Partial purification and characterization of the soluble DNA polymerase (polymerase-α) from seedlings of Pisum sativum L.

Radish seedlings (Raphanus sativus L. Saxa Treib) were grown in the dark with or without added kinetin (2 mg/l=9.29 M). Low-temperature (77°K) fluorescence emission and absorption spectra of etiolated cotyledons were registered at increasing seedling age before and immediately, 30 s and 30 min after one 1-ms flash. Kinetin was found to induce a higher accumulation of the phototransformable protochlorophyll(ide) P_657–650 in the etiolated cotyledons, especially from day 6 to day 10 after germination. The amount of the P_657–650 protochlorophyll(ide) resynthesized during a 30-min dark period after a 1-ms flash decreased with seedling age. It was smaller in cotyledons from kinetin-treated seedlings at day 6 after germination and at that age only. The ability to perform the Shibata shift decreased with increasing seedling age. In cotyledons from 10- and 13-day-old seedlings, the shift was accomplished to a greater extent when the plants were grown in the presence of kinetin.     

Osmotically-regulated trehalose accumulation and cyclic β-(1,2)-glucan excretion by Rhizobium leguminosarum biovar trifolii TA-1

Rhizobium leguminosarum biovar trifolii TA-1 produced high molecular weight extracellular (EPS) and capsular polysaccharides (CPS) as the main carbohydrate products in a medium (10 g of mannitol and 1 g of glutamic acid per liter) with low osmotic pressure of 0.20 MPa. By increasing the osmotic pressure of the medium with the addition of NaCl or other osmolytes up to 1.44 MPa, the synthesis of EPS and CPS was suppressed. Cyclic -(1,2)-glucans were excreted instead. Concentrations of over 1500 mg of glucans/l medium were produced by a biomass of 520 mg protein at 200 mM NaCl (1.20 MPa). Intracellular cyclic -(1,2)-glucan concentrations remained at 45 to 100 mg/g protein during the stationary phase, independent of the osmotic strength of the medium. Parallel to the increasing osmotic pressure of the medium, the disaccharide trehalose accumulated in the cells as osmo-protectant. Concentrations of up to 130 mg/g protein were reached. Strain TA-1 could tolerate 350 mM NaCl.Abbreviations CPS capsular polysaccharide - EPS extracellular polysaccharide - LM_r low molecular weight - HM_r high molecular weight     

Diversity in Guanosine 3′,5′-Bisdiphosphate (ppGpp) Sensitivity among Guanylate Kinases of Bacteria and Plants

The guanosine 3′,5′-bisdiphosphate (ppGpp) signaling system is shared by bacteria and plant chloroplasts, but its role in plants has remained unclear. Here we show that guanylate kinase (GK), a key enzyme in guanine nucleotide biosynthesis that catalyzes the conversion of GMP to GDP, is a target of regulation by ppGpp in chloroplasts of rice, pea, and Arabidopsis. Plants have two distinct types of GK that are localized to organelles (GKpm) or to the cytosol (GKc), with both enzymes being essential for growth and development. We found that the activity of rice GKpm in vitro was inhibited by ppGpp with a K_i of 2.8 μm relative to the substrate GMP, whereas the K_m of this enzyme for GMP was 73 μm. The IC₅₀ of ppGpp for GKpm was ∼10 μm. In contrast, the activity of rice GKc was insensitive to ppGpp, as was that of GK from bakers'' yeast, which is also a cytosolic enzyme. These observations suggest that ppGpp plays a pivotal role in the regulation of GTP biosynthesis in chloroplasts through specific inhibition of GKpm activity, with the regulation of GTP biosynthesis in chloroplasts thus being independent of that in the cytosol. We also found that GKs of Escherichia coli and Synechococcus elongatus PCC 7942 are insensitive to ppGpp, in contrast to the ppGpp sensitivity of the Bacillus subtilis enzyme. Our biochemical characterization of GK enzymes has thus revealed a novel target of ppGpp in chloroplasts and has uncovered diversity among bacterial GKs with regard to regulation by ppGpp.     

Inheritance and genetic mapping of two nuclear genes involved in nuclear–cytoplasmic incompatibility in peas (Pisum sativum L.)

Genetic analysis was performed to finely map and assess the mode of inheritance of two unlinked nuclear genes Scs1 and Scs2 involved in incompatibility of the nuclear genome of the cultivated pea Pisum sativum subsp. sativum with the cytoplasm of the wild pea of the subspecies P. sativum subsp. elatius, accession VIR320. Based on the segregation of genotypes in the progeny of the test-crosses, we concluded that if the cytoplasm was inherited from the wild pea VIR320, the Scs1 allele from the cultivated pea was gametophyte lethal and sporophyte recessive lethal. The Scs2 allele from the cultivated pea reduced male gametophyte viability. In homozygote, Scs2 from cultivated parent brought about nuclear–cytoplasmic conflict manifested as chlorophyll deficiency, reduction of blade organs, and low pollen fertility of about 20%. In heterozygote, Scs1 and Scs2 genes reduced pollen fertility by ca 50 and 30%, respectively. The Scs1 and Scs2 genes involved in nuclear–cytoplasmic incompatibility were genetically mapped. The distance between the markers bordering Scs1 comprised about 2.5 cM on linkage group III. The map distance between the bordering markers in the neighborhood of Scs2 varied substantially from cross to cross in the range of 2.0–15.1 cM on linkage group V.     

The fuc1 gene product (20 kDa FUC1) of Pisum sativum has no α-L-fucosidase activity

An -L-fucosidase purified from pea (Pisum sativum L. cv Alaska) epicotyl was previously described as a cell wall enzyme of 20 kDa that hydrolyses terminal -L-fucosidic linkages from oligosaccharide fragments of xyloglucan. cDNA and genomic copies were further isolated and sequenced. The predicted product of the cDNA and the genomic clone (fuc1), was a 20 kDa protein containing a signal peptide and five cysteines. This was the first -L-fucosidase gene to be cloned in plants but its fucosidase activity has not been demonstrated. Here, our biochemical and immuno analyses suggest that fuc1 does not encode an -L-fucosidase. Pea fuc1 expressed in Escherichia coli, insect cells and Arabidopsis thaliana produced recombinant proteins without -L-fucosidase activity. Pea plants had endogenous -L-fucosidase activity, but the enzyme was not recognised by an antibody produced against recombinant FUC1 protein expressed in E. coli. In contrast, the antibody immunoprecipitated a 20 kDa protein which was inactive. By chromatographic analysis of pea protein extracts, we separated -L-fucosidase-active fractions from the 20 kDa protein fractions. We conclude that the -L-fucosidase activity is not attributable to the 20 kDa FUC1 protein. A new function for fuc1 gene product, now named PIP20 (for protease inhibitor from pea) is proposed.     

A secondary effect of transformation in Rhizobium leguminosarum transgenic for Bacillus thuringiensis subspecies tenebrionisδ-endotoxin (cryIIIA) genes

 By introducing Bacillus thuringiensis subspecies tenebrionisδ-endotoxin genes (cryIIIA) into Rhizobium leguminosarum we have produced strains for the biological control of Sitona larvae. Comparisons between a transgenic and the parent strain show that transformation has induced changes not associated with the intended function of the transgene. Although growth rates in laboratory cultures are similar for both strains, the ability to compete for nodule occupancy is greater in the transgenic than in the non-transformed parent strain. This result demonstrates the importance of studying ecological and agronomic characters of transgenic micro-organisms that could have a bearing on the safety and success of their release into the environment, even if they are not thought to be connected with the transgenes introduced. Received: 20 April 1997 / Accepted: 2 June 1997     

A secondary effect of transformation in Rhizobium leguminosarum transgenic for Bacillus thuringiensis subspecies tenebrionisδ-endotoxin (cryIIIA) genes. Part 2

Summary  Rhizobium leguminosarum strains were produced for the biological control of Sitona larvae by introducing Bacillus thuringiensis subspecies tenebrionis delta-endotoxin genes (cryIIIA). Comparisons between a transgenic and parent strain show that transformation has induced changes not associated with the intended function of the transgene. Although growth rates in laboratory cultures are similar for both strains the ability to compete for nodule sites is greater in the transgenic than in the non-transformed parent strain, a character that has remained stable over 4 years. This increased ability, which was previously observed in axenic culture, is shown here to also occur in non-sterile soil, although the effect is less pronounced than in sterile conditions. Experiments in soil show a highly significant difference from the expected nodule occupancy ratio, assuming no difference between genotypes and with no significant variation between replicates. These results demonstrate that the ecological and agronomic characters of transgenics might be unexpectedly altered by transformation. Such characters might have a bearing on the safety and/ or success of transgenics released into the environment. Received: 1 July 1999 / Accepted: 29 July 1999     

Reverse Relationship between α-Carbon Chirality and Helix Handedness in (αMe)Phe Peptides

Abstract

The crystal-state preferred conformations of two tripeptides, one tetrapeptide, and one pen- tapeptide, each containing a single residue of the chiral, C^α,α-disubstituted glycine C^α-methyl, C^α-benzylglycine [(αMe)Phe], have been determined by X-ray diffraction. The tripeptides are Z-L-(αMe)Phe-(Aib)₂-OH dihydrate and Z-Aib-D-(αMe)Phe-Aib-OtBu, the tetrapeptide is Z-(Aib)₂-D-(αMe)Phe-Aib-OtBu, and the pentapeptide is pBrBz-(Aib)₂-DL-(αMe)Phe-(Aib)₂-OtBu. While the two tripeptides are folded in a β-bend conformation, two such conformations are consecutively formed by the tetrapeptide. The pentapeptide adopts a regular 3₁₀-helix promoted by three consecutive β-bends. This study confirms the strong propensity of short peptides containing C^α-methylated α-aminoacids to fold into β-bends and 3₁₀-helical structures. Since Aib is achiral, the handedness of the observed bends and helices is dictated by the presence of the (αMe)Phe residue. In general, we have found that the relationship between (αMe)Phe chirality and helix handedness is opposite to that exhibited by protein aminoacids. A comparison with the preferred conformation of other extensively investigated C^α-methylated aminoacids is made.     

Isolation of Allo-isoleucic Acid (α-Hydroxy-β-methylvaleric Acid) and β-Hydroxy-β-methylvaleric Acid from Turkish Tobacco Leaves

A new method determining the activity of tannin acyl hydrolase (tannase) was made. This method was based on the change in optical density of substrate tannic acid at 310 mμ. In this method, the error of measurement was about 1~3%, and many samples could be tested at one time because of its simplicity.

The procedure was as follows; To four parts of substrate (0.350 w/v% of tannic acid dissolved in 0.05m citrate buffer, pH 5.5), one part of the enzyme solution was added.

After t minutes reaction at 30°C, 0.1 part of the mixture was added to ten parts of 90% ethanol.

The optical density of the ethanol solution at 310 mμ was measured. Tannase activity (unit/ml) was given by following equation. $u=114\times \frac{E_{t1}?E_{t2}}{t_2?t_1}$

Where E_t1 and E_t2 mean the optical density of the ethanol solution at 310 mμ prepared after t₁ and t₂ minutes reaction, and one unit of the enzyme means the amount of the enzyme which is able to hydrolyze one μ mole of the ester bond in tannic acid in one minute.

The substrate tannic acid used in this determining method was purified. It was composed of one mole of glucose and nine moles of gallic acid, and eight moles of which formed four moles of m-digallic acid.     

Induction of iron(III) and copper(II) reduction in pea (Pisum sativum L.) roots by Fe and Cu status: Does the root-cell plasmalemma Fe(III)-chelate reductase perform a general role in regulating cation uptake?

We investigated the effects of Fe and Cu status of pea (Pisum sativum L.) seedlings on the regulation of the putative root plasma-membrane Fe(III)-chelate reductase that is involved in Fe(III)-chelate reduction and Fe²⁺ absorption in dicotyledons and nongraminaceous monocotyledons. Additionally, we investigated the ability of this reductase system to reduce Cu(II)-chelates as well as Fe(III)-chelates. Pea seedlings were grown in full nutrient solutions under control, -Fe, and -Cu conditions for up to 18 d. Iron(III) and Cu(II) reductase activity was visualized by placing roots in an agarose gel containing either Fe(III)-EDTA and the Fe(II) chelate, Na₂bathophenanthrolinedisulfonic acid (BPDS), for Fe(III) reduction, or CuSO₄, Na₃citrate, and Na₂-2,9-dimethyl-4,7-diphenyl-1, 10-phenanthrolinedisulfonic acid (BCDS) for Cu(II) reduction. Rates of root Fe(III) and Cu(II) reduction were determined via spectrophotometric assay of the Fe(II)-BPDS or the Cu(I)-BCDS chromophore. Reductase activity was induced or stimulated by either Fe deficiency or Cu depletion of the seedlings. Roots from both Fe-deficient and Cu-depleted plants were able to reduce exogenous Cu(II)-chelate as well as Fe(III)-chelate. When this reductase was induced by Fe deficiency, the accumulation of a number of mineral cations (i.e., Cu, Mn, Fe, Mg, and K) in leaves of pea seedlings was significantly increased. We suggest that, in addition to playing a critical role in Fe absorption, this plasma-membrane reductase system also plays a more general role in the regulation of cation absorption by root cells, possibly via the reduction of critical sulfhydryl groups in transport proteins involved in divalent-cation transport (divalent-cation channels?) across the root-cell plasmalemma.     

Community Genomic and Proteomic Analyses of Chemoautotrophic Iron-Oxidizing “Leptospirillum rubarum” (Group II) and “Leptospirillum ferrodiazotrophum” (Group III) Bacteria in Acid Mine Drainage Biofilms

We analyzed near-complete population (composite) genomic sequences for coexisting acidophilic iron-oxidizing Leptospirillum group II and III bacteria (phylum Nitrospirae) and an extrachromosomal plasmid from a Richmond Mine, Iron Mountain, CA, acid mine drainage biofilm. Community proteomic analysis of the genomically characterized sample and two other biofilms identified 64.6% and 44.9% of the predicted proteins of Leptospirillum groups II and III, respectively, and 20% of the predicted plasmid proteins. The bacteria share 92% 16S rRNA gene sequence identity and >60% of their genes, including integrated plasmid-like regions. The extrachromosomal plasmid carries conjugation genes with detectable sequence similarity to genes in the integrated conjugative plasmid, but only those on the extrachromosomal element were identified by proteomics. Both bacterial groups have genes for community-essential functions, including carbon fixation and biosynthesis of vitamins, fatty acids, and biopolymers (including cellulose); proteomic analyses reveal these activities. Both Leptospirillum types have multiple pathways for osmotic protection. Although both are motile, signal transduction and methyl-accepting chemotaxis proteins are more abundant in Leptospirillum group III, consistent with its distribution in gradients within biofilms. Interestingly, Leptospirillum group II uses a methyl-dependent and Leptospirillum group III a methyl-independent response pathway. Although only Leptospirillum group III can fix nitrogen, these proteins were not identified by proteomics. The abundances of core proteins are similar in all communities, but the abundance levels of unique and shared proteins of unknown function vary. Some proteins unique to one organism were highly expressed and may be key to the functional and ecological differentiation of Leptospirillum groups II and III.To understand how microorganisms contribute to biogeochemical cycling, it is necessary to determine the roles of uncultivated as well as cultivated groups and to establish how these roles vary during ecological succession and when environmental conditions change. Shotgun genomic sequencing (metagenomics) has opened new opportunities for culture-independent studies of microbial communities. Examples include investigations of acid mine drainage (AMD) biofilm communities (4, 43, 75), symbiosis in a marine worm involving sulfur-oxidizing and sulfate-reducing bacteria (85), and enhanced biological phosphorous removal by sludge communities (32). From these genomic data sets, it has been possible to reconstruct aspects of the metabolism of individual organisms (32) and coexisting community members (29, 75) and to identify which organisms contribute community-essential functions (75). An interesting question relates to how differences in metabolic potential between organisms from the same lineage allow them to occupy distinct niches. Identification of potentially adaptive traits in closely related organisms is also important from an evolutionary perspective.Genomic data do not reveal how organisms alter their metabolisms in response to the presence of other organisms or environmental conditions. Proteomics methods for analysis of metabolic responses of isolates (16, 17, 42, 80, 81) have been extended to analyze the functioning of the dominant members of natural consortia (56, 69), with strain-level resolution (43, 82). In these studies, peptides are separated by liquid chromatography (LC) and identified by tandem mass spectrometry (MS-MS) through reference to appropriate genomic databases. Proteomic analysis is possible even if the genome sequences are not identical to those of the organisms present (24); however, missing sequence information reduces the resolution of such proteogenomic studies.Due to dominance by a few organism types, chemoautotrophic microbial AMD biofilms from Richmond Mine, Iron Mountain, CA, are tractable model systems used to develop cultivation-independent metagenomic and proteogenomic methods for analysis of community structure, function, and ecology (13). Acidophilic Leptospirillum bacteria dominate this AMD system (15), other AMD systems (54), and bioleaching systems used for recovery of metals (19, 53, 86). These bacteria play pivotal roles in sulfide mineral dissolution because they are iron oxidizers (53, 75), and ferric iron drives sulfide oxidation, leading to formation of metal-rich sulfuric acid solutions. According to a recent microscopy-based study (83), Leptospirillum group II are the first colonists in AMD biofilm communities whereas Leptospirillum group III generally appear later, sometimes partitioned within biofilm interiors. Because only Leptospirillum group III appear to be able to fix nitrogen, they may be keystone species in AMD ecosystems (75). This observation enabled the isolation of one representative, “Leptospirillum ferrodiazotrophum” (76). In prior work, we reported near-complete genome sequences of two Leptospirillum group II types (43, 65), but detailed functional annotations and metabolic analyses have not been published. Genomic data have been used to explore the metabolism of Leptospirillum bacteria in one biofilm community (56), but proteomic and genomic analyses of the same biofilm community have not been performed.Here, we report a near-complete genomic sequence for Leptospirillum group III, derived from a biofilm obtained from the UBA site within the Richmond Mine, Iron Mountain, CA; a detailed functional annotation of the genomes of Leptospirillum groups II and III; and a genomic and proteomic comparison of them. In addition, we report the sequence of an extrachromosomal plasmid associated with these organisms. This study represents the first comprehensive genomics-based analysis of the metabolism of bacteria in the Nitrospirae phylum and the first environmental community proteogenomic study where the genomic and proteomic data were derived from the same sample. We compared the proteomic profiles of three different biofilm communities to evaluate the importance of shared and unique genes and pathways in environmental adaptation.     

The role of the delocalized α,α′-carbanion in vitamin B6 and Al(III) catalyzed transamination of α-amino and α-keto acids

The rates of the transamination reactions of α-amino acids and α-keto acids were followed by measurement of the 200 MHz proton NMR spectra of solution species as a function of time. Reaction systems measured in D₂O at 10 °C consisted of 1:1:1 molar ratios of pyridoxal:α-amino acid:Al(III) or pyridoxamine:α-keto acid:Al(III). Amino and keto acids employed are alanine, α-aminoisobutyric acid, valine, phenylglycine, pyruvic acid, and α-ketobutyric acid. A negatively charged deprotonated Schiff base coordinated to Al(III) was detected in all systems that undergo transamination (i.e., except α-aminoisobutyric acid). The intermediate resembles the aldimine Al(III) chelate with NMR resonances shifted upfield in accordance with its greater negative charge. Its equilibrium concentration is reached in the time required to reach transamination equilibrium and is maintained in solution at a ca. 10–20% of the aldimine Schiff base concentration.     

Gonadotropin-releasing hormone II (GnRH II) mediates the anorexigenic actions of α-melanocyte-stimulating hormone (α-MSH) and corticotropin-releasing hormone (CRH) in goldfish

Intracerebroventricular (ICV) administration of gonadotropin-releasing hormone II (GnRH II), which plays a crucial role in the regulation of reproduction in vertebrates, markedly reduces food intake in goldfish. However, the neurochemical pathways involved in the anorexigenic action of GnRH II and its interaction with other neuropeptides have not yet been identified. Alpha-melanocyte-stimulating hormone (α-MSH), corticotropin-releasing hormone (CRH) and CRH-related peptides play a major role in feeding control as potent anorexigenic neuropeptides in goldfish. However, our previous study has indicated that the GnRH II-induced anorexigenic action is not blocked by treatment with melanocortin 4 receptor (MC4R) and CRH receptor antagonists. Therefore, in the present study, we further examined whether the anorexigenic effects of α-MSH and CRH in goldfish could be mediated through the GnRH receptor neuronal pathway. ICV injection of the MC4R agonist, melanotan II (80 pmol/g body weight; BW), significantly reduced food intake, and its anorexigenic effect was suppressed by ICV pre-administration of the GnRH type I receptor antagonist, antide (100 pmol/g BW). The CRH-induced (50 pmol/g BW) anorexigenic action was also blocked by treatment with antide. ICV injection of CRH (50 pmol/g BW) induced a significant increase of the GnRH II mRNA level in the hypothalamus, while ICV injection of melanotan II (80 pmol/g BW) had no effect on the level of GnRH II mRNA. These results indicate that, in goldfish, the anorexigenic actions of α-MSH and CRH are mediated through the GnRH type I receptor-signaling pathway, and that the GnRH II system regulates feeding behavior.     

Potential implications of the glycosylation patterns in collagen α1(I) and α2(I) chains for fibril assembly and growth

O-Glycosylation of hydroxylysine (Hyl) in collagen occurs at an early stage of biosynthesis before the triple-helix has formed. This simple post-translational modification (PTM) of lysine by either a galactosyl or glucosylgalactosyl moiety is highly conserved in collagens and depends on the species, type of tissue and the collagen amino acid sequence. The structural/functional reason why only specific lysines are modified is poorly understood, and has led to increased efforts to map the sites of PTMs on collagen sequences from different species and to ascertain their potential role in vivo. To investigate this, we purified collagen type I (Col1) from the skins of four animals, then used mass spectrometry and proteomic techniques to identify lysines that were oxidised, galactosylated, glucosylgalactosylated, or glycated in its mature sequence. We found 18 out of the 38 lysines in collagen type Iα1, (Col1A1) and 7 of the 30 lysines in collagen type Iα2 (Col1A2) were glycosylated. Six of these modifications had not been reported before, and included a lysine involved in crosslinking collagen molecules. A Fourier transform analysis of the positions of the glycosylated hydroxylysines showed they display a regular axial distribution with the same d-period observed in collagen fibrils. The significance of this finding in terms of the assembly of collagen molecules into fibrils and of potential restrictions on the growth of the collagen fibrils is discussed.     

Identification of Amino Acid Residues in the α, β, and γ Subunits of the Epithelial Sodium Channel (ENaC) Involved in Amiloride Block and Ion Permeation

The amiloride-sensitive epithelial Nachannel (ENaC) is a heteromultimeric channel made of three αβγ subunits. The structures involved in the ion permeation pathway have only been partially identified, and the respective contributions of each subunit in the formation of the conduction pore has not yet been established. Using a site-directed mutagenesis approach, we have identified in a short segment preceding the second membrane-spanning domain (the pre-M2 segment) amino acid residues involved in ion permeation and critical for channel block by amiloride. Cys substitutions of Gly residues in β and γ subunits at position βG525 and γG537 increased the apparent inhibitory constant (K _i) for amiloride by >1,000-fold and decreased channel unitary current without affecting ion selectivity. The corresponding mutation S583 to C in the α subunit increased amiloride K _i by 20-fold, without changing channel conducting properties. Coexpression of these mutated αβγ subunits resulted in a nonconducting channel expressed at the cell surface. Finally, these Cys substitutions increased channel affinity for block by externalZn²⁺ ions, in particular the αS583C mutant showing a K _i for Zn²⁺of 29 μM. Mutations of residues αW582L or βG522D also increased amiloride K _i, the later mutation generating a Ca²⁺blocking site located 15% within the membrane electric field. These experiments provide strong evidence that αβγ ENaCs are pore-forming subunits involved in ion permeation through the channel. The pre-M2 segment of αβγ subunits may form a pore loop structure at the extracellular face of the channel, where amiloride binds within the channel lumen. We propose that amiloride interacts with Na⁺ions at an external Na⁺binding site preventing ion permeation through the channel pore.     

α1 (Pi) types and subtypes in the Tyrolean population

Since nine patients with infantile liver cirrhosis or hepatopathy associated with the Pi ZZ phenotype had been observed in recent years in the Children's Hospital of the University of Innsbruck, Tyrol, the distribution of the Pi types and the PiM subtypes was determined in the Tyrolean population. Apparently healthy blood donors (868) from different regions of Tyrol were examined. Isoelectricfocusing was used for classification of Pi types. The frequency of the allele PiZ was 0.0138, which corresponded to the range observed in other Middle European populations. The frequencies for the suballeles of PiM were PiM1 = 0.7062, PiM2 = 0.1480, and PiM3 = 0.1037. PiS had a frequency of 0.0225, the other rare alleles occurred with a combined frequency of 0.0058.     

Distinct Subunit-specific α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptor Trafficking Mechanisms in Cultured Cortical and Hippocampal Neurons in Response to Oxygen and Glucose Deprivation

Brain ischemia occurs when the blood supply to the brain is interrupted, leading to oxygen and glucose deprivation (OGD). This triggers a cascade of events causing a synaptic accumulation of glutamate. Excessive activation of glutamate receptors results in excitotoxicity and delayed cell death in vulnerable neurons. Following global cerebral ischemia, hippocampal CA1 pyramidal neurons are more vulnerable to injury than their cortical counterparts. The mechanisms that underlie this difference are unclear. Cultured hippocampal neurons respond to OGD with a rapid internalization of AMPA receptor (AMPAR) subunit GluA2, resulting in a switch from GluA2-containing Ca²⁺-impermeable receptors to GluA2-lacking Ca²⁺-permeable subtypes (CP-AMPARs). GluA2 internalization is a critical component of OGD-induced cell death in hippocampal neurons. It is unknown how AMPAR trafficking is affected in cortical neurons following OGD. Here, we show that cultured cortical neurons are resistant to an OGD insult that causes cell death in hippocampal neurons. GluA1 is inserted at the plasma membrane in both cortical and hippocampal neurons in response to OGD. In contrast, OGD causes a rapid endocytosis of GluA2 in hippocampal neurons, which is absent in cortical neurons. These data demonstrate that populations of neurons with different vulnerabilities to OGD recruit distinct cell biological mechanisms in response to insult, and that a crucial aspect of the mechanism leading to OGD-induced cell death is absent in cortical neurons. This strongly suggests that the absence of OGD-induced GluA2 trafficking contributes to the relatively low vulnerability of cortical neurons to ischemia.