Uptake of taurocholic acid into isolated rat-liver cells.

Binding and transport characteristics for uptake of taurocholic acid by isolated rat liver cells were studied. 1. An adsorption of taurocholate to the cell surface is terminated in less than 15 s. A Ks of 0.55 mM and a total binding capacity of 3.8 nmol/mg cell protein is determined. 2. The rate of uptake of taurocholate follows Michaelis-Menten kinetics with Km = 19 muM and V = 1.7 nmol/mg protein min. 3. There is a broad pH optimum for uptake between pH 6.5 -- 8.0. 4. The activation energy amounts to 29 kcal/mol. At high taurocholate concentration an unusual upward bend is observed in the Arrhenius plot. 5. Taurocholate uptake is competitively inhibited by taurochenodeoxycholate (Ki = 9 muM). It is noncompetitively inhibited by bromosulfophthalein (Ki = 3 muM). 6. At physiological taurocholate concentrations a 200-fold intracellular accumulation of taurocholate is observed. 7. Uptake is inhibited by about 75% by either antimycin A, carbonylcyanide m-chlorophenyl-hydrazone, ouabain. 8. Replacement of extracellular Na+ by either K+ or sucrose results in a 75% decrease of uptake. 9. It is concluded that taurocholate uptake is a carrier-mediated process, and suggested that the energy for intracellular accumulation is made available by cotransport of Na+.     

Permeability of rat liver mitochondria to leucine, tyrosine, alpha-aminoisobutyric acid, and lysine

Radioactive tracer studies show that l-leucine is rapidly taken up by isolated rat liver mitochondria. There is an initial rapid uptake of l-leucine during the first 30 sec of incubation, followed by a slower, progressive increase in l-leucine accumulation over a 10-min incubation. d-Leucinc penetrates the mitochondria rapidly but does not accumulate inside. Both the d and l isomers of tyrosine penetrate the mitochondria relatively more slowly, equilibration being achieved only after several minutes of incubation. The synthetic amino acid, α-aminoisobutyric acid, is shown to rapidly enter the mitochondria. This amino acid is not accumulated within the mitochondria. l-Lysine, a positively charged amino acid, exhibits uptake characteristics similar to those of l-leucine. An examination of the energetic requirements for these amino acid transfer processes reveals no dependence on metabolic energy or on gradients of inorganic cations, and no effects of several reagents known to block other transport mechanisms, under the conditions tested. These findings and previously reported observations are consistent with the view that the membranes of rat liver mitochondria contain a number of different mechanisms mediating uptake of various amino acids.     

Uptake of botulinum neurotoxin into cultured neurons

Botulinum neurotoxins (BoNTs) act within the synaptic terminal to block neurotransmitter release. The toxin enters the neuron by binding to neuronal membrane receptor(s), being taken up into an endosome-like compartment, and penetrating the endosome membrane via a pH-dependent translocation process. Once within the synaptic cytoplasm, BoNT serotypes A and E cleave separate sites on the C-terminus of the neuronal protein SNAP-25, one of the SNARE proteins required for synaptic vesicle fusion. In this study, we measured the effect of brief toxin exposure on SNAP-25 proteolysis in neuronal cell cultures as an indicator of toxin translocation. The results indicate that (1) uptake of both BoNT-A and -E is enhanced with synaptic activity induced by K+ depolarization in the presence of Ca2+ and (2) translocation of BoNT-A from the acidic endosomal compartment is slow relative to that of BoNT-E. Polyclonal antisera against each toxin protect cells when applied with the toxin during stimulation but has no effect when added immediately after toxin exposure, indicating that toxin endocytosis occurs with synaptic activity. Both serotypes cleave SNAP-25 at concentrations between 50 pM and 4 nM. IC50 values for SNAP-25 cleavage are approximately 0.5 nM for both serotypes. Inhibition of the pH-dependent translocation process by pretreating cultures with concanamycin A (Con A) prevents cleavage of SNAP-25 with IC50 values of approximately 25 nM. Addition of Con A at times up to 15 min after toxin exposure abrogated BoNT-A action; however, addition of Con A after 40 min was no longer protective. In contrast, Con A inhibited, but did not prevent, translocation of BoNT-E even when added immediately after toxin exposure, indicating that pH-dependent translocation of BoNT-E is rapid relative to that of BoNT-A. This study demonstrates that uptake of both BoNT-A and -E is enhanced with synaptic activity and that translocation of the toxin catalytic moiety into the cytosol occurs at different rates for these two serotypes.     

Differential inhibition by Bacillus thuringiensis delta endotoxin of leucine and aspartic acid uptake into BBMV from midgut of Manduca sexta.

Pre-incubation of brush border membrane vesicles (BBMV) isolated from the midgut of Manduca sexta with activated Bacillus thuringiensis delta endotoxin for a short period resulted in differential inhibition of K(+)-dependent transport of leucine relative to the effect on K(+)-dependent transport of aspartic acid. The difference in I1/2 (5 fold greater for aspartic acid than for leucine) is interpreted as the result of enhanced binding of the B. thuringiensis delta endotoxin to the leucine transport system.     

Sodium-ion dependence of glycine and lysine transport in chicken erythrocytes genetically selected for high and low leucine transport activity

Amino acid transport was studied in two lines of chickens, one high and the other low uptake, selected for their ability to transport leucine into erythrocytes. On the basis of the number of mol of substrate transferred, medium Na+ was found to be more effective in stimulating glycine and lysine transport into high line cells than into low line cells. Glycine transport in both lines was stimulated by medium Na+ to a greater degree than was lysine transport. In the absence of medium Na+, glycine transport was not significantly different in the two lines. In the absence of medium Na+, lysine transport in the high line was about five-fold greater than in the low line. The transport differences between the lines are probably due to differences in several distinct genetic determinants.     

The interaction of glycine, aspartic acid, and lysine by the protonated macrocyclic ligand 6,19-bis(2-hydroxypropyl)-3,6,9,16,19,22-hexaaza-tricyclo-[22.2.2.2(11,14)]triaconta-11,13,24,26,27,29-hexaene

A new hexaaza macrocyclic ligand (L) bearing two 2-hydroxypropyl pendants, 6,19-bis(2-hydroxypropyl)-3,6,9,16,19,22-hexaaza-tricyclo-[22.2.2.2(11,14)]triaconta-11,13,24,26,27,29-hexaene has been synthesized and characterized. The macrocyclic ligand was isolated as a colorless crystal, monoclinic, P2(1)/n, with a=10.757(2), b=14.214(3), c=13.746(3) A, beta=101.40(3) degrees, V=2060.3(7) A3, Z=2, R1=0.0695, and wR2=0.1538 [I>2sigma(I)]. Potentiometric studies of the macrocyclic ligand and three types of amino acids, glycine (equal numbers of carboxylate and amino groups), aspartic acid (more carboxylate groups than amino group), and lysine (more amino groups than carboxylate group) have been performed. The stability constants for the new macrocycle and binary complexes of the amino acid with the macrocyclic ligand are reported. Binary complexes are formed in aqueous solution as a result of hydrogen bonding interaction and electrostatic attraction between the host and the guest. The binding Schemes for the recognition of amino acids are suggested. From the results, it seems that this new macrocyclic ligand is able to bind three different amino acids with selectivity in aqueous solution, and the strength of binding is of the order lysine < glycine < aspartic acid.     

Direct reprogramming of human astrocytes into neural stem cells and neurons

Generating neural stem cells and neurons from reprogrammed human astrocytes is a potential strategy for neurological repair. Here we show dedifferentiation of human cortical astrocytes into the neural stem/progenitor phenotype to obtain progenitor and mature cells with a neural fate. Ectopic expression of the reprogramming factors OCT4, SOX2, or NANOG into astrocytes in specific cytokine/culture conditions activated the neural stem gene program and induced generation of cells expressing neural stem/precursor markers. Pure CD44+ mature astrocytes also exhibited this lineage commitment change and did not require passing through a pluripotent state. These astrocyte-derived neural stem cells gave rise to neurons, astrocytes, and oligodendrocytes and showed in vivo engraftment properties. ASCL1 expression further promoted neuronal phenotype acquisition in vitro and in vivo. Methylation analysis showed that epigenetic modifications underlie this process. The restoration of multipotency from human astrocytes has potential in cellular reprogramming of endogenous central nervous system cells in neurological disorders.     

Uptake of norepinephrine by isolated neurons of rat pontine reticular formation

Summary Neurons isolated from the nucleus reticularis pontis caudalis and the lateral vestibular nucleus of the rat brain stem have been incubated in buffered sucrose containing norepinephrine. Norepinephrine accumulated intracellularly was then visualized by fluorescence microscopy after formaldehyde condensation.Incubation in sucrose containing 1.0 g/ml norepinephrine resulted in a 2-fold increase in fluorescence that was blocked by 1×10^–5M cocaine. Maximal enhancement of fluorescence, accomplished by incubation of isolated neurons with 40 g/ml norepinephrine, was unaffected by cocaine but effectively blocked by 1×10^–3M metanephrine. Accumulation of norepinephrine was unaffected by pretreatment of animals with reserpine and nialamide.These properties of norepinephrine uptake suggest a neuronal membrane transport system for norepinephrine in isolated neurons similar to Uptake 2 which occurs extraneuronally. Although the intracellular localization of exogenous norepinephrine accumulated is not known, some concentration of norepinephrine in the perikaryal cytoplasm is suggested.This work was supported by U.S. Public Health Service Research Grant NB 07044. The competent technical assistance of Mrs. Gisela Griffith and Miss Susan J. Decker is acknowledged with gratitude.     

Pyruvoyl-dependent histidine decarboxylase from Lactobacillus 30a. Covalent modifications of aspartic acid 191, lysine 155, and the pyruvoyl group

The pyruvoyl-dependent histidine decarboxylase from Lactobacillus 30a is rapidly inactivated by incubation with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and glycine ethyl ester. On 90% of inactivation, 1.3 residues of [14C]glycine ethyl ester are incorporated per alpha subunit; nearly 60% of this is linked to the beta-carboxyl group of Asp-191. Histamine, a competitive inhibitor, protects against this inactivation. The KM value of the modified enzyme for histidine (6.2 mM) is much higher than that of the unmodified enzyme (KM = 0.4 mM); catalytic activity is reduced but not eliminated. Thus, Asp-191 is the most reactive accessible carboxyl group under these conditions and is close to the substrate-binding site, but apparently is not essential for catalysis. At pH 8.0, fluorodinitrobenzene inactivates histidine decarboxylase completely with the incorporation of two dinitrophenyl residues/alpha subunit; the modified residues are Lys-155 and Cys-228. Urocanic acid, a competitive inhibitor, protects against inactivation. Treatment with mercaptoethanol restores the free -SH of Cys-228 but does not restore activity. Conversion of Cys-228 to its cyano derivative slows but does not prevent dinitrophenylation of Lys-155; the resulting derivative is catalytically inactive. Thus, Lys-155 is located within the active site and may play an essential role in catalysis. Finally, histidine methyl ester was shown to inhibit this decarboxylase by forming a Schiff's base with the essential pyruvoyl group.     

Zinc modulation of glycine receptors in acutely isolated rat CA3 neurons

Glycine and GABA are the primary inhibitory neurotransmitters in the spinal cord and brain stem, with glycine exerting its physiological roles by activating strychnine-sensitive ionotropic receptors. Glycine receptors are also expressed in the brain, including the cortex and hippocampus, but their physiological roles and pharmacological properties are largely unknown. Here, we report the pharmacological properties of functional glycine receptors in acutely isolated rat CA3 neurons using conventional whole-cell patch clamp techniques. Both glycine and taurine, which are endogenous agonists of glycine receptors, elicited Cl(-) currents in a concentration-dependent manner. The glycine-induced current (I(Gly)) was inhibited by strychnine, picrotoxin or cyclothiazide in a concentration-dependent manner. At lower concentrations (0.01-1 microM), ICS-205,930 potentiated I(Gly), but at higher concentrations (>10 microM) it inhibited I(Gly). These pharmacological properties strongly suggest that CA3 neurons express functional strychnine-sensitive glycine receptors containing alpha2 subunits. Furthermore, at lower concentrations (1-30 microM), Zn(2+) potentiated I(Gly), but at higher concentrations (>100 microM) it inhibited I(Gly). Considering that Zn(2+) is synaptically co-released with glutamate from mossy fiber terminals that make excitatory synapses onto CA3 neurons, these results suggest that endogenous Zn(2+) modulation of these glycine receptors may have an important role in the excitability of CA3 neurons.     

Intracellular ascorbic acid inhibits transport of glucose by neurons, but not by astrocytes

It has been demonstrated that glutamatergic activity induces ascorbic acid (AA) depletion in astrocytes. Additionally, different data indicate that AA may inhibit glucose accumulation in primary cultures of rat hippocampal neurons. Thus, our hypothesis postulates that AA released from the astrocytes during glutamatergic synaptic activity may inhibit glucose uptake by neurons. We observed that cultured neurons express the sodium-vitamin C cotransporter 2 and the facilitative glucose transporters (GLUT) 1 and 3, however, in hippocampal brain slices GLUT3 was the main transporter detected. Functional activity of GLUTs was confirmed by means of kinetic analysis using 2-deoxy-d-glucose. Therefore, we showed that AA, once accumulated inside the cell, inhibits glucose transport in both cortical and hippocampal neurons in culture. Additionally, we showed that astrocytes are not affected by AA. Using hippocampal slices, we observed that upon blockade of monocarboxylate utilization by alpha-cyano-4-hydroxycinnamate and after glucose deprivation, glucose could rescue neuronal response to electrical stimulation only if AA uptake is prevented. Finally, using a transwell system of separated neuronal and astrocytic cultures, we observed that glutamate can reduce glucose transport in neurons only in presence of AA-loaded astrocytes, suggesting the essential role of astrocyte-released AA in this effect.     

The interaction between aspartic acid 237 and lysine 358 in the lactose carrier of Escherichia coli.

The lacY from Escherichia coli strains 020 and AE43 have been cloned on plasmids which were designated p020-K358T and pAE43-D237N. These lacY mutants contain amino acid substitutions changing Lys-358 to Thr or Asp-237 to Asn, respectively. The charge neutralizing effect of each mutation is associated with a functional defect in melibiose transport which we exploited in order to isolate second site revertants to the melibiose-positive phenotype. Eleven melibiose-positive revertants of p020-K358T were isolated. All contained a second-site mutation converting Asp-237 to a neutral amino acid (8 to Asn, 1 to Gly, and 2 to Tyr). Twelve melibiose-positive revertants of pAE43-D237N were isolated. Two were second-site revertants converting Lys-358 to a neutrally Gln residue, while the remainder directly reverted Asn-237 to the wild-type Asp-237. We conclude that the functional intimate relationship between Asp-237 and Lys-358 suggests that these residues may be closely juxtaposed in three-dimensional space, possibly forming a 'charge-neutralizing' salt bridge.