in heart (except for Thr-devoid diet). The advantage of this situation would be to provide ATP to support growth and maintenance when high-quality protein food (or wealth of high-quality food in general) is available, whereas Thr-deficient diets (or deficient-quality protein food) promote the opposite, increasing mitochondrial uncoupling in liver. The uncoupling with NADH substrates would favor the use of nutrients as energy sources with higher FADH-to-NADH ratios, such as fat, minimizing the first irreversible NADH-dependent catabolism of many amino acids, including Thr, thus enhancing the use of the limiting amino acid for protein synthesis when a low quality protein source is available.

A moderate threonine deficiency differently affects protein metabolism in tissues of early-weaned piglets.

A moderate threonine deficiency may affect differently tissue protein metabolism. In this study, we compared protein metabolism in the small and large intestines, the liver, and the carcass of piglets (Sus scrofa) pair-fed either a control well-balanced diet (C: 9.3 g threonine/kg diet) or a low threonine diet (LT: 6.5 g threonine/kg diet) for 2 weeks. In the small intestine, the LT diet did not modify protein deposition, fractional protein synthesis rate (K(S)) and AA protein composition. Ubiquitin mRNA level, a component of the ubiquitin-dependent proteolytic pathway, was significantly decreased in the jejunum of the LT piglets. Protein deposition measured in the carcass and the colon, and K(S) measured in the semitendinosus muscle and the colon, did not differ between LT and C piglets. Nevertheless, in these compartments, threonine content was reduced indicating deposition of proteins less rich in threonine. In the liver, protein retention was reduced, K(S) was increased and AA protein composition was modified in the LT compared to the C piglets. In conclusion compared to the other compartments, small intestinal protein metabolism seems to be less sensitive to a moderate dietary threonine deficiency. This indicates that dietary threonine extraction by the small intestine may reduce threonine availability for the other tissues when young piglets were fed a diet marginally deficient in threonine.

Adequate oral threonine is critical for mucin production and gut function in neonatal piglets.

In previous experiments, we found that the threonine requirement of neonatal piglets fed parenterally was 40% of that when fed intragastrically; we hypothesized that much of the oral supply of threonine is being used for mucin production. To investigate this hypothesis, intragastrically fed 2-day-old piglets were fed one of three treatments for 8 days: 1) a threonine-adequate diet (IG-A; 0.6 g threonine.kg(-1).day(-1) fed intragastrically); 2) a threonine-deficient diet (IG-D; 0.1 g threonine.kg(-1).day(-1) fed intragastrically); or 3) a threonine-deficient diet with adequate threonine delivered parenterally (IV-A; 0.5 g threonine.kg(-1).day(-1) fed parenterally plus 0.1 g threonine.kg(-1).day(-1) fed intragastrically). IG-D piglets experienced higher nitrogen excretion, higher plasma urea, and lower plasma threonine concentrations versus both of the other groups (P < 0.05), indicating profound threonine deficiency. Mucosal mass and total crude mucin content were lower in the colons of IG-D pigs (P < 0.05). Histopathological analysis showed lower numbers of acidic mucin-producing goblet cells in the duodenum and ileum of IG-D pigs. In IG-D pigs, acidic mucin subtypes were lower in the small intestine but higher in the colon, which corresponded with persistent diarrhea. The parenteral supply of threonine was adequate to maintain most outcome parameters, although IV-A pigs did have smaller colonic goblet cells with more acidic mucins compared with IG-A pigs. Overall, our results suggest that adequate dietary threonine was critical in the production of mucus and that a parenteral threonine supply can ameliorate most of the symptoms of oral threonine deficiency.

Co-localization of phosphorylated extracellular signal-regulated protein kinases 1/2 (ERK1/2) and phosphorylated eukaryotic initiation factor 2alpha (eIF2alpha) in response to a threonine-devoid diet.

The anterior piriform cortex (APC) has been shown to be an essential brain structure for the detection of dietary indispensable amino acid (IAA) deficiency, but little has been known about possible molecular detection mechanisms. Increased phosphorylation of the alpha-subunit of the eukaryotic initiation factor 2alpha (eIF2alpha) has been directly linked to amino acid deficiency in yeast. Recently, we have shown increased phosphorylation of eIF2alpha (p-eIF2alpha) in the rat APC 20 minutes after ingestion of an IAA-deficient meal. We suggest that if phosphorylation of eIF2alpha is an important mechanism in detection of IAA deficiency, then APC neurons that show p-eIF2alpha should also show molecular evidence of potentiation. The present research demonstrates increased expression and co-localization of p-eIF2alpha and phosphorylated extracellular signal-regulated protein kinase 1/2 (p-ERK1/2) in APC neurons, but not in the primary motor or agranular insular cortices in response to an IAA-deficient diet. ERK1/2 is an element of the mitogen-activated protein kinase cascade, an intraneuronal signaling mechanism associated with neuronal activation. The region of the APC that responds to IAA deficiency with increased p-eIF2alpha and p-ERK1/2 labeling ranges from 3.1 to 2.5 mm rostral of bregma. Within this region, only a few neurons respond to IAA deficiency with co-localization of abundant p-eIF2alpha and p-ERK1/2. These chemosensory neurons probably detect IAA deficiency and generate neuronal signaling to other portions of the brain, changing feeding behavior.

Dietary threonine restriction specifically reduces intestinal mucin synthesis in rats.

We determined whether the steady-state levels of intestinal mucins are more sensitive than total proteins to dietary threonine intake. For 14 d, male Sprague-Dawley rats (158 +/- 1 g, n = 32) were fed isonitrogenous diets (12.5% protein) containing 30% (group 30), 60% (group 60), 100% (control group), or 150% (group 150) of the theoretical threonine requirement for growth. All groups were pair-fed to the mean intake of group 30. The mucin and mucosal protein fractional synthesis rates (FSR) did not differ from controls in group 60. By contrast, the mucin FSR was significantly lower in the duodenum, ileum, and colon of group 30 compared with group 100, whereas the corresponding mucosal protein FSR did not differ. Because mucin mRNA levels did not differ between these 2 groups, mucin production in group 30 likely was impaired at the translational level. Our results clearly indicate that restriction of dietary threonine significantly and specifically impairs intestinal mucin synthesis. In clinical situations associated with increased threonine utilization, threonine availability may limit intestinal mucin synthesis and consequently reduce gut barrier function.

Diets deficient in indispensable amino acids rapidly decrease the concentration of the limiting amino acid in the anterior piriform cortex of rats.

Diets deficient in an indispensable amino acid have long been known to suppress food intake in rats. Detection of dietary deficiency takes place in the anterior piriform cortex (APC). Recent studies showed that the response to amino acid deficiency takes as little as 15 min to develop, but few data exist to correlate the concentration of amino acids in the APC with this rapid response. The purpose of this study was to measure the concentration of amino acids in the APC in a behaviorally relevant time frame. Rats were preconditioned by consumption of a basal diet for 7-10 d, and then given a test diet with either a control or deficient amino acid profile. Both the threonine- and leucine-deficient diets reliably depleted threonine and leucine concentration in the APC within 30 min, respectively. The control diets and a diet lacking the dispensable amino acid glycine did not lead to amino acid depletion. In combination with previous studies, the present results show that the decrease in the concentration of indispensable amino acids in the APC may be the initial sensory signal for recognition of dietary amino acid deficiency.

Phosphorylation of Ca2+/calmodulin-dependent protein kinase type ii and the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (ampa) receptor in response to a threonine-devoid diet.

The anterior piriform cortex (APC) functions as a chemosensor for indispensable amino acid deficiency and responds to this deficiency with increased activity, as indicated by observations including averaged evoked-potentials and c-fos expression in the APC. Little is known of the intracellular signaling mechanisms that mediate this deficiency-related increase in neuronal excitability, but previous studies have shown effects on intracellular Ca2+ in deficient APC slices in vitro. In the present study we hypothesized that indispensable amino acid deficiency increases intraneuronal Ca2+, resulting in autophosphorylation of calcium/calmodulin-dependent protein kinase type II (CaMKII) in vivo. Results demonstrated that phosphorylation levels of CaMKII (pCaMKII) in APC neurons increased at 20 and 40 min after a single meal of threonine-devoid diet. Phosphorylation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor subunit (GluR1) at the serine 831 (S831) site was modestly increased in the APC in response to a threonine-devoid meal. The GluR1 subunit also showed increased phosphorylation at the 845 (S845) site, suggesting additional signaling mechanisms. Although phosphorylation of CaMKII was sustained, phosphorylation of the GluR1 subunit returned to control levels by 40 min. These effects of amino acid deficiency did not occur throughout the brain as neither CaMKII nor GluR1 showed increased phosphorylation in the neocortex. These findings support the notion that calcium and glutamate signaling in the APC, but not throughout the brain, are triggered during early responses to amino acid deficiency. They also suggest that longer-term changes in APC neurons in response to such a deficiency may be mediated at least in part by CaMKII.