Cell-to-cell transfer of glial proteins to the squid giant axon: The glia- neuron protein transfer hypothesis

The hypothesis that glial cells synthesize proteins which are transferred to adjacent neurons was evaluated in the giant fiber of the squid (Loligo pealei). When giant fibers are separated from their neuron cell bodies and incubated in the presence of radioactive amino acids, labeled proteins appear in the glial cells and axoplasm. Labeled axonal proteins were detected by three methods: extrusion of the axoplasm from the giant fiber, autoradiography, and perfusion of the giant fiber. This protein synthesis is completely inhibited by puromycin but is not affected by chloramphenicol. The following evidence indicates that the labeled axonal proteins are not synthesized within the axon itself. (a) The axon does not contain a significant amount of ribosomes or ribosomal RNA. (b) Isolated axoplasm did not incorporate [(3)H]leucine into proteins. (c) Injection of Rnase into the giant axon did not reduce the appearance of newly synthesized proteins in the axoplasm of the giant fiber. These findings, coupled with other evidence, have led us to conclude that the adaxonal glial cells synthesize a class of proteins which are transferred to the giant axon. Analysis of the kinetics of this phenomenon indicates that some proteins are transferred to the axon within minutes of their synthesis in the glial cells. One or more of the steps in the transfer process appear to involve Ca++, since replacement of extracellular Ca++ by either Mg++ or Co++ significantly reduces the appearance of labeled proteins in the axon. A substantial fraction of newly synthesized glial proteins, possibly as much as 40 percent, are transferred to the giant axon. These proteins are heterogeneous and range in size from 12,000 to greater than 200,000 daltons. Comparisons of the amount of amino acid incorporation in glia cells and neuron cell bodies raise the possibility that the adaxonal glial cells may provide an important source of axonal proteins which is supplemental to that provided by axonal transport from the cell body. These findings are discussed with reference to a possible trophic effect of glia on neurons and metabolic cooperation between adaxonal glia and the axon.     

Protection of prostaglandin H synthase from trypsin upon binding of heme

The sensitivity of the apo- and holoenzyme forms of prostaglandin H synthase to trypsin have been investigated. Both the cyclooxygenase and peroxidase activities associated with the synthase were rapidly lost from the apoenzyme when incubated with trypsin. However, both activities were resistant to trypsin in the holoenzyme, suggesting that some structural change accompanies heme binding. Inactive protein present in some holoenzyme preparations, although indistinguishable from the synthase subunit by polyacrylamide gel electrophoresis, was also hydrolyzed by trypsin.     

Comparison of the peroxidase reaction kinetics of prostaglandin H synthase-1 and -2.

Prostaglandin H synthase isoforms 1 and 2 (PGHS-1 and -2) each have a peroxidase activity and also a cyclooxygenase activity that requires initiation by hydroperoxide. The hydroperoxide initiator requirement for PGHS-2 cyclooxygenase is about 10-fold lower than for PGHS-1 cyclooxygenase, and this difference may contribute to the distinct control of cellular prostanoid synthesis by the two isoforms. We compared the kinetics of the initial peroxidase steps in PGHS-1 and -2 to quantify mechanistic differences between the isoforms that might contribute to the difference in cyclooxygenase initiation efficiency. The kinetics of formation of Intermediate I (an Fe(IV) species with a porphyrin free radical) and Intermediate II (an Fe(IV) species with a tyrosyl free radical, thought to be the crucial oxidant in cyclooxygenase catalysis) were monitored at 4 degrees c by stopped flow spectrophotometry with several hydroperoxides as substrate. With 15-hydroperoxyeicosatetraenoic acid, the rate constant for Intermediate I formation (k1) was 2.3 x 10(7) M-1 s-1 for PGHS-1 and 2.5 x 10(7) M-1 s-1 for PGHS-2, indicating that the isoforms have similar initial reactivity with this lipid hydroperoxide. For PGHS-1, the rate of conversion of Intermediate I to Intermediate II (k2) became the limiting factor when the hydroperoxide level was increased, indicating a rate constant of 10(2)-10(3) s-1 for the generation of the active cyclooxygenase species. For PGHS-2, however, the transition between Intermediates I and II was not rate-limiting even at the highest hydroperoxide concentrations tested, indicating that the k2 value for PGHS-2 was much greater than that for PGHS-1. Computer modelling predicted that faster formation of the active cyclooxygenase species (Intermediate II) or increased stability of the active species increases the resistance of the cyclooxygenase to inhibition by the intracellular hydroperoxide scavenger, glutathione peroxidase. Kinetic differences between the PGHS isoforms in forming or stabilizing the active cyclooxygenase species can thus contribute to the difference in the regulation of their cellular activities.     

Resonance Raman studies indicate a unique heme active site in prostaglandin H synthase

Prostaglandin H synthase isoforms 1 and 2 (PGHS-1 and -2) catalyze the first two steps in the biosynthesis of prostaglandins. Resonance Raman spectroscopy was used to characterize the PGHS heme active site and its immediate environment. Ferric PGHS-1 has a predominant six-coordinate high-spin heme at room temperature, with water as the sixth ligand. The proximal histidine ligand (or the distal water ligand) of this hexacoordinate high-spin heme species was reversibly photolabile, leading to a pentacoordinate high-spin ferric heme iron. Ferrous PGHS-1 has a single species of five-coordinate high-spin heme, as evident from nu(2) at 1558 cm(-1) and nu(3) at 1471 cm(-1). nu(4) at 1359 cm(-1) indicates that histidine is the proximal ligand. A weak band at 226-228 cm(-1) was tentatively assigned as the Fe-His stretching vibration. Cyanoferric PGHS-1 exhibited a nu(Fe)(-)(CN) line at 446 cm(-1) and delta(Fe)(-)(C)(-)(N) at 410 cm(-1), indicating a "linear" Fe-C-N binding conformation with the proximal histidine. This linkage agrees well with the open distal heme pocket in PGHS-1. The ferrous PGHS-1 CO complex exhibited three important marker lines: nu(Fe)(-)(CO) (531 cm(-1)), delta(Fe)(-)(C)(-)(O) (567 cm(-1)), and nu(C)(-)(O) (1954 cm(-1)). No hydrogen bonding was detected for the heme-bound CO in PGHS-1. These frequencies markedly deviated from the nu(Fe)(-)(CO)/nu(C)(-)(O) correlation curve for heme proteins and porphyrins with a proximal histidine or imidazolate, suggesting an extremely weak bond between the heme iron and the proximal histidine in PGHS-1. At alkaline pH, PGHS-1 is converted to a second CO binding conformation (nu(Fe)(-)(CO): 496 cm(-1)) where disruption of the hydrogen bonding interactions to the proximal histidine may occur.     

PTGS1, PTGS2, ALOX5, ALOX12, ALOX15, and FLAP SNPs: interaction with fatty acids in colon cancer and rectal cancer

Dietary polyunsaturated fatty acids (PUFAs) can be converted to prostaglandins and leukotrienes. Oxygenation of omega-6 PUFAs generally results in the production of pro-inflammatory mediators, whereas oxygenated products of omega-3 (n-3) PUFAs generally have lower inflammatory activity. We hypothesize that elevated n-3 PUFA intakes from fish are associated with lower risk of colorectal cancer among those with genetic variants that result in higher levels of pro-inflammatory mediators. In population-based case–control studies of colon (case n = 1,574) and rectal cancer (case n = 791) and disease-free controls (n = 2,969), we investigated interactions between dietary fatty acid intake and 107 candidate polymorphisms and tagSNPs in PTGS1, PTGS2, ALOX12, ALOX5, ALOX15, and FLAP. The two studies used an identical genotyping protocol. We observed interactions and statistically significant increases in colon cancer risk for low docosahexaenoic acid intake among those with the PTGS1 rs10306110 (−1,053 A > G) variant genotypes (OR = 1.6, 95 % confidence interval = 1.1–2.3, adj. p = 0.06) and rectal cancer risk for low total fat intake among those with the variant PTGS1 rs10306122 (7,135 A > G) (OR_vs.wt = 1.80, 1.02–2.99; adj. p = 0.08). The ALOX15 rs11568131 (10,339 C > T) wild type in combination with a high inflammation score (low EPA intake, high AA intake, no regular NSAID use, high BMI, smoking) was associated with increased colon cancer risk (OR = 2.28, 1.7–3.07). Rectal cancer risk was inversely associated with a low inflammation score among PTGS2 rs4648276 (3,934 T > C) variant allele carriers (OR = 0.49, 0.25–0.75). Overall, these data provide some modest evidence for interactions between dietary fat intake and genetic variation in genes involved in eicosanoid metabolism and colorectal cancer risk.

Electronic supplementary material

The online version of this article (doi:10.1007/s12263-012-0302-x) contains supplementary material, which is available to authorized users.     

High-yield production, purification and characterization of functional human duodenal cytochrome b in an Escherichia coli system

Human duodenal cytochrome b (Dcytb) is a transmembrane hemoprotein found in the duodenal brush border membrane and in erythrocytes. Dcytb has been linked to uptake of dietary iron and to ascorbate recycling in erythrocytes. Detailed biophysical and biochemical characterization of Dcytb has been limited by difficulties in expressing sufficient amounts of functional recombinant protein in yeast and insect cell systems. We have developed an Escherichia coli Rosetta-gami B(DE3) cell system for production of recombinant His-tagged human Dcytb with a yield of ～26 mg of purified, ascorbate-reducible cytochrome per liter of culture. The recombinant protein is readily solubilized with n-dodecyl-β-D-maltoside and purified to electrophoretic homogeneity by one-step chromatography on cobalt affinity resin. The purified recombinant Dcytb has a heme to protein ratio very close to the theoretical value of 2 and retains functional reactivity with ascorbate, as assessed by spectroscopic and kinetic measurements. Ascorbate showed a marked kinetic selectivity for the high-potential heme center over the low-potential heme center in purified Dcytb. This new E. coli expression system for Dcytb offers ～7-fold improvement in yield and other substantial advantages over existing expression systems for reliable production of functional Dcytb at levels suitable for biochemical, biophysical and structural characterization.     

Structural comparisons of arachidonic acid-induced radicals formed by prostaglandin H synthase-1 and -2

Cyclooxygenase catalysis by prostaglandin H synthase (PGHS)-1 and -2 involves reaction of a peroxide-induced Tyr385 radical with arachidonic acid (AA) to form an AA radical that reacts with O₂. The potential for isomeric AA radicals and formation of an alternate tyrosyl radical at Tyr504 complicate analysis of radical intermediates. We compared the EPR spectra of PGHS-1 and -2 reacted with peroxide and AA or specifically deuterated AA in anaerobic, single-turnover experiments. With peroxide-treated PGHS-2, the carbon-centered radical observed after AA addition was consistently a pentadienyl radical; a variable wide-singlet (WS) contribution from mixture of Tyr385 and Tyr504 radicals was also present. Analogous reactions with PGHS-1 produced EPR signals consistent with varying proportions of pentadienyl and tyrosyl radicals, and two additional EPR signals. One, insensitive to oxygen exposure, is the narrow singlet tyrosyl radical with clear hyperfine features found previously in inhibitor-pretreated PGHS-1. The second type of EPR signal is a narrow singlet lacking detailed hyperfine features that disappeared upon oxygen exposure. This signal was previously ascribed to an allyl radical, but high field EPR analysis indicated that ~ 90% of the signal originates from a novel tyrosyl radical, with a small contribution from a carbon-centered species. The radical kinetics could be resolved by global analysis of EPR spectra of samples trapped at various times during anaerobic reaction of PGHS-1 with a mixture of peroxide and AA. The improved understanding of the dynamics of AA and tyrosyl radicals in PGHS-1 and -2 will be useful for elucidating details of the cyclooxygenase mechanism, particularly the H-transfer between tyrosyl radical and AA.     

Oxyferryl heme and not tyrosyl radical is the likely culprit in prostaglandin H synthase-1 peroxidase inactivation

Prostaglandin H synthase-1 (PGHS-1) is a bifunctional heme protein catalyzing both a peroxidase reaction, in which peroxides are converted to alcohols, and a cyclooxygenase reaction, in which arachidonic acid is converted into prostaglandin G2. Reaction of PGHS-1 with peroxide forms Intermediate I, which has an oxyferryl heme and a porphyrin radical. An intramolecular electron transfer from Tyr385 to Intermediate I forms Intermediate II, which contains two oxidants: an oxyferryl heme and the Tyr385 radical required for cyclooxygenase catalysis. Self-inactivation of the peroxidase begins with Intermediate II, but it has been unclear which of the two oxidants is involved. The kinetics of tyrosyl radical, oxyferryl heme, and peroxidase inactivation were examined in reactions of PGHS-1 reconstituted with heme or mangano protoporphyrin IX with a lipid hydroperoxide, 15-hydroperoxyeicosatetraenoic acid (15-HPETE), and ethyl hydrogen peroxide (EtOOH). Tyrosyl radical formation was significantly faster with 15-HPETE than with EtOOH and roughly paralleled oxyferryl heme formation at low peroxide levels. However, the oxyferryl heme intensity decayed much more rapidly than the tyrosyl radical intensity at high peroxide levels. The rates of reactions for PGHS-1 reconstituted with MnPPIX were approximately an order of magnitude slower, and the initial species formed displayed a wide singlet (WS) radical, rather than the wide doublet radical observed with PGHS-1 reconstituted with heme. Inactivation of the peroxidase activity during the reaction of PGHS-1 with EtOOH or 15-HPETE correlated with oxyferryl heme decay, but not with changes in tyrosyl radical intensity or EPR line shape, indicating that the oxyferryl heme, and not the tyrosyl radical, is responsible for the self-destructive peroxidase side reactions. Computer modeling to a minimal mechanism was consistent with oxyferryl heme being the source of peroxidase inactivation.     

His92 and His110 selectively affect different heme centers of adrenal cytochrome b(561)

Adrenal cytochrome b(561) (cyt b(561)), a transmembrane protein that shuttles reducing equivalents derived from ascorbate, has two heme centers with distinct spectroscopic signals and reactivity towards ascorbate. The His54/His122 and His88/His161 pairs furnish axial ligands for the hemes, but additional amino acid residues contributing to the heme centers have not been identified. A computational model of human cyt b(561) (Bashtovyy, D., Berczi, A., Asard, H., and Pali, T. (2003) Protoplasma 221, 31-40) predicts that His92 is near the His88/His161 heme and that His110 abuts the His54/His122 heme. We tested these predictions by analyzing the effects of mutations at His92 or His110 on the spectroscopic and functional properties. Wild type cytochrome and mutants with substitutions in other histidine residues or in Asn78 were used for comparison. The largest lineshape changes in the optical absorbance spectrum of the high-potential (b(H)) peak were seen with mutation of His92; the largest changes in the low-potential (b(L)) peak lineshape were observed with mutation of His110. In the EPR spectra, mutation of His92 shifted the position of the g=3.1 signal (b(H)) but not the g=3.7 signal (b(L)). In reductive titrations with ascorbate, mutations in His92 produced the largest increase in the midpoint for the b(H) transition; mutations in His110 produced the largest decreases in DeltaA(561) for the b(L) transition. These results indicate that His92 can be considered part of the b(H) heme center, and His110 part of the b(L) heme center, in adrenal cyt b(561).