Development of amino acid transport system B0,+ in mouse blastocysts

The capacity of preimplantation mouse blastocysts to express the novel amino acid transport activity provisionally designated system B0,+ increased approximately 3-fold 1 day after administration of estrogen to their progesterone-primed, ovariectomized mothers. Nevertheless, blastocysts obtained 22-25 h after estrogen administration (implanting blastocysts) had to be incubated in vitro for about 20 min before they fully expressed their B0,+ activity. No similar increase in B0,+ activity was observed upon incubation of blastocysts obtained before estrogen administration (diapausing blastocysts). Rapid metabolic changes can be induced in the uterus by massaging it with a blunt instrument while it is receptive to implantation, and this treatment was found to increase the apparent B0,+ activity in implanting but not diapausing blastocysts. In contrast, the activity of an incompletely characterized, Na+-independent system, which accepts L-lysine as a substrate, decreased more than 2-fold when implanting blastocysts were incubated in vitro. No change in Na+-independent lysine uptake was detected during incubation of diapausing blastocysts. It is suggested that both uteri and blastocysts develop the capacity to change rapidly some of their metabolic processes near the time of implantation, and one of the processes which may be subject to rapid change in blastocysts is amino acid transport. These developmental events appear to coincide with and could be required for the decidual cell response and implantation of blastocysts in the uterus.     

Na+-independent transport of basic and zwitterionic amino acids in mouse blastocysts by a shared system and by processes which distinguish between these substrates

Preimplantation mouse blastocysts were found to contain at least three mediated components of Na+-independent amino acid transport. The two less conspicuous components seemed to be selective for either cationic or zwitterionic substrates but were not characterized further or examined for multiple transport activities. L-Leucine and L-lysine competed strongly for uptake by the most conspicuous Na+-independent transport process detected in these conceptuses (referred to as component b0,+), and no further heterogeneity of transport activities was found within this component. A series of inhibitors of various strengths had about the same effect on component b0,+ when either leucine or lysine was the substrate, and uptake of each substrate was not affected significantly by changes in the pH between 6.3 and 8.0. Furthermore, the Ki values for mutually competitive inhibition of transport between leucine and lysine and their Km values for transport via component b0,+ were all on the order of about 100 microM. In addition, the Ki values for competitive inhibition of leucine or lysine uptake by valine were approximately 5 mM in both cases, and alanine appeared to be a similarly weak competitive inhibitor of leucine transport. Based on these results, component b0,+ prefers to interact with bulky amino acids that do not branch at the beta-carbon. Moreover, amino acids that branch at the alpha-carbon, such as the leucine analog 3-amino-endo-bicyclo[3.2.1]octane-3-carboxylic acid, were virtually excluded by this component. The substrate reactivity of component b0,+ is more limited than the Na+-dependent transport system B0,+ in blastocysts which accepts both these branched species and less bulky amino acids relatively well as substrates. Thus, mediated amino acid transport in the mouse trophoblast is clearly distinguishable from that in most other mammalian tissues that have been studied. Not only do component b0,+ and system B0,+ and system B0,+ fail to discriminate strongly between basic and zwitterionic substrates, but their relative reactivity with bicyclic amino acids, such as 3-amino-endo-bicyclo[3.2.1]octane-3-carboxylic acid, is the reverse of transport processes in other cell types where these amino acids react strongly with Na+-independent, but not Na+-dependent, systems.     

Exogenous amino acids regulate trophectoderm differentiation in the mouse blastocyst through an mTOR-dependent pathway.

At the late blastocyst stage, the epithelial trophectoderm cells of the mammalian embryo undergo a phenotypic change that allows them to invade into the uterine stroma and make contact with the maternal circulation. This step can be regulated in vitro by the availability of amino acids. Embryos cultured in defined medium lacking amino acids cannot form trophoblast cell outgrowths on fibronectin, an in vitro model of implantation, but remain viable for up to 3 days in culture and will form outgrowths when transferred into complete medium. The amino acid requirement is a developmentally regulated permissive event that occurs during a 4- to 8-h period at the early blastocyst stage. Amino acids affect spreading competence specifically by regulating the onset of protrusive activity and not the onset of integrin activation. Rapamycin, a specific inhibitor of the kinase mTOR/FRAP/RAFT1, blocks amino acid stimulation of embryo outgrowth, demonstrating that mTOR is required for the initiation of trophectoderm protrusive activity. Inhibition of global protein translation with cycloheximide also inhibits amino acid-dependent signals, suggesting that mTOR regulates the translation of proteins required for trophoblast differentiation. Our data suggest that mTOR activity has a developmental regulatory function in trophectoderm differentiation that may serve to coordinate embryo and uterus at the time of implantation.     

Development of system B0,+ and a broad-scope Na(+)-dependent transporter of zwitterionic amino acids in preimplantation mouse conceptuses

The nature and ontogeny of Na(+)-dependent L-alanine transport was examined in mouse eggs and preimplantation conceptuses. Mediated L-alanine uptake was not detected in fertilized or unfertilized eggs, but a small amount of Na(+)-dependent L-alanine transport was detected in 2-cell conceptuses. Na(+)-dependent alanine transport was more rapid at the 8-cell stage of development, and more than 10-fold faster in blastocysts than in 8-cell conceptuses. Analog inhibition analyses were consistent with the interpretation that L-lysine-sensitive and L-lysine-resistant components of transport were present at the 2-cell, 8-cell and blastocyst stages of development. The range of amino acids and their analogs that inhibited the most conspicuous component of alanine transport in blastocysts was consistent with the conclusion that system B0,+ is largely responsible for L-alanine uptake in these conceptuses. Moreover, system B0,+, but not other known systems in blastocysts, became susceptible to activation as these conceptuses approached the time of implantation, so this activation could be involved in implantation. Although the data are consistent with the possibility that system B0,+ is also present in 2-cell and 8-cell conceptuses, the relatively slow L-alanine transport in conceptuses at these earlier stages of development precluded more detailed study of their ability to take up alanine. Similarly, the less conspicuous L-lysine-resistant component of L-alanine transport in blastocysts also may be present in conceptuses as early as the 2-cell stage. The L-lysine-resistant component of L-alanine transport could not be attributed to residual system B0,+ activity, however, because it was inhibited more strongly by trans-OH-L-proline than L-arginine, whereas the reverse was the case for system B0,+. Similarly, L-tryptophan and L-leucine each inhibited system B0,+ more strongly than L-serine or L-cysteine, whereas all four of these amino acids inhibited the L-lysine-resistant component equally well. Moreover, a Hofstee plot for L-alanine influx was consistent with the interpretation that at least two mediated components of Na(+)-dependent L-alanine transport are present in blastocysts. The less conspicuous component of L-alanine transport in blastocysts was relatively susceptible to inhibition by L-leucine and L-tryptophan, but it resisted inhibition by the 'model' system A substrate, MeAIB, and the system ASC inhibitors, L-penicillamine and cationic amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)     

Changes in the activities of amino acid transport systems b0,+ and L during development of preimplantation mouse conceptuses

Uptake of leucine, lysine, and arginine was predominantly Na(+)-independent in mouse conceptuses through the 8-cell stage of development, and two components of saturable transport were detected for each of these amino acids. Uptake of cationic substrates from solutions near 1 microM was inhibited most strongly by bulky cationic and zwitterionic amino acids whose carbon skeletons do not branch at the alpha or beta positions. By this criterion, system b0,+ accounted for most of the Na(+)-independent arginine and lysine transport in eggs and conceptuses throughout preimplantation development. A small, leucine-resistant, cation-preferring component of amino acid transport was also detected in these cells. Leucine uptake was inhibited most strongly by bicyclic, branched-chain or benzenoid, zwitterionic amino acids in eggs and conceptuses prior to formation of blastocysts. Therefore, it appeared to be taken up mainly by system L, while system b0,+ accounted for a smaller portion of leucine uptake during this developmental period. In blastocysts, in contrast, system L was less conspicuous, and system b0,+ was primarily responsible for Na(+)-independent leucine uptake. The Vmax values for transport of amino acids by system b0,+ increased by up to 30-fold in conceptuses between the 1-cell and blastocyst stages. In contrast, the Vmax value for leucine transport via system L decreased while the Km value increased between these two developmental stages. Although several explanations for these changes are possible, we favor the hypothesis that the density of system L transport sites in plasma membranes decreases while the number of system b0,+ sites increases during development of blastocysts from 1-cell conceptuses.     

Glycine transport in mouse eggs and preimplantation conceptuses

At least two Na+-dependent systems for glycine transport became detectable, while another became undetectable during preimplantation development of mouse conceptuses. Glycine was taken up by a process in eggs and cleavage-stage conceptuses which closely resembles system Gly. Mediated transport at these stages was more rapid at higher Cl- concentrations, sigmoidally related to the exogenous Na+ concentration, and strongly inhibited by sarcosine but not by amino acids with larger side chains. Moreover, neither Li+ nor choline could substitute for Na+ in stimulating glycine transport. System Gly was the only mediated process detected for glycine uptake in unfertilized and fertilized eggs and two-cell conceptuses, but two, less conspicuous, sarcosine-resistant, Na+-dependent components of transport also appeared to be present in eight-cell conceptuses. One of the latter components seemed to remain relatively inconspicuous when conceptuses formed blastocysts, while system Gly became undetectable. In contrast, the other less conspicuous component in eight-cell conceptuses appeared to become the most conspicuous transport process in blastocysts. The latter process, previously designated system B0,+, was shown here also to interact strongly with a broad scope of zwitterionic and cationic amino acid structures. Moreover, transport of glycine via system B0,+ was more rapid at higher Cl- concentrations, and this Na+-dependent process as well as Na+-independent leucine uptake were inhibited by choline. Furthermore, Na+-dependent amino acid transport in two-cell conceptuses and blastocysts was inhibited by 1.0 or 10 mM ouabain, but the inhibition was incomplete at both concentrations. Since Na+/K+-ATPase has not been detected in two-cell conceptuses, inhibition of amino acid transport by ouabain may not have been due solely to an effect on this enzyme. The level of system Gly activity decreased during the development of eight-cell conceptuses from eggs, and this decrease could contribute to an associated decline in intracellular glycine. Since other amino acids begin to compete strongly with glycine for transport when system B0,+ replaces system Gly in conceptuses, this qualitative change in transport activity may help account for a further decrease in the glycine content of conceptuses, reported elsewhere to occur after they form blastocysts.     

Na+-Independent Transporters, LAT-2 and b0,+, Exchange L-DOPA with Neutral and Basic Amino Acids in Two Clonal Renal Cell Lines

The present study examined the functional characteristics of L-DOPA transporters in two functionally different clonal subpopulations of opossum kidney (OKLC and OKHC) cells. The uptake of L-DOPA was largely Na+-independent, though in OKHC cells a minor component (approximately 15%) required extracellular Na+. At least two Na+-independent transporters appear to be involved in L-DOPA uptake. One of these transporters has a broad specificity for small and large neutral amino acids, is stimulated by acid pH and inhibited by 2-aminobicyclo(2,2,l)-heptane-2-carboxylic acid (BCH; OKLC, Ki = 291 mM; OKHC, Ki = 380 mM). The other Na+-independent transporter binds neutral and basic amino acids and also recognizes the di-amino acid cystine. [14C]-L-DOPA efflux from OKLC and OKHC cells over 12 min corresponded to a small amount of intracellular [14C]-L-DOPA. L-Leucine, nonlabelled L-DOPA, BCH and L-arginine, stimulated the efflux of [14C]-L-DOPA in a Na+-independent manner. It is suggested that L-DOPA uses at least two major transporters, systems LAT-2 and b0,+. The transport of L-DOPA by LAT-2 corresponds to a Na+-independent transporter with a broad specificity for small and large neutral amino acids, stimulated by acid pH and inhibited by BCH. The transport of L-DOPA by system b0,+ is a Na+-independent transporter for neutral and basic amino acids that also recognizes cystine. LAT-2 was found equally important at the apical and basolateral membranes, whereas system b0,+ had a predominant distribution in apical membranes.     

The amino acid sensitive TOR pathway from yeast to mammals

The target of rapamycin (TOR) is an ancient effector of cell growth that integrates signals from growth factors and nutrients. Two downstream effectors of mammalian TOR, the translational components S6K1 and 4EBP1, are commonly used as reporters of mTOR activity. The conical signaling cascade initiated by growth factors is mediated by PI3K, PKB, TSC1/2 and Rheb. However, the process through which nutrients, i.e., amino acids, activate mTOR remains largely unknown. Evidence exists for both an intracellular and/or a membrane bound sensor for amino acid mediated mTOR activation. Research in eukaryotic models, has implicated amino acid transporters as nutrient sensors. This review describes recent advances in nutrient signaling that impinge on mTOR and its targets including hVps34, class III PI3K, a transducer of nutrient availability to mTOR.     

Transport of benzenoid amino acids by system T and four broad scope systems in preimplantation mouse conceptuses

We have studied transport of L-tryptophan, L-tyrosine and L-phenylalanine as factors contributing to homeostasis of these amino acids in preimplantation mouse conceptuses. Benzenoid amino acids were transported by the Na(+)-independent systems L and b0,+ in 1-cell conceptuses, and by these systems plus the Na(+)-dependent systems B0,+ and B in blastocysts. In addition, a component of Na(+)-independent tryptophan, tyrosine and phenylalanine transport in 1-cell and 2-cell conceptuses and in blastocysts resisted inhibition by L-leucine. The latter component of transport not only preferred benzenoid amino acids and in particular tryptophan as substrates, but it also was inhibited strongly and competitively by alpha-N-methyl-L-tryptophan. The leucine-resistant component of tryptophan transport also was inhibited strongly by N-ethylmaleimide and D-tryptophan, and it appeared to be inhibited weakly by 3-amino-endo-bicyclo[3.2.1]octane-3-carboxylic acid (BCO) but not by other amino acids tested as inhibitors. By these criteria, the leucine-resistant component of transport of benzenoid amino acids resembled system T in human red blood cells and rat hepatocytes. It is not entirely clear why preimplantation blastocysts have five good systems for transport of tryptophan. It is possible, however, that tryptophan homeostasis is particularly important during preimplantation development since it has been shown elsewhere that tryptophan availability in blood increases within one day after rat eggs are fertilized.     

Transport of cationic and zwitterionic amino acids in preimplantation rat conceptuses

The ability of preimplantation rat conceptuses to take up several amino acids was examined under a variety of conditions, and the characteristics of uptake were compared to those determined previously for mouse conceptuses. Mediated leucine transport in two-cell rat conceptuses is Na(+)-independent and inhibited almost completely by 2-amino-endobicyclo[2.2.1]heptane-2-carboxylic acid (BCH), so it resembles system L which predominates in two-cell mouse conceptuses. System L becomes less conspicuous than homoarginine-sensitive, Na(+)-independent leucine transport (provisionally designated system bo,+) by the time rat conceptuses develop into blastocysts, as is also the case for mouse conceptuses. In contrast to leucine transport, system bo,+ appears to be the most conspicuous transporter of cationic amino acids throughout preimplantation development of both species. A Na(+)-independent cation-preferring amino acid transport process also appears to be present in rat as well as in mouse conceptuses. Moreover, rat conceptuses resemble mouse conceptuses because Na(+)-dependent transport system Gly activity virtually disappears from them by the time they form blastocysts. Unlike mouse conceptuses, however, Na(+)-dependent system Bo,+ activity appears to be present throughout preimplantation development of rat conceptuses, whereas it has not been detected until at least the two-cell stage in the mouse. Although system Bo,+ becomes more conspicuous in mouse than in rat conceptuses by the time they form blastocysts, system Bo,+ activity appears to increase when blastocysts of both species are removed from the uterus just prior to implantation. The latter observation is consistent with the possibility that system Bo,+ activity is controlled, in part, by the uterus near the time of implantation, although further studies are needed to verify this possibility. Similarities as well as differences in the amino acid transport processes present in conceptuses of rats and mice may eventually be understood best in relation to the environments in which they develop in vitro and in situ.     

mTOR is essential for growth and proliferation in early mouse embryos and embryonic stem cells

TOR is a serine-threonine kinase that was originally identified as a target of rapamycin in Saccharomyces cerevisiae and then found to be highly conserved among eukaryotes. In Drosophila melanogaster, inactivation of TOR or its substrate, S6 kinase, results in reduced cell size and embryonic lethality, indicating a critical role for the TOR pathway in cell growth control. However, the in vivo functions of mammalian TOR (mTOR) remain unclear. In this study, we disrupted the kinase domain of mouse mTOR by homologous recombination. While heterozygous mutant mice were normal and fertile, homozygous mutant embryos died shortly after implantation due to impaired cell proliferation in both embryonic and extraembryonic compartments. Homozygous blastocysts looked normal, but their inner cell mass and trophoblast failed to proliferate in vitro. Deletion of the C-terminal six amino acids of mTOR, which are essential for kinase activity, resulted in reduced cell size and proliferation arrest in embryonic stem cells. These data show that mTOR controls both cell size and proliferation in early mouse embryos and embryonic stem cells.     

Oleic acid stimulates system A amino acid transport in primary human trophoblast cells mediated by toll-like receptor 4

Obese women have an increased risk to deliver large babies. However, the mechanisms underlying fetal overgrowth in these pregnancies are not well understood. Obese pregnant women typically have elevated circulating lipid levels. We tested the hypothesis that fatty acids stimulate placental amino acid transport, mediated via toll-like receptor 4 (TLR4) and mammalian target of rapamycin (mTOR) signaling pathways. Circulating NEFA levels and placental TLR4 expression were assessed in women with varying prepregnancy body mass index (BMI). The effects of oleic acid on system A and system L amino acid transport, and on the activation of the mTOR (4EBP1, S6K1, rpS6), TLR4 (IĸBɑ, JNK, p38 MAPK), and STAT3 signaling pathways were determined in cultured primary human trophoblast cells. Maternal circulating NEFAs (n = 33), but not placental TLR4 mRNA expression (n = 16), correlated positively with BMI (P < 0.05). Oleic acid increased trophoblast JNK and STAT3 phosphorylation (P < 0.05), whereas mTOR activity was unaffected. Furthermore, oleic acid doubled trophoblast system A activity (P < 0.05), without affecting system L activity. siRNA-mediated silencing of TLR4 expression prevented the stimulatory effect of oleic acid on system A activity. Our data suggest that maternal fatty acids can increase placental nutrient transport via TLR4, thereby potentially affecting fetal growth.     

Inhibition of transport system b0,+ in blastocysts by inorganic and organic cations yields insight into the structure of its amino acid receptor site

The most conspicuous, Na(+)-independent amino acid transport process in preimplantation mouse blastocysts was provisionally designated system b0,+ because it accepts some cationic and zwitterionic amino acids about equally well as substrates. Although system b0,+ is not Na(+)-stimulated, it has not been determined if it is inhibited by Na+, or if its activity is affected by most other ions. Therefore, we measured uptake of amino acids by blastocysts in isotonic solutions of different ionic and nonionic osmolites. Na(+)-independent L-leucine uptake was unaffected by the ion concentration, but L-lysine transport was several-fold faster in isotonic solutions of non-electrolytes than in similar solutions of inorganic and organic salts or zwitterions. The Km value for 'Na(+)-independent' L-lysine transport was about 10-fold higher in isotonic salt solutions than in solutions of nonionic osmolites, whereas the Km value for L-leucine transport was about the same in either type of solution. Therefore, inorganic and organic cations and the cationic portions of zwitterions appear to compete with cationic but not zwitterionic amino acids for system b0,+ receptor sites. The cation, harmaline, was a particularly strong competitive inhibitor of 'Na(+)-independent' L-lysine uptake by system b0,+, even in isotonic salt solutions, whereas it inhibited L-leucine uptake noncompetitively. Moreover, harmaline appeared to compete with inorganic cations for the lysine receptor sites of system b0,+. Harmaline also has been found by other investigators to competitively inhibit L-lysine uptake by the Na(+)-independent system asc1 in horse erythrocytes, whereas it noncompetitively inhibits alanine uptake by the same system. Similarly, harmaline noncompetitively inhibits L-alanine uptake by the Na(+)-dependent system ASC in human erythrocytes, but it appears to compete for binding with L-alanine's cosubstrate, Na+. In addition, others have found that the positively-charged side chains of cationic amino acids seem to take the place of Na+ needed near side chains in order for zwitterionic amino acids to be transported by systems ASC and y+. We conclude that system b0,+ may be similar to systems asc1, ASC and y+, and that each of these systems may be a variant of the same ancestral transport process. We speculate that since it appears to accept a broader scope of substrates and to interact with a wider variety of cations than do systems asc1, ASC or y+, system b0,+ may more closely resemble the proposed ancestral transport process than any of the other contemporary systems.     

Morphological and radiochemical evidence for the metabolism of exogenous proteins by the preimplantation sheep blastocyst.

The ability of the trophoblast of the ovine preimplantation blastocyst to take up and metabolise proteins has been investigated using two experimental approaches, microscopical and radiochemical. The ultrastructure of the expanded blastocyst obtained from 14 and 17 day pregnant ewes was examined. The morphology of tissues maintained in culture for 24 h has been compared with that of fresh tissues. After culture, the cellular morphology of the explants was well preserved. Fresh and 24 h cultured tissues were incubated with horse-radish peroxidase and ferritin and these proteins subsequently were found to be localized in coated pits, caveolae and secondary lysosomes of the trophoblast. Comparison of the uptake of [3H]dextran and of 125I-labelled bovine serum albumin indicated that proteins could be taken up by cultured tissue by mechanisms in addition to simple fluid phase endocytosis. During culture of explants of blastocyst with 125I-labelled bovine serum albumin, a large fraction of the radioactivity taken up by the tissue appeared in the TCA-soluble fraction of the culture medium indicating that cultured trophoblast hydrolysed proteins. That amino acids released from captured protein could be used for protein synthesis by the trophoblast was indicated by the labelling of tissue and medium proteins after culturing explants with beta-lactamase labelled with [14C]leucine. A major product (Mr approximately 17 x 10(3) present in the medium was likely to have been ovine trophoblast protein-1. It is concluded that, during the expansion of the ovine blastocyst, the trophoblast has the ability to take up proteins, transport them to lysosomes and degrade them to amino acids which are used for protein synthesis. Thus proteins, as well as free amino acids, present in the histotrophe may be an important source of nitrogen for the sheep conceptus in the critical period just prior to implantation.     

Active amino acid transport in plasma membrane vesicles from Simian virus 40-transformed mouse fibroblasts. Characteristics of electrochemical Na+ gradient-stimulated uptake.

Selectively permeable membrane vesicles isolated from Simian virus 40-transformed mouse fibroblasts catalyzed Na+ gradient-coupled active transport of several neutral amino acids dissociated from intracellular metabolism. Na+-stimulated alanine transport activity accompanied plasma membrane material during centrifugation in discontinuous dextran 110 gradients. Carrier-mediated transport into the vesicle was demonstrated. When Na+ was equilibrated across the membrane, countertransport stimulation of L-[3H]alanine uptake occurred in the presence of accumulated unlabeled L-alanine, 2-aminoisobutyric acid, or L-methionine. Competitive interactions among neutral amino acids, pH profiles, and apparent Km values for Na+ gradient-stimulated transport into vesicles were similar to those previously described for amino acid uptake in Ehrlich ascites cells, which suggests that the transport activity assayed in vesicles is a component of the corresponding cellular uptake process. Both the initial rate and quasi-steady state of uptake were stimulated as a function of a Na+ gradient (external Na+ greater than internal Na+) applied artificially across the membrane and were independent of endogenous (Na+ + K+)-ATPase activity. Stimulation by Na+ was decreased when the Na+ gradient was dissipated by monensin, gramicidin D or Na+ preincubation. Na+ decreased the apparent Km for alanine, 2-aminoisobutyric acid, and glutamine transport. Na+ gradient-stimulated amino acid transport was electrogenic, stimulated by conditions expected to generate an interior-negative membrane potential, such as the presence of the permeant anions NO3- and SCN-. Na+-stimulated L-alanine transport was also stimulated by an electrogenic potassium diffusion potential (K+ internal greater than K+ external) catalyzed by valinomycin; this stimulation was blocked by nigericin. These observations provide support for a mechanism of active neutral amino acid transport via the "A system" of the plasma membrane in which both a Na+ gradient and membrane potential contribute to the total driving force.     

Intracellular sensing of amino acids in Xenopus laevis oocytes stimulates p70 S6 kinase in a target of rapamycin-dependent manner.

Amino acids exert modulatory effects on proteins involved in control of mRNA translation in animal cells through the target of rapamycin (TOR) signaling pathway. Here we use oocytes of Xenopus laevis to investigate mechanisms by which amino acids are "sensed" in animal cells. Small ( approximately 48%) but physiologically relevant increases in intracellular but not extracellular total amino acid concentration (or Leu or Trp but not Ala, Glu, or Gln alone) resulted in increased phosphorylation of p70(S6K) and its substrate ribosomal protein S6. This response was inhibited by rapamycin, demonstrating that the effects require the TOR pathway. Alcohols of active amino acids substituted for amino acids with lower efficiency. Oocytes were refractory to changes in external amino acid concentration unless surface permeability of the cell to amino acids was increased by overexpression of the System L amino acid transporter. Amino acid-induced, rapamycin-sensitive activation of p70(S6K) was conferred when System L-expressing oocytes were incubated in extracellular amino acids, supporting intracellular localization of the putative amino acid sensor. In contrast to lower eukaryotes such as yeast, which possess an extracellular amino acid sensor, our findings provide the first direct evidence for an intracellular location for the putative amino acid sensor in animal cells that signals increased amino acid availability to TOR/p70(S6K).     

Changes in neutral amino acid transport activity in myeloid leukemia cells differentiated by lipopolysaccharide

M1 cells derived from mouse myeloid leukemia have been reported to differentiate to macrophage-like cells upon treatment with substances such as lipopolysaccharide. Previously we found that in mouse peritoneal macrophages most of the neutral amino acids were taken up through a unique Na+-independent system. In this paper we have investigated the neutral amino acid transport in M1 cells and in those treated with lipopolysaccharide. In M1 cells serine, alanine and proline were taken up mainly by Na+-dependent transport systems, and leucine was largely transported by a Na+-independent system. By treating the cells with lipopolysaccharide, the activities of the Na+-dependent systems markedly decreased, whereas the activity of the Na+-independent system was little affected. The amino acid concentrations in the cells and the culture medium were measured. As a whole, the intracellular to extracellular distribution ratios for neutral amino acids that are preferred substrates for Na+-dependent systems were decreased on lipopolysaccharide treatment, whereas those for amino acids that are mainly transported by a Na+-independent system were slightly increased. From these results we conclude that M1 cells treated with lipopolysaccharide tend to differentiate to macrophage-like cells with respect to the neutral amino acid transport.     

Amino acids and insulin control autophagic proteolysis through different signaling pathways in relation to mTOR in isolated rat hepatocytes

Autophagy, a major bulk proteolytic pathway, contributes to intracellular protein turnover, together with protein synthesis. Both are subject to dynamic control by amino acids and insulin. The mechanisms of signaling and cross-talk of their physiological anabolic effects remain elusive. Recent studies established that amino acids and insulin induce p70 S6 kinase (p70(S6k)) phosphorylation by mTOR, involved in translational control of protein synthesis. Here, the signaling mechanisms of amino acids and insulin in macroautophagy in relation to mTOR were investigated. In isolated rat hepatocytes, both regulatory amino acids (RegAA) and insulin coordinately activated p70(S6k) phosphorylation, which was completely blocked by rapamycin, an mTOR inhibitor. However, rapamycin blocked proteolytic suppression by insulin, but did not block inhibition by RegAA. These contrasting results suggest that insulin controls autophagy through the mTOR pathway, but amino acids do not. Furthermore, micropermeabilization with Saccharomyces aureus alpha-toxin completely deprived hepatocytes of proteolytic responsiveness to RegAA and insulin, but still maintained p70(S6k) phosphorylation by RegAA. In contrast, Leu(8)-MAP, a non-transportable leucine analogue, did not mimic the effect of leucine on p70(S6k) phosphorylation, but maintained the activity on proteolysis. Finally, BCH, a System L-specific amino acid, did not affect proteolytic suppression or mTOR activation by leucine. All the results indicate that mTOR is not common to the signaling mechanisms of amino acids and insulin in autophagy, and that the amino acid signaling starts extracellularly with their "receptor(s)," probably other than transporters, and is mediated through a novel route distinct from the mTOR pathway employed by insulin.     

Action of aldosterone on cultured renal collecting duct cells during the latent period.

The effects of aldosterone on protein synthesis in the latent period were investigated on cultured renal collecting duct cells from neonatal rabbit kidneys. Tissue was incubated with radioactively labelled uridine and amino acids and then precipitated with trichloroacetic acid in order to determine the intracellular precursor pool and identify new synthesis of RNA and protein. During the latent period, aldosterone increased the intracellular radioactive uridine pool and total radioactive RNA content already 20 and 60 min after its application; conversely 40 min after aldosterone introduction, no stimulation was found. Further experiments revealed that the intracellular radioactive amino acid pool was generally increased by aldosterone after 20, 40 and 60 min, while a distinct increased radioactive protein content was found to be induced by aldosterone only after 40 min. This indicates that aldosterone increases the uptake of RNA and protein precursors and the new synthesis of RNA and proteins. These events seem to to be regulated not continuously but intermittently. The induced proteins possibly take part in the mediation of the early hormone response. Experiments with the aldosterone antagonist, spironolactone, provide evidence for the specificity of the described hormone effects. The results after application of the Na+ channel blocker, amiloride, and the Na+/K(+)-ATPase inhibitor, G-strophanthin, indicate that the aldosterone effects are controlled by Na+ channels and Na+ pumps and therefore by the intracellular Na+ content. The inhibitory effect of cycloheximide on the aldosterone-induced protein synthesis indicates the role of these proteins on the hormone-stimulated Na+ transport.     

Leucine and arginine regulate trophoblast motility through mTOR-dependent and independent pathways in the preimplantation mouse embryo

Uterine implantation is a critical element of mammalian reproduction and is a tightly and highly coordinated event. An intricate and reciprocal uterine-embryo dialog exists to synchronize uterine receptivity with the concomitant activation of the blastocyst, maximizing implantation success. While a number of pathways involved in regulating uterine receptivity have been identified in the mouse, less is understood about blastocyst activation, the process by which the trophectoderm (TE) receives extrinsic cues that initiate new characteristics essential for implantation. Amino acids (AA) have been found to regulate blastocyst activation and TE motility in vitro. In particular, we find that arginine and leucine alone are necessary and sufficient to induce TE motility. Both arginine and leucine act individually and additively to propagate signals that are dependent on the activity of the mammalian target of rapamycin complex 1 (mTORC1). The activities of the well-established downstream targets of mTORC1, p70S6K and 4EBP, do not correlate with trophoblast motility, suggesting that an independent-rapamycin-sensitive pathway operates to induce trophoblast motility, or that other, parallel amino acid-dependent pathways are also involved. We find that endogenous uterine factors act to induce mTORC1 activation and trophoblast motility at a specific time during pregnancy, and that this uterine signal is later than the previously defined signal that induces the attachment reaction. In vivo matured blastocysts exhibit competence to respond to an 8-hour AA stimulus by activating mTOR and subsequently undergoing trophoblast outgrowth by the morning of day 4.5 of pregnancy, but not on day 3.5. By the late afternoon of day 4.5, the embryos no longer require any exposure to AA to undergo trophoblast outgrowth in vitro, demonstrating the existence and timing of an equivalent in vivo signal. These results suggest that there are two separate uterine signals regulating implantation, one that primes the embryo for the attachment reaction and another that activates mTOR and initiates invasive behavior.