Source of amino acids for tRNA acylation. Implications for measurement of protein synthesis

Dual proteolytic pathways govern glycolysis and immune competence

Proteasomes and lysosomes constitute the major cellular systems that catabolize proteins to recycle free amino acids for energy and new protein synthesis. Tripeptidyl peptidase II (TPPII) is a large cytosolic proteolytic complex that functions in tandem with the proteasome-ubiquitin protein degradation pathway. We found that autosomal recessive TPP2 mutations cause recurrent infections, autoimmunity, and neurodevelopmental delay in humans. We show that a major function of TPPII in mammalian cells is to maintain amino acid levels and that TPPII-deficient cells compensate by increasing lysosome number and proteolytic activity. However, the overabundant lysosomes derange cellular metabolism by consuming the key glycolytic enzyme hexokinase-2 through chaperone-mediated autophagy. This reduces glycolysis and impairs the production of effector cytokines, including IFN-γ and IL-1β. Thus, TPPII controls the balance between intracellular amino acid availability, lysosome number, and glycolysis, which is vital for adaptive and innate immunity and neurodevelopmental health.

MudPIT analysis of alkaline tolerance by Listeria monocytogenes strains recovered as persistent food factory contaminants

Alkaline solutions are used to clean food production environments but the role of alkaline resistance in persistent food factory contamination by Listeria monocytogenes is unknown. We used shotgun proteomics to characterise alkaline adapted L. monocytogenes recovered as persistent and transient food factory contaminants. Three unrelated strains were studied including two persistent and a transient food factory contaminant determined using multilocus sequence typing (MLST). The strains were adapted to growth at pH 8.5 and harvested in exponential phase. Protein extracts were analysed using multidimensional protein identification technology (MudPIT) and protein abundance compared by spectra counting. The strains elicited core responses to alkaline growth including modulation of intracellular pH, stabilisation of cellular processes and reduced cell-division, independent to lineage, MLST or whether the strains were transient or persistent contaminants. Alkaline adaptation by all strains corresponded to that expected in stringent-response induced cells, with protein expression supporting metabolic shifts concordant with elevated alarmone production and indicating that the alkaline-stringent response results from energy rather than nutrient limitation. We believe this is the first report describing induction of a stringent response in different L. monocytogenes strains by alkaline pH under non-limiting growth conditions. The work emphasises the need for early intervention to avoid persistent food factory contamination by L. monocytogenes.

Nutritional homeostasis and indispensable amino acid sensing: a new solution to an old puzzle

Indispensable amino acids are neither synthesized nor stored in animals and are rapidly depleted when not provided by the diet. To maintain homeostasis, organisms must sense deficiency of an indispensable amino acid and implement a repletion strategy. In rats and birds, the anterior piriform cortex houses the detector, but its mechanism has evaded description for >50 years. Recently, rapid detection of amino acid depletion was shown behaviorally when naïve animals, pre-fed a low nitrogen diet, terminated their first deficient meal within 20 min. The general amino acid control system of yeast, which is activated by amino acid deprivation via deacylated tRNA, was found to be active in rodent brain, showing conservation of amino acid sensory mechanisms across eukaryotic species.

Reevaluation of amino acid stimulation of protein synthesis in murine- and human-derived skeletal muscle cells assessed by independent techniques

Murine L6 and human rhabdomyosarcoma cells were cultured standardized in low (0.28 mM) and normal (9 mM) amino acid (AA) concentrations to reevaluate by independent methods to what extent AA activate initiation of protein synthesis. Methods used were incorporation of radioactive AA into proteins, distribution analysis of RNA in density gradient, and Western blots on initiation factors of translation of proteins in cultured cells as well as in vivo (gastrocnemius, C57Bl mice) during starvation/refeeding. Incorporation rate of AA gave incorrect results in a variety of conditions, where phenylalanine stimulated the incorporation rate of phenylalanine into proteins, but not of tyrosine, and tyrosine stimulated incorporation of tyrosine but not of phenylalanine. Similar problems were observed when [35S]methionine was used for labeling of fractionated cellular proteins. However, the methods entirely independent of labeled AA incorporation indicated that essential AA activate initiation of translation, whereas nonessential AA did not. Branched-chain AA and glutamine, in combination with some other AA, also stimulated initiation of translation. Starvation/refeeding in vitro agreed qualitatively with results in vivo evaluated by initiation factors. Insulin at physiological concentrations (100 microM/ml) did not stimulate global protein synthesis at low or normal AA concentrations but did so at supraphysiological levels (3 mU/ml), confirmed by independent methods. Our results reemphasize that labeled AA should be used with caution for quantification of protein synthesis, since the precursor pool(s) for protein synthesis is not in complete equilibrium with surrounding AA. "Flooding" tracee experiments did not overcome this problem.

Transfer ribonucleic acid charging in rat brain after consumption of amino acid-imbalanced diets

Recognition of an amino acid-imbalanced diet (IMB) is thought to occur in the anterior piriform cortex (APC) of the brain in response to a decrease in the limiting amino acid. We hypothesized that tRNA charging is decreased after ingestion of IMB and that this is part of the mechanism by which a decrease in the limiting amino acid is recognized. We investigated this question by determining levels of charged and uncharged tRNA using the periodate oxidation method and also by using high performance liquid chromatography (HPLC) analysis of amino acids acylated to brain tRNA. Using the periodate method, we found that isoleucyl-tRNA in both whole brain and APC of rats fed an isoleucine-IMB was increased, rather than decreased, in comparison to the basal diet and the corrected diet. Using HPLC analysis, we found that the absolute amount of tRNA charged with the limiting amino acid was not altered by dietary treatment. These two experimental approaches measure different aspects of tRNA charging, but the results clearly indicate that a reduction in tRNA charging is unlikely to be the signal by which a limiting amino acid is recognized in the brain 2 h after ingestion of IMB.

Aminoacyl-tRNA and tissue free amino acid pools are equilibrated after a flooding dose of phenylalanine

The flooding dose method, which is used to measure tissue protein synthesis, assumes equilibration of the isotopic labeling between the aminoacyl-tRNA pool and the tissue and blood free amino acid pools. However, this has not been verified for a phenylalanine tracer in an in vivo study. We determined the specific radioactivity of [(3)H]phenylalanine in the aminoacyl-tRNA and the tissue and blood free amino acid pools of skeletal muscle and liver 30 min after administration of a flooding dose of phenylalanine along with [(3)H]phenylalanine. Studies were performed in neonatal pigs in the fasted and refed states and during hyperinsulinemic-euglycemic-amino acid clamps. The results showed that, 30 min after the administration of a flooding dose of phenylalanine, there was equilibration of the specific radioactivity of phenylalanine among the blood, tissue, and tRNA precursor pools. Equilibration of the specific radioactivity of the three precursor pools for protein synthesis occurred in both skeletal muscle and liver. Neither feeding nor insulin status affected the aminoacyl-tRNA specific radioactivity relative to the tissue free amino acid specific radioactivity. The results support the assumption that the tissue free amino acid pool specific radioactivity is a valid measure of the precursor pool specific radioactivity and thus can be used to calculate protein synthesis rates in skeletal muscle and liver when a flooding dose of phenylalanine is administered.

Insulin-like growth factor (IGF-I) induces myotube hypertrophy associated with an increase in anaerobic glycolysis in a clonal skeletal-muscle cell model

Insulin-like growth factor-I (IGF-I) is an important autocrine/paracrine mediator of skeletal-muscle growth and development. To develop a definitive cultured cell model of skeletal-muscle hypertrophy, C2C12 cells were stably transfected with IGF-I and clonal lines developed and evaluated. Quantitative morphometric analysis showed that IGF-I-transfected myotubes had a larger area (2381+/-60 micrometer2 versus 1429+/-39 micrometer2; P<0.0001) and a greater maximum width (21.4+/-0.6 micrometer versus 13.9+/-0.3 micrometer; P<0.0001) than control C2C12 myotubes, independent of the number of cell nuclei per myotube. IGF-I-transfected myotubes had higher levels of protein synthesis but no difference in DNA synthesis when compared with control myotubes, indicating the development of hypertrophy rather than hyperplasia. Both lactate dehydrogenase and alanine aminotransferase activities were increased (3- and 5-fold respectively), and total lactate levels were higher (2.3-fold) in IGF-I-transfected compared with control myotubes, indicating an increase in anaerobic glycolysis in the hypertrophied myotubes. However, expression of genes involved in skeletal-muscle growth or hypertrophy in vivo, e.g. myocyte nuclear factor and myostatin, was not altered in the IGF-I myotubes. Finally, myotube hypertrophy could also be induced by treatment of C2C12 cells with recombinant IGF-I or by growing C2C12 cells in conditioned media from IGF-I-transfected cells. This quantitative model should be uniquely useful for elucidating the molecular mechanisms of skeletal-muscle hypertrophy.

Lactase phlorhizin hydrolase turnover in vivo in water-fed and colostrum-fed newborn pigs

We have estimated the synthesis rates in vivo of precursor and brush-border (BB) polypeptides of lactase phlorhizin hydrolase (LPH) in newborn pigs fed with water or colostrum for 24h post partum. At the end of the feeding period, piglets were anaesthetized and infused intravenously for 3h with L-[4-3H]- phenylalanine. Blood and jejunal samples were collected at timed intervals. The precursor and BB forms of LPH were isolated from jejunal mucosa by immunoprecipitation followed by SDS/PAGE, and their specific radioactivity in Phe determined. The kinetics of precursor and BB LPH labelling were analysed by using a linear compartmental model. Immunoisolated LPH protein consisted of five polypeptides [high-mannose LPH precursor (proLPHh), complex glycosylated LPH precursor (proLPHe), intermediate complex glycosylated LPH precursor (proLPH1i) and two forms of BB LPH]. The fractional synthesis rate (Ks) of proLPHh and proLPHc (approx. 5%/min) were the same in the two groups but the absolute synthesis rate (in arbitrary units, min-1) of proLPHh in the colostrum-fed animals was twice that of the water-fed animals. The Ks values of proLPHi polypeptides were significantly different (water-fed, 3.89%/min; colostrum-fed, 1.6%/min), but the absolute synthesis rates did not differ. The Ks of BB LPH was not different between experimental treatment groups (on average 0.037%/min). However, the proportion of newly synthesized proLPHh processed to BB LPH was 48% lower in colostrum-fed than in water-fed animals. We conclude that in neonatal pigs, the ingestion of colostrum stimulates the synthesis of proLPHh but, at least temporarily, disrupts the processing of proLPH polypeptides to the BB enzyme.

Influence of targeted asparagine starvation on extra- and intracellular amino acid pools of cultivated Chinese hamster ovary cells

For the development of an expression system with an amino-acid-inducible promoter, the influence of extracellular stress, by starvation of the non-essential amino acid asparagine, on the extra- and intracellular amino acid pool was investigated. Therefore a widely used nontransformed CHO cell line was cultivated in a serum-free and optimized DMEM/F12 medium in repeated batch mode. During the last repeat the medium contained no asparagine. The cells could compensate totally for this lack by an increased conversion of aspartate, glutamate, asparagine, serine, glutamine and arginine, while almost the whole intracellular pool of amino acids decreased. By this enhanced metabolic activity the maximum growth rate rose from 0.8 day-1 in complete medium to 1.1 day-1 in asparagine-free medium. The exceptional increase in asparagine biosynthesis points to a strong activation of asparagine synthetase, the key enzyme within the asparagine biosynthesis pathway. The regulation mechanism for the asparagine synthetase at the transcription level had to be analysed further in detail and will lead to an asparagine-sensitive promotor. To investigate reaction cascades that influence the protein synthesis or the overall gene expression, one had to look carefully at intracellular amino acid levels, because of their importance for polypeptide synthesis and energy supply, but also because of their obvious sensitivity to extracellular stresses.

Pentagastrin increases pepsin secretion without increasing its fractional synthetic rate

We studied the effects of increasing doses of pentagastrin on gastric secretion of pepsin and on incorporation of L-[1-13C]leucine into gastric aspirate protein as an index of pepsin synthesis. Pentagastrin (0.25-4.0 micrograms.kg-1.h-1) significantly increased pepsin output from basal 76 mg/h to < or = 181 mg/h but did not significantly alter incorporation of L-[1-13C]leucine from the basal fractional synthetic rate of 3.63 +/- 0.05%/h. In four subjects in whom infusion of tracer leucine was continued for > 1 day, aspiration of pepsin between 24 and 27 h demonstrated that plateau 13C labeling of leucine in pepsin had been attained, but at a value that was only 48% of the 13C labeling of plasma alpha-ketoisocaproic acid (alpha-KIC) [0.730 +/- 0.02 (SE) vs. 1.520 +/- 0.14 atoms %excess]. This suggests that actual rates of pepsin synthesis were approximately double those calculated on the basis of alpha-KIC labeling. The results are consistent with an interpretation that increasing doses of pentagastrin cause increased secretion of pepsinogen by recruitment of gastric chief cells, each synthesizing pepsinogen at an unaltered rate. Plateau 13C enrichment of alpha-KIC may not be a valid surrogate for plateau 13C leucine enrichment when fractional synthetic rates of some secreted proteins are calculated.